Forecast Discussions mentioning any of "HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/30/20

Area Forecast Discussion

National Weather Service La Crosse WI

558 PM CST Tue Dec 29 2020

.SHORT TERM...(This evening through Wednesday Morning)
Issued at 247 PM CST Tue Dec 29 2020

At 1930Z, snow was spreading into the area with a quick drop to 1
mile or less visibility in most places. The forecast continues to
be on track...with a minor adjustment down in the snow totals
from this morning...about 1 inch.

Intense moisture surge of over 200% normal precipitable water now
has its leading edge around I-80 in Iowa. This is reflected by
higher reflectivity echo and will shift just south of the forecast
area. This is transitioning to heavy snow as it moves
north and its track, or northern extent, is just south of
previous forecasts. The northern side of this schield should track
into the warning area, but the CAMS and 29.12Z SPC HREF target
higher 1-2"/hr snow rate probabilities slightly south of the
forecast area. As Bob Uecker says, "Just a bit outside". But, the
northern extent will certainly have influence to reach warning
snows. When this area hits locally, it will really snow heavy for
the 5-9 pm window. 280K isentropic surface is beautiful today via
the RAP and really provides a nice visual of the upglide, moisture
plume, 40-50kt winds in the 800-900 mb layer - as well as the sharp
north-south boundary shifting to subsidence.

Also evident is the sharper back edge to the precipitation axis in
eastern SD/NE. This will continue to shift east and end
precipitation earlier across MN than previously thought. Thus, have
lowered snow amounts some in that area. With the major moist plume
shifting to just south of the area, have also lowered amounts
on the NW edge of the warning...but only by an inch.

Overall, have decided to end the Warnings and Advisories 3 hours
earlier based on the increase in speed. The bulk of accumulations
should be done by midnight. Travel conditions should be improving
rapidly Wednesday morning.

.LONG TERM...(Wednesday night through Tuesday)
Issued at 247 PM CST Tue Dec 29 2020

Wednesday afternoon into Thursday...system exits to the east of the
forecast area and surface ridge builds into central Wisconsin and
eastern Iowa by 12z Thursday. Drier air advects into the forecast
area and skies will clear from west to east Wednesday
night...providing mostly sunny skies and near normal temperatures
Thursday.

Focus then turns to late Thursday night into New Year`s day. The
29.12z GFS/ECMWF/NAM in decent agreement in lifting upper level low
over the Southern Plain States into the Great Lakes Region New
Year`s Day. The main difference is the NAM is further north with
surface low...hence advects warmer air further north into the
southern forecast area. However...the latest 29.12z GEFS members
suggest the NAM to be an outlier and will follow more the 29.12z
GFS/ECMWF/GEFS track. This will keep the wintry mix across the far
southern part of the forecast area. Ice accumulation at this time
look to be up to a tenth of an inch and snow amounts up to 2 inches.

Main forecast concerns Saturday through Tuesday are temperatures
through the period. The 29.12z GFS/ECMWF/GEM are in good agreement
in developing west to east zonal flow across the continental United
States through the period. The deterministic models suggest weak
impulses embedded in flow aloft to track across the northern tier
states through period. However...forcing/lift is weak with each
piece of energy and looks to be mainly dry through the period.
Temperatures through period will be above normal...as the
deterministic models indicate 925mb minus 2 to plus 3 degrees
celsius and the 29.12z NAEFS show temperatures plus 1 to near plus 2
standard anomalies at 850mb by Monday and Tuesday. Expect high
temperatures to be in the lower to middle 30s Monday and Tuesday
across forecast area.

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 558 PM CST Tue Dec 29 2020

MVFR to IFR conditions are expected into tonight, with snow tapering
off from west to east over the next few hours. There may be a couple
of hours of fog or very light freezing drizzle behind the snow, but
conditions should improve later tonight as the low pressure system
pushes east. Guidance does suggest lower ceilings eventually
sneak in from the west, and have brought IFR ceilings into KRST
later tonight as a result. Ceiling and visibility trends will have
to be monitored for possible adjustments, however. Low level
moisture looks to stick around through much of Wednesday, so
confidence is low on whether low clouds will be able to clear out
during the day. Winds will turn from southeast to westerly
tonight, with gusts near 20 kts possible early this evening and
again Wednesday.

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Storm Warning until 9 AM CST Wednesday for WIZ053>055-061.

Winter Weather Advisory until 9 AM CST Wednesday for WIZ017-029-
034-042>044.

Winter Weather Advisory until 6 AM CST Wednesday for WIZ032-033-
041.

MN...Winter Weather Advisory until 6 AM CST Wednesday for MNZ079-
086>088-094>096.

IA...Winter Storm Warning until 9 AM CST Wednesday for IAZ009>011-018-
019-029-030.

Winter Weather Advisory until 6 AM CST Wednesday for IAZ008.

SHORT TERM...Baumgardt
LONG TERM...DTJ
AVIATION...Lee

Area Forecast Discussion

National Weather Service Buffalo NY

1002 PM EST Tue Dec 29 2020

.SYNOPSIS...
High pressure ridge from New York to the North Carolina coast will
provide dry weather most of the overnight. A warm front will move
through the area Wednesday morning with a chance of some light snow
possibly mixed with sleet. The warm front will usher in warmer
temperatures for Wednesday afternoon, with an approaching cold front
then bringing a period of rain showers late Wednesday and Wednesday
night. The rain may be mixed with or take form of a wintry mix
especially in the interior Southern Tier and east of Lake Ontario.
Any rain will then end as some wet snow and lake effect snow showers
Wednesday night into Thursday.

.NEAR TERM /THROUGH WEDNESDAY/...
Still have some lake effect clouds off southeast Lake Ontario and to
east of Lake Erie with just thin high clouds streaming in over all
but North Country. Clear and calm conditions east of Lake Ontario to
St. Lawrence and temps have plummeted to near zero there, so lowered
mins there for the rest of the night. Temps elsewhere will stabilize
and even rise slightly as clouds thicken from west ahead of warm
front aloft that moves in toward daybreak.

Warm front aloft traverses the Lower Lakes Wednesday morning. Brief
saturation occurs especially over Niagara Frontier and across
Southern Ontario/western Lake Ontario. Seems that only looking at a
hundredth or two of QPF per HRRR which is matching upstream radar
trends to this time. Point soundings for this leading edge of precip
indicate snow or maybe some sleet as it ends. Given very dry layer
seen on BUF sounding this evening, would expect good wet bulb
cooling and trended forecast toward ptype of snow with a chance
of sleet through mid morning.

Will become breezy and warmer in wake of this morning precip and
ahead of sfc cold front that approaches in the afternoon. Expect
highs to reach the low to mid 40s along the Lake Plains in the
afternoon. By that time, more widespread precip is expected to move
in from the west as upper trough and associated sfc low and cold
front slide across lower Great Lakes. Though most areas will see
only rain as ptype in the afternoon, colder air will be harder to
displace from the interior Southern Tier as well just inland across
the North Country. There are differences in models in just how warm
the warm layer aloft around H85/5kft AGL becomes, with the NAM/GFS
still warmest and Canadian coldest. Where temps stay cold in the
interior Southern Tier could see rain/sleet or even freezing rain.
Seems the better chance of more frozen precip in form of sleet and
snow will be across the North Country. Greatest confidence in seeing
enough snow/sleet to cause some issues will be over the North
Country as seen in latest HRRR output as well with at least a couple
inches of snow/sleet, but this is very late on Wednesday. Meanwhile
due to questions with warm layer/ptype, there is lesser confidence
in seeing only freezing rain over interior Southern Tier. Overall at
this point, put a mention in HWO of possible wintry mix both of
these areas, but given uncertainty opted for no advisory headlines.
Mid shift can take another look at this with new guidance to see if
advisory will be needed.

.SHORT TERM /WEDNESDAY NIGHT THROUGH THURSDAY NIGHT/...
The cold front shifts through Wednesday night. With its
passage, west-northwest flow drops 850 hPa temperatures
enough to start to get a lake response. This is reflected well
in the Canadian, so the forecast was pushed largely toward that
guidance through Thursday. However, as large scale moisture departs,
lake effect will not have the gusto that it has had recently.

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Low pressure over the lower Mississippi Valley will return the cold
front as a warm front, which will slowly work its way across the
area on Friday. Some light snow may develop out ahead of the warm
front as it approaches from the south Friday morning. As the warmer
air aloft noses into the area, model soundings indicate a better
sustained shallow surface freezing layer on the higher terrain in
the Southern Tier and the Tug Hill Plateau. This may facilitate the
formation of freezing rain Friday afternoon for the Southern Tier
and later Friday evening for the Tug Hill, though there still
remains uncertainty with timing and duration this far out. The area
should see all rain by Friday evening, which will gradually taper
off through the day Saturday.

A Southern Stream shortwave trough trails behind the low pressure
system, passing to our south during the day Sunday. 12z model
guidance has hinted at this shortwave possibly phasing with a
northern stream longwave trough, which could nudge it far enough
north to bring another round of precip for the area. High pressure
builds in as this system passes to our southeast, bringing dry
weather through Tuesday.

.AVIATION /03Z WEDNESDAY THROUGH SUNDAY/...
Westerly flow will support a few lake effect clouds east and
southeast of Lake Erie and Lake Ontario through midnight. Dry
weather will prevail with VFR conditions.

On Wednesday, CIGS and VSBY will remain VFR until late in the day
with approaching cold front. Expect LLWS to develop at all TAF sites
in the morning with gusty sfc winds over 25 knots following in the
afternoon. A chance of light snow, mixed with sleet in the morning
will be followed by mainly rain mid to late afternoon. However,
there could be a wintry mix over the interior Southern Tier in the
afternoon and east of Lake Ontario late Wednesday into Wednesday
evening. CIGS will eventually trend MVFR by late day. Best chance of
IFR or MVFR VSBY late on Wednesday will be east of Lake Ontario
where ptype may stay mainly snow.

Outlook...

Wednesday night and Thursday morning...MVFR/IFR with rain likely,
possibly ending as wet snow. Improvement to VFR Thursday afternoon.
LLWS possible Wednesday night.
Friday...IFR/MVFR with rain likely. Possibly starting as a wintry
mix at onset.
Saturday...MVFR with chance of snow.
Sunday...MVFR with a chance of rain and snow showers.

.MARINE...
Southerly winds will rapidly increase Wednesday ahead of a cold
front, producing another round of Small Craft Advisory conditions
for the eastern Great Lakes, which will then last into Thursday.

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...None.
MARINE...Small Craft Advisory from 10 AM Wednesday to 7 AM EST Thursday
for LEZ040-041.
Small Craft Advisory from 1 PM Wednesday to 1 PM EST
Thursday for LOZ042-043.
Small Craft Advisory from 10 AM Wednesday to 7 PM EST
Thursday for LOZ044-045.

SYNOPSIS...Fries/Hitchcock/JLA
NEAR TERM...Fries/JLA
SHORT TERM...Fries
LONG TERM...PP
AVIATION...Fries/JLA
MARINE...Fries/Hitchcock/JLA

Area Forecast Discussion

National Weather Service Corpus Christi TX

545 PM CST Tue Dec 29 2020

.AVIATION /00Z TAFs/ ...

VFR conditions are currently prevailing across all terminals with
the exception of VCT. Isolated showers will continue to impact
the Victoria Crossroads this evening. Brief MVFR/IFR conditions
can be expected with any passing shower. Ceilings will lower to
MVFR levels overnight as low level stratus builds back in. Gusty
southeasterly winds will weaken slightly tonight before
restrengthening by mid day tomorrow with gusts from 25-30 knots
possible across the eastern 3 terminals. VFR conditions are
expected to return by mid morning. Included a mention of VCSH at
CRP and VCT tomorrow as isolated showers are possible during the
day. Chances for showers and storms will increase just beyond this
TAF window as a cold front nears the region.

.PREV DISCUSSION... /Issued 348 PM CST Tue Dec 29 2020/

SHORT TERM (Tonight through Wednesday Night)...

Rich moisture continued to overspread the region this afternoon on
very breezy southeasterly winds. Satellite and RAP mesoanalysis
reveal a moist axis extending from the western Gulf NNW to the
Victoria Crossroads coincident with a band of low-topped showers.
Even though a 45-50 knot southerly LLJ develops tonight over the
Coastal Plains, additional moisture advection from this jet appears
negligible given its drier continental source region. However, a
broad stratus shield should still develop overnight which combined
with breezy SE winds will keep lows much milder than recent nights.

PWATs on Wednesday rise to between 1 and 1.3 inches from west to
east as winds aloft back more southerly ahead of a vigorous upper
low crossing northern Mexico. Throughout much of the daytime
Wednesday, background ascent appears rather muted with only
some showers and a few thunderstorms at times. This picture should
completely change by Wednesday night as significant forcing
arrives - not with just a strong cold front but also large height
falls, a 120 knot upper jet, and favorable upper diffluence east
of the approaching upper low. Models and CAMs agree nicely with
the strongest ascent occurring generally from 10 PM until just
before daybreak Thursday. An axis of MLCAPEs of 1000-1300 J/kg
along the Coastal Bend north to the Crossroads will be the focus
for some brief severe modes as the cold front drops south,
although this threat may take a back seat to heavy rain/minor
flooding given PWATs of 1/2 to 3/4 inch above normal, deep
southerly flow and cell training modes evident on various CAMs.
Latest QPF from the HREF mean depicts a core of 2-3 inches across
the Victoria Crossroads which could still cause some flooding
concerns despite rather dry soils.

LONG TERM (Thursday through Tuesday)...

Rain and thunderstorm chances are expected to linger into Thursday
morning across South Texas, before drying out in the afternoon as
moisture decreases behind the front and the upper level low exits
the region to our north. At this time, a warm thermodynamic
profile continues to support rainfall as the primary precip type
across the area on Thursday, with the frozen precip remaining
farther north over the Texas Hill Country. However, if the upper
low shifts slightly southward than anticipated, then can`t rule
out brief isolated sleet or graupel across the northern Rio Grande
Plains and Brush Country. But, little to no impacts are expected
if that were to occur due to the warm surface temperatures and
quick melting.

Breezy to windy conditions can also be expected behind the front on
Thursday which may approach Advisory criteria by Thursday afternoon.
In addition to the gusty winds, expect much cooler temperatures in
the wake of the front on Thursday, with highs struggling to get out
of the 50s due to the thick cloud cover and the arrival of CAA.

New Year`s Eve looks much drier and calmer as PWATs are progged to
drop to below a half an inch, with a clearing in the cloud cover
expected to occur from southwest to northeast. The combination of
the clear skies, lighter winds and a much drier and cooler mass in
place will allow for optimal radiational cooling to occur by New
Year`s morning, with the lows forecast to fall into the 30s across
most of the area. A light freeze will be possible over portions of
the Coastal Plains, but it`s not anticipated to be widespread and
rather more localized over low-lying and drainage areas. These dry
and cooler conditions are expected to persist through most of the
weekend, with the exception of a few spotty showers here and there
with the arrival of a couple of weak disturbances in the mid/upper
flow. Winds will become onshore on Sunday leading to a gradual warm-
up and moisture increase through early next week. Rain chances may
return late in the period with the arrival of deeper moisture and a
coastal trough.

MARINE...

Moderate to strong southeasterly flow continues tonight through the
day on Wednesday. A Small Craft Advisory remains in effect through
Wednesday evening, and may continue through Wednesday night ahead of
a strong cold front. Following isolated to scattered showers tonight
along with a few thunderstorms on Wednesday, numerous showers and
thunderstorms are expected Wednesday night ahead of the front. The
cold front is expected to move across the coastal waters Thursday
morning with the winds shifting to the west and strengthening
through the day. Strong to very strong offshore flow with frequent
gusts to gale force is expected to develop by Thursday afternoon
and continue Thursday night. The offshore flow will diminish
Friday with weak to moderate north winds expected by Friday
afternoon and continuing through Saturday night. A weak to
moderate onshore flow will resume Sunday afternoon into Sunday
night. Showers and thunderstorms will begin to diminish by
Thursday afternoon. Isolated showers will be possible again Friday
night.

.PRELIMINARY POINT TEMPS/POPS...
Corpus Christi 65 80 51 60 39 / 20 40 100 50 0
Victoria 64 78 51 59 37 / 40 50 100 80 0
Laredo 62 80 43 56 38 / 0 10 80 30 0
Alice 64 83 48 58 37 / 20 30 100 50 0
Rockport 67 76 52 61 42 / 30 40 100 70 0
Cotulla 62 78 43 50 34 / 10 20 100 50 0
Kingsville 64 83 49 59 37 / 20 30 100 50 0
Navy Corpus 68 78 52 61 45 / 20 40 100 60 0

.CRP WATCHES/WARNINGS/ADVISORIES...
TX...None.
GM...Small Craft Advisory until 6 PM CST Wednesday For the following
zones: Bays and Waterways from Baffin Bay to Port Aransas...
Bays and Waterways from Port Aransas to Port O`Connor...
Coastal waters from Baffin Bay to Port Aransas out 20 NM...
Coastal waters from Port Aransas to Matagorda Ship Channel
out 20 NM...Waters from Baffin Bay to Port Aransas from 20
to 60 NM...Waters from Port Aransas to Matagorda Ship
Channel from 20 to 60 NM.

TC/95...AVIATION

Area Forecast Discussion

National Weather Service Des Moines IA

553 PM CST Tue Dec 29 2020

...Updated Aviation Discussion...

.DISCUSSION.../Tonight through Tuesday/
Issued at 301 PM CST Tue Dec 29 2020

Forecast Highlights:

-- Ongoing winter storm having significant impact on travel,
especially over central and southern Iowa.
-- High snow rates have led to rapid accumulation in some areas
-- Transition to wintry mix in the south is ongoing and will
continue into this evening
-- Late week wintry precipitation still on track

Details:

GOES-East upper level water vapor shows the plume of moisture
ahead of the shortwave trough that has ejected out of the base
of the western US longwave trough. Sporadic and embedded lightning
has occurred with the precipitation shield, especially over
Kansas into northern Missouri earlier this morning and more
recently in Adams County within the last 30 minutes. QPF has been
raised to the 75th percentile of guidance, which helped to up QPF
amounts in a narrow band from Taylor/Ringgold Counties up toward
Warren/Lucas and Poweshiek/Mahaska. This is a trend noted in the
12z guidance from HREF, HRRR, and RAP. This along with negative
EPV and steep mid- level lapse rates noted on area soundings in
HRRR and RAP guidance will lead to continued chances of thunder-
wintry precipitation, which will locally enhance rates. 12z HREF
guidance is all in on greater than 1"/hr rates from Des
Moines/Polk County and surrounding counties mid-afternoon moving
east-northeastward into this evening and over east central
Iowa/Dubuque area. The highest probabilities and even some less
than 40% probabilities of 2"/hr rates are over east central Iowa
and that is where WFO Quad Cities has amounts around double
digits. Given incoming reports this afternoon, the 1"/hr rates are
more than verifying and probably in some cases approaching 2"/hr.
With these high snow rates making it challenging for road crews,
worst impacts will be into at least early this evening and
lingering longer as lighter precipitation continues.

Farther south, spotter reports as well as observational trends in
surface and radar data have shown a changeover to sleet or
freezing rain as the warm nose aloft moves over southern Iowa.
KMCI had around 0.15" of ice so far with freezing rain and sleet
across northern Missouri according to our neighboring office WFO
Pleasant Hill/Kansas City. Thus, have tried to increase freezing
rain in the far south. NBM 50th percentile supportive of amounts
to around that value or a bit higher. Some deterministic output is
quite high with freezing rain amounts, but there is uncertainty
in accretion given high rate of rainfall. A recent spotter report
from Murray indicated heavy freezing rain keeping concerns of
accretion a concern going into the rest of this afternoon. Farther
north where there is the transition to sleet, this will hamper
snow amounts with a tricky transition zone likely near or south of
the I-80 corridor, especially east of Des Moines. At this point,
no change in headlines given the wintry mix bag the south has had
with predominately snow north of Highway 30.

As drier mid-level air works in as the precipitation begins to
depart this evening, there is the possibility of a little
freezing drizzle. Amounts will be light from a glaze to perhaps a
few hundredths. By daybreak Wednesday, the precipitation will be
over for central Iowa, though roadway impacts will linger.

The next weather system to impact our area will be late this
week/next year. A shortwave trough that is digging over the
Southern California coast this afternoon into the base of the
longwave trough will translate across northern Mexico and then
lift northward quickly late this week. Surface low pressure will
passes southeast of the state with Gulf of Mexico moisture being
pulled back into the deformation zone. This will yield a mainly
snow event for areas farther northwest in the state while a wintry
mix may occur over southeastern sections of the state. The most
likely area for wintry precipitation will be over southeast Iowa
into central Iowa with chances tapering to nothing over far
northwest Iowa.

.AVIATION.../For the 00Z TAFS through 00Z Wednesday evening/
Issued at 543 PM CST Tue Dec 29 2020

Heavy snow band mixed with some freezing drizzle and even sleet
looks to mainly impact DSM, OTM and ALO this evening. Still
looking at IFR ceilings for much of the night into early Wednesday
morning before gradually lifting to MVFR overnight and then with
the CAA developing anticipating ceilings to lift to VFR late in
the TAF period.

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning until 6 AM CST Wednesday for IAZ017-025>028-
034>039-044>050-057>062-070>075-081>086-092>097.

Winter Weather Advisory until 6 AM CST Wednesday for IAZ004>007-
015-016-023-024-033.

DISCUSSION...Ansorge
AVIATION...Podrazik

Area Forecast Discussion

National Weather Service Detroit/Pontiac MI

830 PM EST Tue Dec 29 2020

.UPDATE...

Amplified upper level ridge axis over the Central Great Lakes early
this evening, as 00z DTX Raob checks in with 560 DAM height at 500
MB, close to the 90th percentile based on climatology. The sounding
also revealed some very dry low levels, as 700 MB dew pt comes in at
-38 C.

Consolidating northern stream upper level PV/main mid level
circulation over the Dakotas, which will be tracking across Lake
Superior Tomorrow. Main forcing will be associated with the nose of
a strong low-mid level jet (65-75 knots) over the Midwest which will
veering a bit tonight and tracking through Lower Michigan early
tomorrow morning, allowing for an excellent surge of moisture
(specific humidity 4.5+ g/kg) to overcome the dry low levels. The
inherited forecast looks to be in good shape as far as I can tell,
with the narrow warm advection arm lifting through late this
evening/around midnight. Best forcing up around 600 MB, where temps
reside in the negative lower teens. The strong, but short lived
forcing lifts north overnight, leading to lull for southern two
thirds of the CWA. The track of the low level jet and continued
isentropic ascent likely helps maintain snow across Tri-Cities
region and northern Thumb region. A 1-3 inch snowfall generally captures
the flavor of the forecast for tonight, with the 1 inch confined to
southern areas (I-94 south) and the 3 inches confined to far northern
sections (M-46 north), with Tri-Cities region seeing additional snow
early tomorrow morning to push totals to 4 inches or slightly
greater.

Tomorrow morning`s freezing rain impact will be dictated by exactly
where the surface temps reside. Should be looking at the marginal
30-32 degree range vs more significant under 30 degrees for ice
accumulation. It looks like the NAM has a bit of a cold bias (outside
of the moderate snow areas) when comparing the temps over the Ohio
Valley this evening. Once surface winds flip around to the south-
southwest, right around 12z, will lose that dry near surface feed
/wet-bulbing potential. The 12z European was one of the more
aggressive solutions with the warm air moving in, leading to high
confidence in a switch to light rain for most or all of southeast
Michigan late in the morning/by Noon. Once again, inherited forecast
and Winter Weather Message/Advisory looks in line and no significant
deviations anticipated.

.PREV DISCUSSION...
Issued at 630 PM EST Tue Dec 29 2020

AVIATION...

Accumulating snow remains on target to develop late this
evening/near midnight across southeast Michigan. Ceilings and
visibilities should drop abruptly, with vsbys of 1/2-1SM and cigs
aob 1000 feet. A dry slot will spread in overnight from south to
north, ending snow overnight for most of the TAFS, MBS being the
exception. Precipitation will then move back in 10-12z, and looks to
be mostly in the form of freezing rain as warm and moist air surges
into the state with increasing southerly winds, which look to gust
aoa 20 knots during Wednesday. Higher dew pts in the mid to upper
30s should translate to IFR visibility and cigs and light rain for
bulk of the day. Cold front/wind shift will occur late in the day
(centered around or just after 20z), ending any precipitation. Still
enough residual moisture with the brisk westerly winds to maintain
high MVFR cigs through the evening.

For DTW...Looking for up to an inch of snow accumulation centered
around midnight, with snow ending between 2-3 AM. Freezing rain then
expected to move in 6-7 AM, changing to rain 9-10 AM.

DTW THRESHOLD PROBABILITIES...

* Moderate for ceilings at or below 5000 feet around midnight
tonight. High after 12z Wednesday.

* High for precipitation type to start as all snow late tonight.

* Moderate for freezing rain with some sleet 11-15z, then rain.

PREV DISCUSSION...
Issued at 419 PM EST Tue Dec 29 2020

DISCUSSION...

Tranquil conditions across Southeast Michigan this afternoon with
just a veil of transparent cirrostratus overhead. Strongly
anticyclonic flow trajectories are in place between 925-575mb which
is driving active subsidence within the layer. Antecedent dry air is
forecasted to hold through approximately 02Z.

The consistent trend in the model data has been towards a colder
solution for the event tonight/Wednesday. Current observations are
certainly eyebrow raising with surface dewpoints running in the
teens. First part of the event will begin this evening as lead 800-
600mb warm advection wing lifts into Southeast Michigan. Plan view
and cross section analysis depict sharp frontal structure on the 298-
305Ke surfaces that system relative isentropic ascent will be
maximized on. X-section also show that a good connection exists
between this midlevel frontal forcing with upper level jet forcing
aloft. Very low static stability above the frontal surface with
perhaps the models even suggesting some window for convectively
unstable lapse rates at 600mb. Tally of QPF from the breadth of
model solutions point to a brief 2 hour period of Moderate to Heavy
snowfall rates from south to north between 02-08Z. There remains
some concern that best frontal forcing and UVVS could bypass the far
southern/eastern sections of the cwa. However, the HRRR and Rap
remain very bullish tonight for Wayne/Monroe counties. 3 hour
snowfall rates offered by the latest model data suggests upwards of
1 to 2 inches snow will be possible across northern Metro Detroit
northward. Expectations are for a quick 1/2 to 1 inch southern metro
Detroit. Cold temperatures in the middle to upper 20s overnight will
maximize the accumulating potential.

A lull in the precipitation is expected across the southern 2/3rds
of the cwa overnight mainly between 06-10Z. For areas north of I 69
and M 46 forecast data shows main elevated frontal boundary lining
out, parallel to the upwind flank of the +60 knot low level jet.
Impressive signal for inflection point to 850-500mb 2d frontogenesis
then persisting over the Tri Cities and northern Thumb region right
through 15Z. NAM and RAP soundings are quite cold throughout the
entire event, casting doubt on whether or not precipitation type
will changeover to liquid. Given the look of the sounding, the
feeling is could see more sleet than freezing with the cold air
holding. Total snow accumulations of 2 to 4 inches will be possible
in the Tri Cities and Thumb with locally higher amounts in northern
Midland and Bay counties. The GFS is considered an outlier at this
time. Farther south, the warm nose will be collocated with return of
precipitation mainly after 10-11Z. A strong consensus exists that
warm nose of greater than 3C will exist above 3000 ft agl with
surface temperatures solidly holding below freezing through roughly
14Z. Virtually all model data is suggesting QPF amounts of up to
0.10 in freezing rain possible. Icy travel with snow covered roads,
perhaps some icy untreated roadways.

The decision was made to go with Winter Weather Advisory for all of
Southeast Michigan from tonight through the Wednesday morning
commute. Hazarouds travel conditions are expected during the
Wednesday morning commute due to slippery roadways.

The main trend for the New Year`s Day system is a slower onset of
precipitation. Current storm track continues to favor a Gulf of
Mexico low lifting into western Illinois 00Z Saturday the rocketing
to far eastern Ontario and Quebec by 12Z Saturday. Latest
information suggests a period of freezing rain will occur at the
front end of the precipitation now well after 12Z. Very warm air is
forecasted to lift through the region with 850mb temperatures
potentially exceeding 8C right at the Lake Erie/Ohio stateline.

MARINE...

High pressure currently overhead departs tonight as the center moves
from the Ohio Valley into the Northeast. The next low pressure
system quickly follows lifting through the Upper Great Lakes
Wednesday bringing with it rain and snow. Southerly flow increases
Wednesday morning with gusts up to around 30kts across Lake Huron.
There could be a brief couple hour period around 12Z where gusts
could touch low-end gales, however, the duration is not expected to
be long enough to require a warning. Winds rotate to southwesterly
and eventually westerly over the course of the day with the cold
front crossing the central Great Lakes Wednesday evening. This
frontal passage will bring another uptick in wind strength with
gusts reaching up to around 30kts, particularly over the northern
half of Lake Huron. The pressure gradient over the region rapidly
weakens Wednesday night as the low moves deeper into Canada and a
new high begins to build over the Great Lakes. Light winds and dry
weather are expected Thursday under this high before another large
low lifts into the Great Lakes Friday.

.DTX WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory until noon EST Wednesday for MIZ047>049-
053>055-060>062.

Winter Weather Advisory until 9 AM EST Wednesday for MIZ063-068>070-
075-076-082-083.

Lake Huron...Small Craft Advisory from 4 AM to 4 PM EST Wednesday for LHZ442-443.

Small Craft Advisory from 4 AM Wednesday to 4 AM EST Thursday for
LHZ421-441.

Lake St Clair...NONE.
Michigan waters of Lake Erie...Small Craft Advisory from 4 AM to 4 PM EST Wednesday for LEZ444.

UPDATE.......SF
AVIATION.....SF
DISCUSSION...CB
MARINE.......KDK

You can obtain your latest National Weather Service forecasts online
at www.weather.gov/detroit.

Area Forecast Discussion

National Weather Service Grand Rapids MI

843 PM EST Tue Dec 29 2020

LATEST UPDATE...
Update

.SYNOPSIS...
Issued at 228 PM EST Tue Dec 29 2020

- Impactful winter storm expected tonight into Wednesday morning

- Wintry mix looking likely for Friday

- Above normal temperatures over the weekend into early next week

.UPDATE...
Issued at 842 PM EST Tue Dec 29 2020

At 0125Z KGRR radar trends and sfc obs indicate that the leading
edge of the snow extends from around KMKG to KGRR. The snow will
continue to advance to the northeast and quickly increase in
intensity with snowfall rates up to around an inch per hour
expected in the heaviest bands. Therefore hazardous travel
conditions will continue to develop quickly across our area from
sw to ne this evening.

No significant fcst changes are needed. A consensus blend of
latest high res short range guidance continues to show potential
for around 4 to 7 inches of snow near to north/northwest of KGRR.
We cannot rule out locally higher amounts to around 8 inches based
on latest runs of the HRRR and overall guidance trends the past
12-24 hrs.

Snowfall amounts south to southeast of KGRR will be considerably
lighter where thermal profiles will moderate sufficiently enough
overnight to cause pcpn to change to sleet and possibly a brief
period of light freezing rain as well until sfc temps climb to
above the freezing mark. So the mixed pcpn will limit potential
for higher snow accumulations south/se of KGRR.

.DISCUSSION...(This evening through next Tuesday)
Issued at 228 PM EST Tue Dec 29 2020

- Impactful winter storm expected tonight into Wednesday morning

Looking upstream across Central IL into IA...the leading edge of
the expanding area of snow has featured some heavier snow rates.
Several sites are 1/4 mi or less. High res models do spread this
snow into the CWA this evening. Trends have been for a little
quicker arrival of the snow. Earlier today...we did bump up the
start time of the winter weather advisory. It does look like
southern zones could see an inch or two of snow quickly this
evening before the main axis of precipitation shifts north towards
Holland...Grand Rapids and Alma. At that point the band of steady
to heavy snow stalls out. Thus...highest accumulations are
projected to be along and north of a Holland to Grand Rapids to
Alma line. This axis of heavier snow is a little further south
than previous runs. A period of sleet or freezing rain will likely
move into southern zones overnight...but qpf values are projected
to be limited. Still...slick travel conditions are forecasted for
the morning commute in all areas as temperatures are unlikely to
go above freezing until after 8 or 9 am. The precipitation is
expected to taper off from west to east late morning to early
afternoon.

- Wintry mix looking likely for Friday into Saturday

The latest guidance continues to show a wintry mix of
precipitation moving in Friday morning from the south. The
precipitation then persists into Friday night...tapering off
Saturday morning. The precipitation will not be steady...but it
will occur in several rounds. This next storm appears to be more
of a freezing rain/sleet event rather than a mostly snow event.
Impacts look likely. Icy roads are expected to generate some of
the impacts. As ice builds up on trees/power lines...some
scattered power outages are possible. We will continue to message
on the storm and provide social media guidance on it.

- Above normal temperatures over the weekend into early next week

Stronger warm air advection will occur across the Canadian
Prairies over the weekend. This will act to shove the arctic air
unusually far north for this time of the year. As a
result...without any stronger cold fronts pushing down into
MI...the region will see a period of above normal temperatures
over the weekend and into next week. With some snow on the
ground...temperatures should not stray too far above normal.

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 527 PM EST Tue Dec 29 2020

Conditions will deteriorate rather rapidly to IFR during the
mid to late evening hours tonight as snow moves in. Primarily
IFR conditions will then continue at all the terminals
overnight but there will be some variability during the early
morning hours Wednesday as pcpn becomes mixed with and/or
changes over to sleet and possibly light freezing rain.

Steady mixed pcpn Wednesday morning will gradually taper off
Wednesday afternoon but conditions will likely remain IFR to
perhaps MVFR by late in the day.

.MARINE...
Issued at 228 PM EST Tue Dec 29 2020

The onshore winds over 15 knots Wednesday into Wednesday night will
support the hazardous conditions for small craft. The pressure
gradient does weaken through Thursday so the winds and waves will
likely decrease then.

.GRR WATCHES/WARNINGS/ADVISORIES...
MI...Winter Weather Advisory until 1 PM EST Wednesday for MIZ037>040-
043>046-050>052-056>059-064>067-071>074.

LM...Small Craft Advisory until 10 AM EST Thursday for LMZ844>849.

UPDATE...Laurens
SYNOPSIS...MJS
DISCUSSION...MJS
AVIATION...Laurens
MARINE...MJS

Area Forecast Discussion

National Weather Service Lincoln IL

837 PM CST Tue Dec 29 2020

.SYNOPSIS...
Issued at 225 PM CST Tue Dec 29 2020

A storm system will move across the region through early
Wednesday. A winter weather advisory in in effect northwest of the
Illinois River for up to inch or two of snow and a tenth or two of
ice as warmer air transitions the precipitation from snow to
freezing rain this evening. Precipitation will go over to all rain
after midnight as the warmer air filters into central Illinois.

.UPDATE...
Issued at 828 PM CST Tue Dec 29 2020

Freezing rain is about to overspread areas northwest of the
Illinois River Valley with icy accumulations expected over the
next few hours. At this time, no major changes in thinking with up
to a couple tenths of an inch of ice accumulations possible on
untreated surfaces. Surface temps remain in the 28-32 range across
this area with dew points in the lower to mid 20s giving a little
room for temps to dip slightly due to evaporative cooling despite
warm air advection occurring. RAP indicates the layer max 3C temp
contour in the 925-700 layer has overspread the entire CWA so
temperatures aloft are warm enough for full melting. Surface temps
should rise above freezing between roughly midnight and 4 am.

Further south, temperatures from IJX/SPI/AAA are 34/35/35,
respectively, and mesonet sites across Schuyler and Menard
counties show temps of 32-33 degrees. As such, the southern
segment of the advisory should be able to expire on schedule at
9pm. This will include Schuyler, Mason, Cass, Scott, and Morgan
counties.

.SHORT TERM...(This evening through Wednesday)
ISSUED AT 225 PM CST Tue Dec 29 2020

Strong isentropic lift and 900-700 frontogenetic forcing from the
NAM still suggests that best lift will occur late afternoon and
early this evening over the far northwest portion of the forecast
area with precip mainly in the form of snow. NBM and GEFS
ensemble suggest while there is some potential for greater than a
quarter inch of ice accumulation once warmer temperatures aloft
advect into the area later this evening, However, overall
thickness will likely remain lighter as the rain falls on snow
cover already in place.

Temperatures should rise above freezing by 07z throughout the
region as the warm sector of the cyclone advects north of the
forecast area. By sunrise, the cold front should begin to
progress southeastward pushing the area of highest PoPs with it.
By 00z Thursday, chance PoPs should be limited to near and south
of I-70. Temperatures will are expected to fall behind the front
and high temps will likely be during the morning.

.LONG TERM...(Wednesday night through Tuesday)
ISSUED AT 225 PM CST Tue Dec 29 2020

Broad southern stream trough centered along the California Coast
is expected to dig into northeast Mexico over the next 24 hours
before ejecting northward into the Midwest. Gulf of Mexico will be
open and high water vapor transport values are expected to bring
between a half inch and 1.5 inches of rain to the area Thursday
Night and New Years Day. The warm sector surges northward as well
and expect for the very start of precipitation, the phase should
be mostly liquid with highs reaching the middle 30s in the GBG
area to into the 50s near and southeast of I-70.

Lots of uncertainty as to how fast the precip pulls out with
deterministic GFS and ECMWF suggesting some snow/rain may stick
around through Saturday, while several ensemble members push the
precip south and east of the forecast area. For now will favor the
drier scenario given the relative progressive flow.

Arctic air cutoff from Illinois in the mostly zonal flow next week
should keep temperatures mostly above normal at least into
midweek.

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 526 PM CST Tue Dec 29 2020

A warm front is lifting across central Illinois this evening with
SE winds veering to southerly and strengthening aloft to 40-55kt
resulting in LLWS. Meanwhile, spotty light precip will continue
particularly at PIA where freezing rain is possible for several
hours. Temps will eventually warm enough for all rain as precip
coverage increases ahead of an approaching cold front. Moderate to
heavy rain and IFR/LIFR conditions is expected overnight, then
the cold front will mark the back edge of precip with winds
turning NW and conditions improving slightly to MVFR.

.ILX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 4 AM CST Wednesday for ILZ027>031-
036-037.

Winter Weather Advisory until 9 PM CST this evening for ILZ040-
041-047-049-050.

UPDATE...Deubelbeiss
SYNOPSIS...Barker
SHORT TERM...Barker
LONG TERM...Barker
AVIATION...Deubelbeiss

Area Forecast Discussion

National Weather Service Jacksonville FL

808 PM EST Tue Dec 29 2020

.UPDATE...

00z soundings show a shallow moist layer around 5kft above the
ground, beneath a big subsidence inversion. Radar has been
recently showing a band of very light showers/sprinkles from
coastal waters into Duval County...weakening as they spread
further inland. Latest HRRR guidance suggests this pattern will
continue through the night, with another very light band possibly
developing over our southeastern GA coast. Keeping POP values low
as likely most areas affected by band will only get Trace amounts.

With surface dew points in the 40s to lower 50s...not expecting
current temperatures to drop too much more, so not planning any
significant changes to low temperature forecast in update.

.AVIATION...
[Through 00Z Thursday]

Satellite shows areas of CIGS BKN040-050 moving inland over eastern
counties. Latest guidance suggests some lowering of CIGS down to
BKN025-030 during the night...continuing during the day Wednesday,
especially eastern TAF sites.

.MARINE...

Winds will veer easterly tonight as high pressure
elongates along the Atlantic Seaboard. An inverted trough of low
pressure will develop in the easterly flow, leading to increase
shower chances and cloud cover on Wednesday. Winds will shift
southeasterly as high pressure builds into the Atlantic on Thursday
ahead of the next frontal system. A cold front will approach from the west and
eventually stall over the region late this week and into the
weekend.

RIP CURRENTS: NE FL & SE GA - Moderate Risk through Wednesday

.PRELIMINARY POINT TEMPS/POPS...

AMG 47 66 54 76 62 / 10 20 20 30 20
SSI 53 66 57 71 62 / 20 20 20 20 10
JAX 53 71 58 77 64 / 20 20 10 10 10
SGJ 57 71 61 76 65 / 10 20 20 10 0
GNV 50 75 58 81 62 / 0 10 10 10 10
OCF 52 77 60 82 64 / 0 10 10 10 10

.JAX WATCHES/WARNINGS/ADVISORIES...
FL...None.
GA...None.
AM...Small Craft Advisory from 4 AM Wednesday to 10 PM EST Friday for
Waters from Altamaha Sound GA to Fernandina Beach FL from
20 to 60 NM-Waters from Fernandina Beach to St. Augustine
FL from 20 to 60 NM-Waters from St. Augustine to Flagler
Beach FL from 20 to 60 NM.

Area Forecast Discussion

National Weather Service Chicago/Romeoville, IL

934 PM CST Tue Dec 29 2020

.UPDATE...
934 PM CST

Our band of heavy snow earlier this evening is now transitioning
over to a wintry mix. This trend will continue over the next few
hours as strong warm air advection, via a 50-60kt low-level jet,
continues to transport warmer air off the surface. This will
support a period of sleet and freezing rain over the Chicago area
for the next hour or so before the warm layer aloft becomes
strong enough to support total melting of the hydrometeors, at
which point we will see primarily freezing rain. More of a wintry
mix is expected to continue up near the WI state line where the
warm layer aloft could remain a bit weaker.

It appears the Chicago area will have about a 3 to 5 hour window
in which freezing precipitation will be favored before
temperatures warm above freezing overnight. We are already seeing
sites across central IL inch just above freezing, so it is just a
matter of time before these warmer surface temperatures shift
northward into the area. Unfortunately, until we climb above
freezing we will see accumulating freezing rain, with a tenth or
two possible before the change to rain occurs overnight.

No changes are planned to headlines, but expect some improving
conditions prior to the current ending times of these headlines.

KJB

.SHORT TERM...
320 PM CST

Through Wednesday night...

As expected, top-down saturation process initially slow through
mid day, has progressed rapidly within the last hour or so. Seeing
moderate to heavy snow automated observations rather far south
with this initial wave. Also received reports of snow almost
immediately accumulating on roads upon onset of steady snow,
backed up by area webcam observations. In what has been a month
devoid of winter weather and associated impacts, this will be a
higher impact event given the high snowfall rates into early this
evening, followed by ice accumulations in the mid to late evening
into the overnight as precipitation transitions over to brief
sleet then freezing rain.

The main change to the forecast to expand the Winter Storm Warning
to include Kane, Lake (IL), and northern Cook (added as of just
after 3pm) counties, per observational trends supporting another
slight southward adjustment in the heavier snow axis, plus up to
1/10 to 2/10" ice accums tonight. Did not expand the Winter
Weather Advisory to include any additional counties yet, but as
mentioned in late morning AFD Update, may need to include Newton
and Jasper Counties in Indiana for ice accumulation potential.
However, confidence remains lower there, so will allow evening
shift to closely monitor observational trends.

/Technical Discussion/...

The challenges with the forecast remain similar, namely the
position of the heaviest snow axis into this evening, and then
timing of expected transition over to sleet and then freezing rain
in the mid-late evening into the overnight. As we`ve been
advertising, the ingredients for moderate to heavy snowfall rates
have most certainly materialized with the initial thump of warm
advection snow rapidly onsetting this afternoon and continuing
through the early to mid evening. Seeing numerous obs of 1/4 to
1/2 mile visibility and webcams and road reports with quickly snow
covered roads. This is owing to radar indicating transient bands
of embedded 30 to 35 dBZ echoes. The rapid onset of steady to
heavy snow has quickly resulted in hazardous travel conditions
down south of I-80. Snow band should be more transient in this
area, so most of the snow for the event will be what has fallen
and will fall over the next couple hours. Area of concern for
perhaps overperforming snow totals and impacts would be northern
half of LaSalle County, so will need to monitor for any headline
changes there.

As alluded to in previous update in the late morning, 12z OAX
RAOB sampled steep (>7C/km) lapse rates above 550 mb within the
DGZ, and did see some TSSN recently upstream over eastern Iowa.
Seasonably high moisture (0.71" PWAT on SGF sounding) rapidly
advecting northeastward is overlapping with the period of strong
large scale (mid-level impulse and right entrance region of upper
jet) and mesoscale (strong transient 850-700 mb f-gen) forcing
through about 02z this evening. With what we`re already seeing,
expect that the window of 1"/hr to even briefly 2"/hour snowfall
rates will be about 2 to 4 hours, longest I-88 and north and
shortest south of I-80. The high snowfall rates and periods of
sharply reduced visibility are resulting in hazardous travel for
the evening commute. Impressive alignment with the available
moisture and strong forcing did also yield increased snow amounts
in the heaviest axis across portions of far northern Illinois,
with 5-9" messaged and localized 10" amounts possible closer to
the WI state line in north central Illinois, depending on how long
the heavy snowfall rates last.

Earlier concern with respect to our headlines is the potential
mentioned of the heaviest snow axis shifting a bit southward, and
we did reflect this in the updated official forecast, along with
the upgrade to warning for Kane, Lake (IL) and northern Cook
County. The HRRR had consistently depicting this scenario (high
liquid equivalent QPF as snow, brief intense hourly rates
supported by forecast soundings). Confidence remains high that the
850 mb warm nose will be able to surge in this evening into the
early overnight from southwest to northeast, and cause the p-type
transition over to brief sleet and then freezing rain.

While again the snow side is performing to earlier expectations
or perhaps exceeding it, there`s signs on the observational trends
back across southern Iowa and near-term guidance that the
mix/change to brief sleet then freezing rain could occur a bit
earlier this evening. This would be followed by steady rates, and
potential for localized ice accums north of I-80 up to 0.2 to
0.25". This was also accounted for in our decision to upgrade some
northern IL counties to a warning, and we`ll be watching closely
for possibly needing any other short-fused changes.

Otherwise, temperature trends will also be key tonight into early
Wednesday, as longer duration of temps at or below freezing could
spread ice accum risk a bit farther south, and necessitate
expansion of WWA to Newton and Jasper Counties, though confidence
is still low there. Temperature trends in northern Illinois will
be key as to magnitude of travel impacts lingering into Wednesday
morning, with some potential for locations in the far north
staying close to 32F for longer. Most of the additional precip
into the day on Wednesday will be out ahead of the approaching
cold frontal trough and within areas that should be safely above
freezing by then. The front will move through early in the
northwest CWA and progress southeast from there, so highs only
look to top out in the 33-36 range along and north/northwest of a
Peru to Aurora to Highland Park line, with upper 30s southeast of
that, and lower to mid 40s in the far southeast CWA.

Expansive high pressure will move overhead Wednesday night, and
low clouds will be scoured out. Should mid and high cloud cover be
less than expected, forecast lows could end up colder over the
remaining deeper snow covered areas of northern Illinois. For now,
have close to seasonable lows, teens to lower 20s, in the official
forecast.

Castro

.LONG TERM...
224 PM CST

Thursday through Tuesday...

Confidence continues to increase that yet another winter weather event
will impact our area centered on New Year`s Day as a low pressure
system from the Gulf of Mexico lifts northward into the Great Lakes.
Forecast thermal profiles continue to suggest initially light snow
early Friday will transition to freezing rain after daybreak across
much of the area, perhaps continuing well into the afternoon. A
transition to "plain" liquid rain is also possible depending on
the exact track of the low pressure system as well as how far
north the freezing line manages to get (with a fresh snowpack from
the system today and a dry/cooling northeasterly wind acting to
keeping a good chunk of the area below freezing). Early numbers
from the National Blend of Models (NBM) demonstrate that a glaze
of ice is likely across the entire area with a fairly high signal
(>70%) for a zone of more than a tenth of an inch somewhere across
northern Illinois. Also, looking at this system with the CIPS
Analogs (reanalysis based on the GFS forecast), 30-40 percent of
the top 15 matches have at least three hours of freezing rain
across northern Illinois. So, the key points for now are that
we`ll have limited time to clean up after our system today/tonight
and to prepare for another round of sloppy wintry weather late
Thursday night into Friday.

How quickly the system departs the region Friday Night-Saturday is an
unknown (as it may slow). At any rate, long range guidance indicates a
warming and dry conditions into the first full week of the new year.

MTF/Borchardt

.AVIATION...
For the 00Z TAFs...

Aviation forecast concerns:

* Period of moderate/heavy snow early this evening with LIFR
conditions.

* Snow transitions to a mix of snow/sleet/freezing rain mid to
late evening, then to rain as surface temps warm above freezing
pre-dawn. IFR ceilings prevail overnight into Wednesday morning.

* Blustery southeast to winds gusting over 25 kt at times this
evening, eventually shift southwest pre-dawn and west late
Wednesday morning.

* MVFR ceilings gradually improve through Wednesday evening.

A band of moderate to heavy snow has lifted over the terminals as
expected during the late afternoon hours, with snowfall rates of
an inch per hour or more at times and visibilities at or below
1/2SM. Snow intensity/rates will begin to diminish by mid-evening,
and may become mixed with sleet at times as warm air spreads in
aloft. As this warming aloft continues, sleet/snow will transition
to a period of freezing rain during the late evening with
accretion rates of a few hundredths per hour. Surface temperatures
will eventually climb slowly above freezing during the pre-dawn
hours, roughly 08-10Z, with light rain then lingering through
mid-morning Wednesday before ending. In total, snowfall across the
Chicago terminals should range from about 1 to 4 inches (highest
toward DPA and ORD), with ice amounts from a heavy glaze to around
a tenth of an inch and possibly locally higher.

For Rockford, moderate snow will persist through mid-evening with
visibility in the 1/2 to 3/4SM range. Warming aloft should also
produce a transition to a snow/sleet mix by mid-late evening and a
mix of freezing rain/sleet and snow after midnight, before
tapering to drizzle before ending during the pre-dawn hours. Total
snow amounts of over 6 inches are likely at KRFD, with a glaze of
ice.

Surface winds across the terminals will be blustery from the
southeast early this evening, then becoming south late. Gusts of
25 kts or more are possible. Winds become southwest after
midnight, then turn westerly mid-morning Wednesday as the cold
front moves through. Breezy west winds will then persist into
Wednesday evening.

Ratzer

.MARINE...
303 PM CST

South to southeast winds will increase over the next few hours
with gale force winds likely by late evening north of Chicago.
Overnight, winds will gradually become southerly and then
southwesterly by daybreak Wednesday with magnitudes decreasing in
tandem. A Gale Warning remains in effect from Chicago northward
through daybreak Wednesday.

Borchardt

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Winter Storm Warning...ILZ003-ILZ004-ILZ005-ILZ006-ILZ008-ILZ010-
ILZ011-ILZ012-ILZ103 until 9 AM Wednesday.

Winter Weather Advisory...ILZ013-ILZ019-ILZ020-ILZ104-ILZ105-
ILZ106 until 9 AM Wednesday.

Winter Weather Advisory...ILZ021-ILZ023-ILZ032-ILZ107-ILZ108
until 6 AM Wednesday.

IN...Winter Weather Advisory...INZ001-INZ002 until 6 AM Wednesday.

LM...Gale Warning...Winthrop Harbor to Northerly Is. until 6 AM Wednesday.

Small Craft Advisory...LMZ742 until 6 AM Wednesday.

Small Craft Advisory...IN nearshore waters until 3 PM Wednesday.

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Area Forecast Discussion...Updated

National Weather Service Saint Louis MO

829 PM CST Tue Dec 29 2020

.UPDATE...
Issued at 822 PM CST Tue Dec 29 2020

The forecast remains about on track for tonight. Temperatures have
warmed to above freezing everywhere except Knox county where it
was still hanging around 29 degrees at 830 PM. Will continue to
monitor, but I don`t think it will be necessary to extend the
Winter Weather Advisory. The RAP is showing the wind turning to
the southeast over the next hour or two up there which should
speed up the warming trend up in Knox county. The rest of the
night just looks rainy ahead of the cold front as it moves slowly
eastward. I don`t anticipate any big changes to the going forecast
through Wednesday morning.

Carney

.SHORT TERM... (Through Tonight...)
Issued at 314 PM CST Tue Dec 29 2020

An area of precipitation continues across the northern CWA. Quincy
is beginning to changeover to freezing rain or sleet from the snow
that has been reported since late this morning. This trend will
continue over northeast Missouri and west central Illinois into
early this evening when warmer air over eastern Kansas and southern
Missouri moves north and changes all precipitation into all rain.
Will continue the advisory through mid-evening over the far
northwest counties temperatures will rise above freezing in these
counties early this evening.

Mass fields from the short term models and latest runs of the CAMS
still point to a lull in the precipitation for all but the far
northwest part of the CWA later this afternoon and early this
evening before a band of rain spreads back across the CWA from
northwest to southeast tonight. This will occur as the upper trough
currently over the Plains moves east tonight causing the attendant
cold front also over the Plains to move into central and northeast
Missouri by early tomorrow morning. Warm air advection ahead of the
front will cause temperatures to rise the rest of the afternoon and
then slowly rise or remain steady tonight.

Britt

.LONG TERM... (Wednesday through Next Tuesday)
Issued at 314 PM CST Tue Dec 29 2020

Early Wednesday, the northern shortwave will be tracking across the
upper Midwest with its associated surface low positioned across the
Great Lakes. The surface low`s cold front will begin pushing across
the CWA with widespread rainfall occurring along the front. As this
front and associated precipitation continue pushing southeastward,
cold air filtering in behind the front will allow for a transition
to light snow on the back end of the precipitation during the
evening and night. Only minor accumulations of around 0.1 are
expected.

As the front plunges into the Tennessee and Ohio vallies, it will
stall, lingering the chance of precipitation across portions of
southeastern Missouri and southwestern Illinois late Wednesday night
and early Thursday morning. Here, temperature profiles during this
portion of the period will support sleet and freezing rain, however,
dry air advecting into the CWA behind the front may limit this
threat. An additional factor that introduces a degree of uncertainty
is the positioning of the front - more progressive movement will
result in reduced precipitation chances. Current thinking is that
sleet/ice accumulations of 0.1-0.2 inches are possible across
portions of Reynolds, Iron, and Madison counties.

During Thursday, our focus quickly turns to the next system. A
shortwave digging southward across the western United States will
kick a closed low located over northern Mexico into the Southern
Plains. This closed low will continue to move nearly due north into
the Middle Mississippi Valley, developing and bringing a surface low
with it. The effects of this system will begin to be felt across
southern portions of the CWA late Thursday afternoon and the entire
CWA overnight as the precipitation shield advances northward. A warm
layer seen in both deterministic and ensemble guidance will fully
melt ice crystals that fall through the layer, and surface air will
be at or just below freezing, bringing the threat of freezing rain
through the CWA. However, there are several factors that will hinder
ice accretion to some degree. First, the depth and temperature of
the warm layer, which is forecast to be +5 to +10C, will couple with
warming surface and ground temperatures. Second, high precipitation
rates are expected, which is unfavorable for accretion. All that
being said, it does look like there will be a window for ice
accumulations of around 0.1-0.2 primarily across portions of central
and northeastern Missouri and west-central Illinois where cooler
surface temperatures will reside longer.

As the surface low nudges into Missouri, precipitation will switch
over to all rain across the CWA by Friday morning, and the axis of
heavy snowfall will remain well to the west. Widespread rainfall is
expected through mid-day Friday, with values of 1-1.5 inches
possible in total. Runoff into waterways is greater this time of
year, but river flooding continues to look unlikely.

As the system exits the CWA later on Friday, it still appears
possible that deformation zone may clip the northern portion of
the CWA, bringing with it the chance for light snow. If this
scenario occurs, light snow accumulations would be possible across
portions of northeastern Missouri and west-central Illinois.
Another chance for light snow is possible Friday night and
Saturday as the aforementioned "kicker" shortwave moves through
the Middle Mississippi Valley late Friday. Guidance has increased
the amount of available moisture slightly, but confidence in
precipitation is still low, so I have continued to keep this
portion of the forecast dry.

For Sunday through the end of the period, quiet weather is
anticipated, and a warming trend will begin as temperatures increase
to around 10 degrees above normal.

Elmore

.AVIATION... (For the 00z TAFs through 00z Wednesday Evening)
Issued at 543 PM CST Tue Dec 29 2020

MVFR ceilings are overspreading the area from the west and
southwest. Expect lowering ceilings tonight as rain pushes from
western and north central Missouri east across our area. Ceilings
will eventually get down to IFR across the area by Wednesday
morning as the front moves through. There will be a little bit
more wintry precip across northeast Missouri and west central
Illinois for the first few hours this evening, but temperatures
are warming ahead of the cold front, so the threat for freezing
rain should end by 02-04Z. Only rain is expected thereafter.

Carney

.LSX WATCHES/WARNINGS/ADVISORIES...
MO...Winter Weather Advisory until 9 PM CST this evening for Knox MO-
Lewis MO-Marion MO-Shelby MO.

IL...Winter Weather Advisory until 9 PM CST this evening for Adams IL-
Brown IL-Greene IL-Pike IL.

WFO LSX

Area Forecast Discussion

National Weather Service Pocatello ID

204 PM MST Tue Dec 29 2020

.SHORT TERM...Through Thursday night. Fog and stratus remains a
forecast concern tonight. This morning`s area of Fog has lifted
into a well-defined stratus layer that is well entrenched within
the entirety of the Treasure and Magic Valleys as well as the
Snake Plain. Although the stratus layer has eroded a bit from the
south, with slight warming above the stratus layer indicated on
forecast soundings tonight, this will only serve to strengthen
inversions and ensure that the stratus remains locked in. The HRRR
and Nam3km performed admirably in their depictions of fog
development and evolution last night, and these have been used to
outline the fog areas once again tonight. Based on their
depiction, fog should be somewhat less widespread tonight, and
this makes sense given the dominance of the stratus layer. The
best chance for dense freezing fog tonight looks to reside across
the higher terrain of Lincoln and Minidoka counties north and
east through the INL, Dubois and Island Park areas. Additional
areas of freezing fog appear likely across valleys of the eastern
highlands as well. Although the threat for fog along the
Interstate corridors is lessened tonight with stratus preferred,
we can`t rule out a few areas of fog here as well. Above the
stratus layer, high level clouds will begin to gradually increase
from the northwest after midnight in advance of our first in a
series of incoming storm systems.

Snow showers in association with the lead disturbance/warm
advection regime will begin across the central Idaho mountains and
Island Park region early Wednesday afternoon. Have increased
snowfall totals and snowfall potential across the upper Snake
Plain Wednesday afternoon, more in line with high-resolution
guidance, as they indicate the seeder-feeder mechanism will be
activated across this region. As a result, a quick 1-2" is
possible from Idaho Falls north through Island Park, a few tenths
more than the NBM depiction. Through the Wednesday evening hours,
snow showers will gradually overspread the eastern highlands from
north to south. No snow is expected Wednesday afternoon and
evening across the eastern Magic Valley and extreme southern Snake
Plain. Late Wednesday evening, a more robust round of snow showers
will intensify across the central Idaho mountains, associated with
the a second, stronger area of low pressure advancing southeast
through Oregon. Snow associated with this wave will gradually
expand south and east across the eastern Magic Valley, southern
Snake Plain and south hills through the evening, continuing
overnight. On Thursday, snow showers will be common across mainly
high terrain areas of central and eastern Idaho, with northwest
facing slopes favored. These snow showers will gradually come to
an end Thursday evening and overnight, with dry conditions
expected area-wide by daybreak Friday. 1-3 inches with as much as
5 inches over windward slopes is expected across higher terrain
areas with this system, with generally 1" or less across the Snake
Plain. An exception will be from Idaho Falls northward and in the
Burley area, where 1-2" will be favored. Patchy freezing fog and
stratus will once again be possible Friday morning on the heels of
Thursday`s snow shower activity.

.LONG TERM...Friday through Tuesday. Friday brings in a weak high
pressure ridge building in between two areas of low pressure.
Abundant moisture remains on track this weekend from an atmospheric
river event. Snow showers will arrive in Central Idaho early
Saturday with a chance for snow showers the remainder of the day in
East Idaho. More boisterous snowfall remains on track for Sunday
across our area; particularly in Central Idaho. Blowing snow is also
likely to creep up as a weather impact across our area going through
the day. Yet another shortwave trough will pass through the area
Monday into Tuesday, bringing another period of snow showers. Yes,
an Idaho winter pattern is upon us!

.AVIATION...Fog and low stratus challenges will persist in today and
tonight`s forecast. The fog/low stratus will not completely go
away but it is unlikely to impact all TAF sites throughout the
entirety of the evening. Expect the bank to begin spreading back
out overnight/early tomorrow morning once again. There is less
certainty in the bank reaching BYI tomorrow and perhaps better
chances for fog at DIJ.

.PIH WATCHES/WARNINGS/ADVISORIES...
None.
&&