Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/16/20
Area Forecast Discussion...UPDATED
National Weather Service Albuquerque NM
439 PM MST Tue Dec 15 2020
00Z TAF CYCLE
Lingering lcl MVFR cigs and nly winds gusting to around 35kt east of
the Pecos Valley to diminish after 01Z as well as the gusty nw winds
noted over far nw NM. Not anticipating low clouds to develop
overnight as a drier airmass overspreads the forecast area.
Relatively strong nly drainage winds to develop at KSAF aft 10Z and
nw winds at mt top level are forecast to increase somewhat aft 06Z,
impacting the higher terrain along the Contdvd and along the central
mt chain and ern slopes.
.PREV DISCUSSION...309 PM MST Tue Dec 15 2020...
Cold and dry will continue through the remainder of the week and into
the weekend. Bouts of clouds and some flurries for the northern
mountains will move through with a weak, moisture starved Pacific
storm system on Friday. Otherwise temperatures slightly below
average for mid December along with dry conditions will be the rule
through the weekend.
.SHORT TERM...(TONIGHT THROUGH WEDNESDAY NIGHT)...
The 21Z RAP analysis and satellite loop indicate the upper low has
migrated its way into parts of KS/OK. The brunt of the snowfall has
ended across northeast NM, although some orographically-driven very
light snow/flurries is lingering over the Sangres. Meanwhile, 700mb
speed max across the eastern plains will gradually weaken and shift
southeast of the CWFA later this afternoon/early evening. Once the
atmosphere decouples, strong surface winds will relax in the plains.
As a result, will allow the ongoing Wind Advisory to expire at 00Z.
With backing winds aloft, this will be a typical setup for breezy to
windy conditions tonight through Wednesday at KCQC.
A notably warmer Wednesday is on tap for the Land of Enchantment as
500mb heights increase about 10dam under mostly sunny skies. An upr
level ridge axis will roll across the state for Wednesday night and
this will support robust overnight temperature inversions. DPorter
LONG TERM...(THURSDAY THROUGH MONDAY)...
Zonal flow aloft Thursday will give way to a lee side trough Thursday
and southwest breezes along and east of the central mountain chain.
By Friday, models agree that a moisture starved upper-level trough
near the CO border Friday. Flurries or light snow will favor the west
slopes of the northern mountains Friday afternoon. A trailing short-
wave trough is forecast to drop in on brisk northwest flow aloft to
bring spotty light snow to portions of the northern mountains Friday
The upcoming weekend continues to look quiet and slightly warmer as
high pressure builds over southern CA and southwest AZ with northwest
to northerly flow aloft over NM. High pressure translates east over
NM early next week for continues quiet weather but likely strong
Weather prediction models continue to forecast another moisture
starved Pacific trough/storm system will slide through NM. It could
get more interesting if the Madden-Julian Oscillation (MJO) breaks
free and begins translating eastward once again. It`s been stuck in
phase 5 for what seems like an eternity (thanks in large part to La
Nina). Approximately 6-7 out of 20 GEFS members now strengthen the
MJO as it propagates eastward into phases 6 and 7 for the last week
of December. How would this change things? The resulting tropical
convection in the EPAC would draw a consolidated polar jet farther
south as compared with the split and weakened form currently moving
through the Western U.S.
A cold Tuesday night is expected behind the departing storm system
with modest to excellent overnight RH recoveries. Winds will weaken
this evening, excluding the areas lee of the Sandia/Manzano Mts. A
warmer regime is expected for Wednesday/Thursday as a ridge of high
pressure rolls from west to east across the region. This will also
support poor ventilation rates and decent overnight temp inversions.
The next storm system will approach the region Thursday night into
Friday, and this will increase the winds once again. Precipitation
should be rather meager with flurries and light snow favored across
the northern high terrain. Ventilation rates will briefly, at least
for one day, climb into the good to excellent range along/south of
Interstate 40 on Friday. Dry north/northwest flow is expected this
weekend with poor ventilation plaguing the forecast area once again.
Area Forecast Discussion
National Weather Service La Crosse WI
1000 PM CST Tue Dec 15 2020
.SHORT TERM...(This evening through Wednesday)
Issued at 212 PM CST Tue Dec 15 2020
As surface high pressure retreats into eastern Canada, a surface
ridge was still extending back into the forecast area this
afternoon, preserving our stretch of quiet weather. Meanwhile a
deepening upper low was crossing the Southern Plains, with its
associated mid-level moisture streaming into the Upper Mississippi
Valley. That`s brought plenty of clouds to the area today, keeping
temperatures on the chilly side for a second day in a row despite a
light southeast wind today.
The clouds will stick around tonight, which introduces another night
of low temperature complexities. Last night`s clouds prevented
temperatures from drastically plummeting across the area, and with
tonight`s clouds a bit lower and thicker, expect an even greater
insulating effect. Forecast lows range from round 15 to 20, perhaps
a smidge colder in marshy cranberry bog areas. As the system to our
south lifts toward the Ohio Valley late tonight, models show the
resultant easterly flow driving low-level moisture westward from
Lake Michigan. This low cloud layer looks fairly shallow as it
spreads in from the east late tonight into Wednesday morning. Can`t
entirely rule out isolated flurries as some very weak isentropic
ascent noses in from the southeast, but have kept forecast dry for
the time being as omega looks very weak and RAP continues to show a
layer of drier air near the surface. A drier wedge of air may slowly
invade from the northwest through the day Wednesday to allow for a
bit of sun, mainly west of the Mississippi River. Expect highs in
the upper 20s to lower 30s.
.LONG TERM...(Wednesday night through Tuesday)
Issued at 212 PM CST Tue Dec 15 2020
A shortwave trough swings through the area Wednesday night
into Thursday, but weak forcing and model soundings that only just
briefly approach saturation do not bode well for much in the way of
measurable precipitation from this. 15.12Z deterministic
GFS/Canadian/ECMWF all show light QPF staying entirely northwest,
and 15.12Z GEFS plumes have only 2 members out of 30 showing a tenth
of an inch of snow just east of KMDZ. Given this, it`s currently
looking more like flurries for the area or perhaps a light dusting
on the heavier end of things, and so have kept the forecast on the
dry side. A better chance of light rain and snow comes Friday night
into Saturday, as a larger trough approaches from the west. Ensemble
guidance is a tad wetter with this, with most GEFS/EPS members
suggesting a few tenths of an inch of snow up to an inch or two
possible, though some are still dry. Things also look like they
could be breezy on Friday into Saturday with a tight pressure
gradient ahead of and behind this system. Beyond this, there are
signs that a couple of shortwaves could impact the area Sunday into
early week in a relatively active flow aloft, but it is unclear at
this time whether they will be able to produce much in the way of
precipitation, with much of the ensemble guidance keeping amounts
light. Highs look to be in the 30s during the rest of the week into
the weekend, warmest on Friday when highs could approach 40 for
.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1000 PM CST Tue Dec 15 2020
Light E/SE winds will persist through the period with periods of
mid/high clouds. There is increasing potential for SCT/BKN MVFR
clouds, especially at KLSE, possibly by as early as late tonight.
Recent runs of the RAP and the 16.00Z NAM indicate some stratus
development into at least into KLSE by 07Z to 11Z. The 16.00Z
HREF also indicates higher probabilities for the developing MVFR
stratus field to expand westward towards the MS River, although it
would suggest the higher chances would be later Wednesday
Area Forecast Discussion...UPDATED
National Weather Service Corpus Christi TX
636 PM CST Tue Dec 15 2020
Updated the forecast specifically for the Victoria Crossroads
this evening for fog and increasing clouds. Surface and satellite
obs show a backdoor front extending from Matagorda Bay north to
Victoria edging slowly SW. The HRRR is the only model doing this
boundary justice and depicts low stratus and some fog reaching the
Victoria area from 8-11 PM, before the greater of the two cold
front shoves SE ahead of much drier NW winds and clearing skies
toward midnight. No other updates at this time.
.PRELIMINARY POINT TEMPS/POPS...
Corpus Christi 42 58 36 63 49 / 0 0 0 0 0
Victoria 39 55 32 61 41 / 0 0 0 0 0
Laredo 38 62 36 67 47 / 0 0 0 0 0
Alice 40 61 33 67 45 / 0 0 0 0 0
Rockport 44 59 41 63 50 / 0 0 0 0 0
Cotulla 35 62 32 67 43 / 0 0 0 0 0
Kingsville 41 61 35 66 46 / 0 0 0 0 0
Navy Corpus 47 58 47 62 55 / 0 0 0 0 0
GM...Small Craft Advisory from midnight tonight to 3 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.
Area Forecast Discussion
National Weather Service State College PA
1040 PM EST Tue Dec 15 2020
High pressure will drift northeast of the region tonight.
A significant coastal storm will affect central Pennsylvania
Wednesday afternoon into Wednesday night with widespread heavy
snow, creating difficult to near impossible travel conditions on
un-treated/un-plowed roads late Wednesday into early Thursday.
.NEAR TERM /UNTIL 9 AM WEDNESDAY MORNING/...
Making only minor tweaks to the overnight grids. Main tweak was
to inch temps a little cooler and dewpoints a little lower.
That might have implications for both start time of the snow
(drier air maybe delay start by an hour), and SLRs (drier at
first might make for higher, fluffier snow).
GOES Vis Sat loop shows the last vestige of the ribbons of
shallow lake effect clouds and flurries across the northern
tier counties of PA this mid afternoon. The stratocu will
dissipate by dusk, leaving a layer of gradually thickening
cirrostratus streaming ENE across the Commonwealth for the rest
of this afternoon and evening.
NNW winds of 5 to 10 kts will veer around to the NE tonight then
east toward daybreak as the aforementioned ridge axis shifts
off to our NE.
20Z temps ranging from the mid-upper 20s up north to the mid and
upper 30s in the southern valleys will dip to the uteens across
the north and low to mid 20s central and southern PA.
Expect lowering/thickening cirrostratus/altostratus overnight,
but continued dry conditions.
.SHORT TERM /9 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
Main changes to the short term grids (the main event) were to
nudge the axis of the heaviest accums to the NW by 20-30 miles
and add a little, too. This is in line with a slight shift west
of the dry slot and best slant-wise instability (for the banded
structures). Latest model QPF and SLRs argue for even higher
numbers with State College and Williamsport in an axis of 24+"
accums. We will not take it to that extreme of a change in
either axis of the highest accums nor the SF totals. But, a
nudge in that direction seems prudent.
For the afternoon update...we converted the Winter Storm Watch
to a Warning for an additional layer of counties in the NW/Ncent
Mtns, while issuing a Wint Wx Advisory for Warren and Mckean,
where a general 3-7 inches is expected by daybreak Thursday.
Elsewhere, all model guidance is in good agreement, advertising
a Winter Storm for the memories across much of Central PA, The
Susq Valley and east to the Poconos and I-81 corridor. Expect a
high probability for over 1 foot of snow within the region
bounded by I-81 and I-80, with as much as 20 inches in spots
from Scent PA, to the Western Poconos and Endless Mtns region NE
This target area will see a one-two (maybe even three) punch of
warm advection heavy snow later Wednesday afternoon and
evening, followed by a period of peak CSI-banded heavy snow with
the potential for Thunder snow later Wed Evening through a few
hours after midnight as the nose of a strong 50-60 kt easterly
850 mb jet and 130 kt upper jet focuses hefty uvvel and
slantwise instability near and just to the NW of I-81.
Main change to the upcoming nor`easter QPF and Snowfall amounts
was to shift the axis of the heaviest snow about 30-50 NM to
the NW as a host of 12Z Op Model/Hi-Res EFS data indicates the
likelihood of a mid/upper level dry slot surging NE across the
region near and to the east of the RT 15/I-81 corridor in the
Lower Susq Valley and Western Poconos later Wed evening through
the first half of Wed night.
After the surge of WAA moderate to heavy snow, some sleet
should mix in near and to the southeast of a York-Lancaster line
Wed evening, trimming the snowfall amounts by several inches or
This will be the time when snowfall rates near and just to the
NW of the I-81 corridor will be 2-3 inches per hour in
developing CSI bands.
Thundersnow is possible early Wednesday night near and to the NW
of the I-81 corridor.
Cold Conveyor belt/FGEN moderate snows with near 1 inch per hour
snowfall rates should linger until around, or shortly after
daybreak from KUNV to KIPT and points NE, leading to the axis of
heaviest snow near or just to the SE of that axis with some
areas seeing 15-18 inches for high-end storm total amounts.
A major winter storm will impact central PA Wed afternoon
through Early Thu morning, with snow accumulations of 1-2 feet
expected, and snow rates as high as 2-4 inches per hour Wed
Snow will move into the southern tier by late morning and then
overspread the rest of central PA through the afternoon. Snow
will start off light but quickly become moderate in intensity,
as the primary low moving into the Ohio Valley forces large
scale WAA and isentropic ascent over the cold air associated
with a nearly 1040mb high pressure system in southern Quebec.
This setup has all the hallmarks of a textbook Northeast snow
storm, with dewpoints in central PA starting in the low teens
Wed morning and a clear signal for cold air damming keeping a
large wedge of cold air at low levels.
Heavy snow bands will take shape during the evening hours on
Wednesday. The ECMWF and NAM deterministic models suggest the
easterly jet to the north of the intensifying 850mb low may
intensify to as strong as 60-70 kts as it moves over eastern PA
by 06z Thu. Historically, many of the heaviest snow events in
this area have occurred just west of strong easterly wind
anomalies, where convergence and frontogenesis forces intense
upward motion within cold air. As snow bands move through
central PA Wednesday night, localized snow rates of 2 to 4
inches per hour are a reasonable expectation.
Overall, a consensus/mean of all deterministic and ensemble
models has been remarkably consistent in painting the stripe of
heaviest snow along an axis from south central PA northeastward
into interior New England. We bumped this area up into the
18-24 inch range in our latest grid update. However, individual
deterministic models continue to vary on the exact placement of
the heaviest snow. It is certainly possible that the location
of the axis of heaviest snow may be realized farther
north/west... closer to the I-80/I-99 corridor... similar to
what the 00z ECMWF deterministic run suggested. Snow to liquid
ratio (SLR) may also be conservative here, as the National Blend
of Models suggests a SLR near 11, but other techniques suggest
the SLR may be as high as 15:1 along and north of Interstate 80.
If this is the case, and QPF ends up being as high as the 00z
ECMWF suggests, max totals in excess of 20 inches could be
realized in the State College area. On the other hand, the "bust
potential" in the Lower Susq (esp. S of Harrisburg) might be
that less snow ends up falling due to the presence of a nearby
dry slot, or warm nose of above freezing temperatures that
changes the snow over to sleet for a period early Wed night. The
3km NAM and HRRR have hinted at these "flies in the ointment"
in recent runs. For now, these are still just things to consider
as worst case/best case scenarios. We will continue to monitor
trends closely, but overall confidence remains high for 1-2 feet
of snow across a large portion of central PA.
The storm will be relatively quick moving, with most places
seeing precip for 18-24 hours. Nonetheless, with cold
temperatures in place, good SLRs, and very strong frontogenesis,
this should end up being the most significant snowstorm for
most of the area since November 2018... and for some areas,
perhaps one of the heaviest snow events in the past 5+ years. It
also may rival some of the record 1-day and 2-day December snow
totals. For reference, the December 2-day record snowfall at
Harrisburg is 13.9 inches, set in 1961.
.LONG TERM /THURSDAY THROUGH TUESDAY/...
The atmosphere for the long term period will be quiet. Coastal
low and associated snow should be exiting eastern Pa by Thursday
morning. The rest of Thursday looks chilly. Some clearing behind
the departing snow shield will fill back in, especially across
the Alleghenies, as upper trough swings into the region. Have
kept the mention of scattered snow showers Thursday night in the
Laurels. The off-lake fetch favors the Laurels vs the rest of
the Alleghenies for any accum of snow as the flow off the lower
lakes is from the NE and would take the SHSN a long path through
OH to get back into PA. Inversion drops pretty low through the
night, so any accums would be minor. Plenty of snow on the
ground at the ski areas by then, anyway.
Confidence remains high for fair and seasonable conditions
Friday/Saturday, as upper ridge builds over the area. Latest GEFS
and ECENS support a chance of snow showers late Sat into early
Sun for the NW with the passage of a weak and mainly dry cold
front. Not much precip will get past the Allegheny Front. A low
tracking to our west and trailing cold front could bring our
next chance of either rain or snow showers by late Monday or
Monday night of next week - but only worthy of a 20-30 PoP at
.AVIATION /04Z WEDNESDAY THROUGH SUNDAY/...
At 03z, VFR conds were found across central PA with a layer of
cirrus at 20-25 kft passing overhead.
Cloud bases will slowly drop overnight to around or just under
10 kft by daybreak Wed morning.
The leading edge of the snow will spread from SW to NE across
the commonwealth during the late morning and early afternoon
hours on Wed. Conditions will quickly drop to IFR and LIFR
shortly after the snow begins and will remain there into Wed
night as the snow continues /heavy at times/.
Conditions will gradually improve on Thursday, as the snowstorm
pulls east of the area and the snow tapers off.
Fri-Sat...Mainly VFR conditions.
Sat night-Sun...Restrictions possible in scattered snow/rain
The last time there was 12+" of snow over a two day period (as
many storms will cross from one date into another):
State College: 6-7 Feb 2010 = 14.0"
Harrisburg: 20-21 Mar 2018 = 14.2"
Williamsport: 14-15 Mar 2017 = 18.4"
Last time there was an 24+" storm (not sayin`, just sayin`):
State College: 3-4 Mar 1994 = 27.7"
Harrisburg: 22-23 Jan 2016 = 30.2"
Williamsport: 12-13 Jan 1964 = 24.1" (the only two-foot storm
Highest two-day total:
State College: 29-30 Mar 1942 = 30.5"
Harrisburg: 22-23 Jan 2016 = 30.2" (that one, again)
Williamsport: 12-13 Jan 1964 = 24.1" (that one, again)
Winter Storm Warning from noon Wednesday to 10 AM EST Thursday
Winter Storm Warning from 10 AM Wednesday to 7 AM EST Thursday
Winter Weather Advisory from noon Wednesday to 7 AM EST
Thursday for PAZ004-005.
AREA FORECAST DISCUSSION
National Weather Service Hastings NE
930 PM CST Tue Dec 15 2020
Issued at 923 PM CST Tue Dec 15 2020
Despite the expiration of the Winter Weather Advisory, several
forecast challenges remain.
Firstly, we`ve continued to see bands of light snow continue to
develop over central Nebraska. These were not well-predicted by
models, but fortunately should only amount to a dusting to a few
tenths of an inch in most spots. The final band of snow stretches
from Holdrege to Central City and will push southeastward and out
of the area over the next couple hours.
Additionally, patchy fog has started to develop over western
portions of the area as skies begin to clear. For now, the dense
fog is very patchy, but will likely continue to expand eastward.
Therefore went ahead with the Dense Fog Advisory, and it is
possible that this will need to be expanded later.
.DISCUSSION...(This evening through Tuesday)
Issued at 308 PM CST Tue Dec 15 2020
So far today has been a cold and snowy December day. Radar indicates
that snowfall is beginning to taper off, with one last band having
just formed over Buffalo and Phelps counties. Snow should
continue to die down over the coming hours (into the evening and
overnight) as this system continues to move eastward. A Winter
Weather Advisory is still in effect until 6pm this evening.
Satellite shows skies beginning to clear on the backside of this
system, over the NE panhandle. If skies continue to clear out from
west to east overnight with the departing low, then there is a
chance that some fog could develop. Recent runs of the HRRR are
showing a large swath of lower visibilities over the western
portions of the CWA towards morning. It is also important to note
that our forecast lows for tonight would be dropping well below
our current afternoon dewpoints...meaning saturation would be
nearly a given. However with that being said, I`ve decided to hold
off on putting any fog in for right now for a few reasons...
1. Skies may not clear out and we could just end up with a low
stratus deck instead, and so temps may not drop enough. A few
models are suggesting this as a possibility.
2. Though sfc winds will be rather light, they will generally be
out of the SW by morning...this would advect in drier air
3. Midlevel winds (around 850mb) would be out of the northwest,
so once again that westerly component could lead to drier air.
Clearing skies would also allow for overnight lows
to easily drop into the single digits and lower teens across the
entire area...just something else to keep an eye on.
As this winter system exits the region we transition into NW flow
aloft before brief ridging passes over Thursday afternoon/evening.
Friday a shortwave aloft makes its way over the region along with an
accompanying sfc low and cold front. Models are in good agreement
with the placement and timing of this wave, however there is some
uncertainty as to whether any precip will be produced. The ECMWF,
GFS, and NAM all indicate at least some QPF signal Friday
evening...so have decided to include some weak PoPs from 18Z Friday
till 00Z Saturday to account for this possibility. This wave on
Friday would be the only other chance for precip through the rest of
the 7-day forecast period. Conditions are expected to return to the
dry and mild weather we`ve been seeing as of late. Also, after today
temperatures are expected to gradually warm each day through the
end of the weekend, and could even reach the lower 50s by Monday.
.AVIATION...(For the 00Z KGRI/KEAR TAFS through 00Z Thursday)
Issued at 504 PM CST Tue Dec 15 2020
Snow is tapering off across the area, but there is increasing
confidence that we will see fog and/or low stratus after midnight.
The remaining question will be how far east this fog develops. For
now, it looks like EAR has a better chance to be impacted by fog
Skies will start to clear out mid Wednesday morning.
NE...Dense Fog Advisory until 9 AM CST Wednesday for NEZ060-061-072-
Area Forecast Discussion
National Weather Service Marquette MI
655 PM EST Tue Dec 15 2020
.SHORT TERM...(This evening through Wednesday)
Issued at 259 PM EST TUE DEC 15 2020
Water vapor imagery and RAP analysis reveals a split flow across
much of the CONUS with the Upper Great Lakes in the confluent flow
of the northern branch jet. Sfc high pressure which built over the
area last night remains today ensuring dry conditions for much of
the area other than some lingering light LES along the eastern
shoreline of Lake Superior. With flow backing more southerly this
afternoon, this LES is now lifting offshore. Despite WAA mid-high
clouds filling in on the back side of high pressure center, high
temps today will still reach to around 20F into the lower 20s.
By this evening, with the southerly flow increasing across the area,
LES could organize some off Lake Michigan and move into the eastern
counties. Lake Michigan temperatures are around 5C to 7C and 850 mb
temperatures will be around -12C or so, so there will be enough
instability with lake-850 mb delta-t near 18C to get pure lake
effect snow showers forming off Lake Michigan and then LES
continuing into Wed. Big negative for any meaningful snow
accumulation though will be attendant dry air in place at low-levels
as noted on the inverted-V look to fcst soundings over Lake Michigan
and Manistique and large scale anticyclonic flow. With light south-
southeast flow, expect light LES accumulation into east half
portions of the CWA. If a more organized band does form and persist
over any one location for several hours, maybe an inch or two of
accumulation could be possible, but believe most locations will see
less than an inch.
Temps tonight under mostly cloudy skies and a light southerly flow
will range from the lower to mid teens over the interior to upper
teens to around 20F near the Great Lakes shores. Under partly to
mostly skies, expect highs Wednesday ranging from the mid 20s to
the lower 30s.
.LONG TERM...(Wednesday night through Tuesday)
Issued at 358 PM EST TUE DEC 15 2020
The long term period looks progressive with a series of low
amplitude waves moving through. The first in a series of shortwaves
passes to our south on Wednesday with another weak wave digging to
our south on Thursday. Shortwave ridging builds in Thursday night
into Friday ahead of a cold front on Friday night that should bring
widespread light snow. A low level jet ahead of the cold front could
result in a brief period of breezy weather Friday afternoon with
potential gales over marine areas. Mostly zonal flow over the
weekend should result in dry weather with warming temperatures ahead
of a deeper trough next Monday/Tuesday.
Light return flow west of the departing arctic high pressure starts
the long term period. 850 mb temperatures around -10C will support
some lake effect snow for southerly snow belts across the southeast
UP. Snowfall amounts appear light due to dry lower levels, but the
NMM/ARW show a brief period of elevated precipitation rates in
Schoolcraft county around 06z Thursday. The next wave tracks just
south of the CWA during the day Thursday. However, moisture is
lacking so widespread cloud cover, flurries, and perhaps a couple
snow showers for southeastern zones.
Shortwave ridging builds in Friday ahead of a shortwave and
associated cold front. The LLJ ahead of this front has potential for
a brief period of gale force winds/gusts Friday night, but 12z GFS
backed off a bit so capped grids at 37 knots. Model guidance
supports a couple hundredths of QPF falling as snow with the cold
front passage. The airmass behind the cold front is not particularly
cold so lake effect is unlikely. Mostly zonal flow over the weekend
should result in dry weather and a warming trend for Sunday into
A more substantial low pressure system should pass through the
region on Monday. Ensemble guidance indicates a low pressure system
will track north of Lake Superior next Monday with a substantially
colder air mass filling in behind the cold front. The NBM blend
indicates steadily falling temperatures with several inches of snow
leading up to Christmas. However, NBM temperature spread increases
rapidly beyond Monday indicating a low confidence forecast so don`t
get too excited yet.
.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 654 PM EST TUE DEC 15 2020
VFR conditions should prevail thru this fcst period at KIWD/KCMX. At
KSAW, VFR conditions will prevail for much of the night. Then,
developing upslope sse wind from Lake Michigan will bring lake
effect clouds late tonight/Tue, resulting in MVFR cigs. There could
be some flurries in the morning as well, but do not anticipate any
.MARINE...(For the 4 PM Lake Superior forecast issuance)
Issued at 259 PM EST TUE DEC 15 2020
The wind will remain at or below 25 knots into early Friday before
it increases to south gales of 35 to 40 knots over the east half Fri
afternoon and evening ahead of a cold front approaching from the
Plains. The wind then drops back down to northwest 15 to 25 knots on
Saturday behind the cold front and then will shift southwest at 15
to 25 knots on Sunday ahead of the next approaching frontal system
from the Northern Plains. Friday looks to be the windiest day for
AREA FORECAST DISCUSSION
National Weather Service Morristown TN
927 PM EST Tue Dec 15 2020
Forecast on track this evening as we await our next weather
system. Clouds continue to increase and precip is slowly
advancing east-northeast toward our area from out of middle TN
and MS/AL. Latest few runs of the HRRR keep most of the freezing
rain on the other side of the Appalachians and only show very
little across extreme northeast TN and portions of southwest VA.
Our current freezing rain totals may be overdone but I am hesitant
to lower them before the event has even started. Though I will
note that it is several degrees warmer at the current hour across
the aforementioned areas than what was previously forecast.
Anyways, no changes to the forecast as of now but will be
watching trends closely over the next few hours.
00Z TAF DISCUSSION.
Our next weather system moves in tonight. VFR conditions
currently in place but CIGs will lower as this system approaches
and a combination of IFR/LIFR can be expected by tomorrow and
persist through the end of the period.
/ISSUED 333 PM EST Tue Dec 15 2020/
SHORT TERM...(Tonight and Wednesday)
1. Rain begins late tonight and will begin as light freezing rain
and sleet across far NE TN, SW VA, the mountains, and SW NC.
2. Precipitation will transition to all rain by mid day Wed, but
some freezing rain will continue in the E TN mountains and high
elevations of SW VA during the afternoon.
3. Ice amounts will range between a trace and a tenth of an inch
with the greatest amounts in Russell and Washington Counties VA
and Johnson County TN.
Temps have warmed into the upper 40`s to near 50 as of 2 PM as the
mid/upper shortwave ridge slides east ahead of a mid/upper
trough/closed H5 low moving across the S Plains. High cirrus clouds
will continue to increase through the evening as the closed low gets
closer and resultant moisture advection increases.
Tonight and Wednesday...
Tricky forecast during this period as classic Miller B storm system
evolves over the eastern CONUS. The aforementioned closed H5 low
over the S Plains will open up and eject E into the TN Valley
tonight and phase with a northern stream H3 jet max over eastern
Canada. The original surface and 850 mb lows will track through the
western TN Valley tonight and into the lower OH Valley Wed while a
new coastal low develops off the SE coast tonight and moves up the
eastern seaboard while rapidly deepening Wed. WAA and isentropic
ascent will increase across E TN and the S Appalachian region
tonight ahead of the surface and 850 lows passing to the NW, and a
120-135 kt H3 jet streak developing overhead early Wed will further
strengthen the warm and moist advection in the lower levels through
the morning with a warm front slowly advancing NE before the cold
front moves through on the backside of the system Wed afternoon.
Precip Type and Amounts
There are two challenges to precip type and amounts. First, despite
the aforementioned dynamics, moisture advection will not be as
strong as it could be. This is due to the coastal low taking over
early in the day which blocks some of the Gulf and Atlantic
moisture. Second, a large and strong ~1035 mb surface high over
Quebec will feed cold air into the E side of the Appalachians
creating classic cold air damming/wedge over VA and the Carolinas.
Guidance has struggled with how far SW this cold low-level air
bleeds into our CWA over the past few days, but the trend has been
warmer, which makes sense due to the climatology of these wedge
events. With all of this in mind, used a blend of WPC and NBM hourly
QPF for late tonight through Wed which keeps amounts fairly light
(0.50 to 0.70 inches on average). Precip will begin in the S Valley
and Plateau after 06Z gradually spreading NE, but downslope S to SE
flow as low-level WAA increases will delay the start of precip in
far NE TN and SW VA until after 09 or 10Z. Forecast soundings also
suggest virga for many areas at the onset until the column can
saturate. Soundings suggest that mixed precip will be confined to
far NE TN, SW VA, the E TN mountains, and SW NC. A warm nose between
about 800 and 900 mb with surface temps near or just below freezing
will allow for primarily light freezing rain in these areas, but
could see some pockets of sleet. This will end over SW NC by 10Z as
the warm layer deepens to the surface, but it will take until mid to
late morning for the warm layer to deepen across far NE TN and SW
VA, so these areas will see the longest period of freezing rain. The
relatively light QPF and short duration of freezing rain will keep
ice amounts a trace to 0.02 inches in most areas, but could see
locally higher amounts in far NE TN and SW VA where a Winter Weather
Advisory was issued. Believe the amounts will be light enough
farther south and west for an SPS. Widespread rain will continue
ahead of the cold front through mid afternoon so have PoPs
categorical. The afternoon hours will be primarily rain except for
the highest peaks of the E TN and SW VA mountains. The steadiest
rain will exit behind the front in the late afternoon, but kept
chance to likely PoPs as a deformation band wraps overhead and a
second trough approaches from the NW. Highs will reach the low/mid
40`s ahead of the cold front.
LONG TERM...(Wednesday Night through Tuesday)
The long term fcst period kicks off on Wednesday night amidst broad
H5 troughing across the central/eastern CONUS, while a higher
amplitude shortwave traverses the mean flow atop the OH/TN Valley
into the Central Appalchians, while a second shortwave digs across
the Upper Midwest. At the surface, a cyclone associated with the
ejecting upper shortwave noted above will deepen as it slides up the
eastcoast while high pressure builds over the Plains and stretches
east into the MidSouth region. At fcst initialization, the
shortwave axis should be pivoting atop the Upper TN Valley with any
synoptically forced precipitation coming to and end as the upper jet
and associated QG forcing advects east. By that point, am expecting
any of this initialization precipitation to be all rain in the
Valley, perhaps with a ptype phase change to sleet/snow beginning at
the higher elevations across southwest VA and the East TN mountains.
However, although forecast profiles cool, they also dry out rather
quickly aloft (through much of the column actually) as the synoptic
wave ejects northeast. Thus, into Thursday morning am expecting a
shift in precipitation forcing to orographic lift with any/all
precipitation focused along the upslope regions per a weak NWF
regime. Thursday morning will reveal accumulating snowfall across
portions of the Cumberland Plateau (light accums here) as well as
the higher elevations of southwest VA and the mountains of East TN
(possibly as far south as the NC mountains). Also during this
timeframe, the base of the longwave flattens slightly as the
aforementioned Upper Midwest secondary shortwave slowly approaches
the OH/TN Valley region. By Thursday afternoon, the fcst will
feature dwindling snow showers across the terrain as moisture is
increasingly starved and H85 temperatures warm slightly. All in
all, think this brief round of NWFS will be hindered by overall
moisture content, depth, and weak flow altogether. Snow totals over
the mountains should remain at a couple inches or less.
Moving on, by Friday morning the synoptic pattern looks to shift as
upper heights rise over the eastern CONUS with sprawling high
pressure dominant over the OH/TN Valley regions beneath confluent
flow aloft. Meanwhile to the west, a longwave trough will be
sliding east across the Rockies, ejecting into the Plains by Friday
evening. In response, lee cyclogenesis is favored which will foster
warm advection into the Plains and MS Valley increasing
baroclinicity. However closer to home, the TN Valley and southern
Appalachians will remain cool/dry with highs topping out a few
degrees below normal amidst mostly sunny skies. Looking at Friday
night into Saturday morning, the deepened plains trough and
associated prefrontal convection will start to make impacts
downstream as sky cover increases amidst enhanced waa thanks to the
repositioned surface ridge to the east over the Carolinas. Therefore
on Saturday, waa will be offset somewhat by increasing cloudiness
allowing temperatures to only warm a few degrees (still just below
normal) from Friday. As for precipitation, there remains some
discontinuity in the operational guidance as the ECMWF favors an
unphased split flow regime, while the GFS keeps things consolidated
and phased. With that, the GFS likes a fropa overnight into Sunday
morning per its dominant northern stream cyclone and consequent
front, while the ECMWF lacks that strong northern stream component,
favoring the southern stream GOM coastal wave. Nevertheless, pops
will increase Saturday afternoon/evening to chance levels, holding
there through Sunday, tapering Sunday night. At this range, and
given the uncertainty described above, any wintry ptype
determination is done with low confidence. However, will say that
overall it looks as if profiles will be too warm for widespread
wintry precip, with the possible exception being at the higher
elevations where rain could change to snowfall Sunday night.
Lastly, moving into next week, the pattern looks rather uneventful
as nearly the entire CONUS is consumed by a very broad yet low
amplitude H5 trough, while sprawling dry/cool high pressure
dominates atop the southcentral CONUS on Monday, only to be
reinforced by a cooler/drier high pressure airmass advection
southeast along the mean H5 flow out of Canada into Tuesday.
Therefore, the start of the new work week looks dry with
temperatures near normal on Monday, slightly cooler into Tuesday.
.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN 38 47 32 44 27 / 80 90 20 0 0
Knoxville McGhee Tyson Airport, TN 35 44 33 40 26 / 70 90 40 10 0
Oak Ridge, TN 34 44 32 40 25 / 60 90 40 10 0
Tri Cities Airport, TN 32 43 32 39 26 / 40 100 50 10 0
TN...Winter Weather Advisory from 4 AM to noon EST Wednesday for
VA...Winter Weather Advisory from 4 AM to noon EST Wednesday for
Area Forecast Discussion
National Weather Service New York NY
1050 PM EST Tue Dec 15 2020
High pressure will build to the north overnight into early
Wednesday. Low pressure will then quickly intensify into a
major winter storm just off the Mid Atlantic coast Wednesday
night into Thursday. High pressure will then build in from the
west through the first half of the weekend before moving
offshore late in the day Saturday. A series of weak disturbances
will move across the area Sunday and Monday, followed by a
stronger cold frontal passage Monday night into Tuesday.
.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
For this update have only adjusted cloud cover up for the
overnight as high clouds are abundant on the IR imagery. The
clouds will be quite high, around 25 kft so minimal to minor
impacts on temperatures are expected.
Strong (1037 mb) arctic high pressure north of the Great Lakes
will move east over southeastern Canada overnight. This
introduces a cold air mass. Lows will range from the teens in
the normally colder spots, to the lower and middle 20s
elsewhere. This roughly 5 to 8 degrees below normal.
.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH THURSDAY/...
A shortwave over the northern Gulf States will become
negatively tilted overnight night tonight, and induce an area of
low pressure along the mid-Atlantic coast by Wednesday
afternoon. This low parallel the coast and then appears to make
an eastward jog just south of Long Island. The arctic high will
provide enough cold air for the area to see snow by late
Wednesday afternoon, which will spread from southwest to
northeast overnight Wednesday. With dew points expected to start
off in the single digits, there will likely be a fair amount of
virga to start off, and perhaps the start time of the
precipitation is earlier than when it actually occur by a couple
Precipitation is expected to start off as all snow everywhere,
with a period of heavy snow during the first half of the night
(between 7 pm Wednesday and 1 am Thursday). Snowfall rates of
1-3 inches per hour are possible during this time frame (even
after 1 am for areas away from the coast). This will be thanks
to mid-level frontogenesis and EPV less than 0 noted in the NAM
during this timeframe that could set up a heavy band of snow
that moves in from the southwest. This feature is also seen in
the HRRR reflectivity, which has been pretty consistent from run
to run. This burst of heavy snow right after the onset will be
where most of the coastal sections get their share of heavy
snow. Thereafter, warmer air aloft at around at around 750 mb
will move in from east to west, mainly along the coast. There is
still uncertainty as to where this warm air advects inland, but
the NAM has been pretty progressively bringing this warm air
aloft closer to the coast. While this is an outlier, it may be
a sign of how the other models may trend. After 1 am, a
transition to a wintry mix (snow and sleet mainly) is expected
across southern Long Island (including the Twin Forks) and
southeastern Connecticut. This will cut snow totals for these
areas. But, even with a track closer to the coast, there is high
enough confidence for Watches to be converted everywhere, except
for southern Nassau, southwestern Suffolk, the Twin Forks and
coastal southeast Connecticut, where as the previously
mentioned uncertainty with the warm air aloft will lowered
confidence for warning criteria there. If models trend colder
overnight, then we can issue a Winter Storm Warning for those
areas, but if the trend continues, then an Advisory (or perhaps
even nothing) will be issued.
Another uncertainty to monitor will be the development of a dry
slot that will cut down totals as well, which if the HRRR is
right, could happen as early as 10 pm. Precipitation may become
more convective after that, also lowering widespread snow
As the low makes it`s eastward trek, winds will shift more to
the north and allow colder air to work into the region. Changing
any mixed precipitation or rain back to all snow along the
coast. Some additional accumulation is possible during this time
frame which would be early Thursday morning.
Storm total snow amounts will be 12-16 inches across portions of
northeast New Jersey, the Lower Hudson Valley, and southern
Connecticut. 8 to 12 inches for New York City and western Long
Island, and 3 to 10 across eastern Long Island and southeastern
Connecticut. This large range is due to continued uncertainty.
Winds will also increase Wednesday night, as the pressure
gradient increases between the strong high and the developing
low pressure. Northeast winds of 25 to 35 mph with gusts as
high as 50 are possible (some isolated wind gusts of 55 mph are
also possible) mainly along the coast.
The low pulls away Thursday (much of the area dries out by the
afternoon, with some lingering snow for eastern areas), with
winds diminishing (though it will still be windy in the morning)
skies clearing into the afternoon, and well below normal
.LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/...
A progressive, yet amplified upper flow will feature a longwave
trough moving offshore to close out the week with ridging for the
first half of the weekend. However, a broad upper trough will once
again reestablish itself for the second half of the weekend into
early next week.
At the surface, gusty north winds (up to 20 mph) will linger into
the first half of Friday as low pressure slowly pulls away across
the north Atlantic, while a south to north elongated ridge builds
toward the eastern seaboard. The latter of which will build across
the area Saturday and then offshore by evening.
For Sunday into Monday, a disjointed upper trough will send a series
of weak shortwaves into the area with a chance of some light
precipitation Sunday, with mainly rain at the coast and a rain/snow
mix inland. Due to mild temperatures and the light nature of the
precipitation, any accumulation will be minor at best. A stronger
cold front will then move across the area Monday night into Tuesday
with another shot of some light precipitation.
Temperatures will gradually moderate through the period, with
readings 5 to 10 degree below normal at the start, to slightly above
normal readings by early next week.
.AVIATION /04Z WEDNESDAY THROUGH SUNDAY/...
High pressure builds over the region overnight and into early
Wednesday morning before shifting northward. This will give way to
an approaching low pressure system on Wednesday, with impacts
expected by Wednesday evening and into the remainder of Wednesday
VFR conditions through Wednesday morning and then MVFR to IFR
conditions develop as snow begins Wednesday late afternoon and picks
up in intensity going into Wednesday evening.
Winds will be out of the N at 5-10 kt overnight. The winds become
more NE Wednesday and increase to 10-15 kt with gusts of 20-25 kt
developing late in the day.
...NY Metro (KEWR/KLGA/KJFK/KTEB) TAF Uncertainty...
Start time of snow on Wednesday could be 1-2 hours off.
.OUTLOOK FOR 6Z THURSDAY THROUGH SUNDAY...
.Wednesday night...Conditions will range from IFR to VLIFR at times
in moderate to heavy snow. The precip can possibly mix with ice
pellets for the city terminals, and mix with ice pellets and rain
for the the more eastern coastal terminals late. NE wind around 15
to 25 kt, increasing to 30-40kt at night. LLWS at 50 to 55 kts for
.Thursday...IFR to MVFR in snow / snow showers, ending during the
late morning for NYC terminals and N/W, early afternoon east of NYC
terminals, with improvement to VFR. N wind G30-40kt in the morning,
subsiding 20-30kt by around noon. Storm total snowfall forecast of 6
to 14 inches, with highest total across northwestern most terminals.
For southeastern most terminals 4 to 8 inches is expected for KJFK
and KISP terminals.
Detailed information, including hourly TAF wind component forecasts,
can be found at: http:/www.weather.gov/zny/n90
High pressure builds in tonight. The high will depart on
Wednesday morning as low pressure approaches from the southwest
on during the day Wednesday. Winds should ramp up quickly late
day Wed into Wed night, with NE gales developing on all waters
Wed night, and potential for storm force gusts on the ocean
waters as a developing low approaches the water from the
Gale Watches have been converted to Gale Warnings as
gusts to 40 to 45 kt are expected. Occasional gusts to 50 kts
are possible. There is uncertainty as to how high the winds
actually get and a Storm Warning may be issued overnight if
Seas build to 10 to 15 ft over the ocean waters late Wednesday
night into Thursday morning, then diminish late Thursday morning
and thereafter. 3 to 6 ft seas are expected in the western
sound, and up to 8 ft for the extreme eastern sound, before
NE-N gales on all waters Thu AM should gradually diminish later
Thu into Thu night, with gales only on the ocean by afternoon
and SCA on the ocean and eastern Sound/bays by Thu evening.
Northerly winds will continue to gradually ramp down Thursday night
into Friday as high pressure builds in from the west. Sub-SCA seas
are expected on the ocean by Saturday morning and will then remain
there through the remainder of the weekend.
There is an increasing potential for significant snowfall
Wednesday afternoon into Thursday morning, with liquid
equivalent QPF of 1-1.5 inches. Any melting should be slow as
high temperatures remain below 40 until the weekend.
Surge is expected to build to 2 to 2 1/2 ft in most spots
across Lower NY/NJ harbor, W LIS, and south shore bays of W LI
on NE winds ramping up to solid gale Wed PM. Widespread minor
coastal impacts are expected along the south shore bays of
Nassau County. Along W LI Sound, combo of 3-5 ft surf and
elevated water levels will have potential for local moderate
coastal impacts for shoreline roads and properties. Elsewhere,
potential for localized minor coastal impacts. If guidance
continues to trend upward with surge, with GFS wind speeds
trending towards NAM strength wind fields, may have to expand
coverage of advisories for Wed PM.
Storm surge will likely peak out at 4-5 ft in spots Wednesday
Night during time of low tide, but then is likely to settle back
to 2 1/2 to 3 1/2 ft during the times of high tide as winds
shift to the north. These water levels, during a higher
astronomical tide Thu AM, will bring potential for more
widespread minor to moderate coastal impacts with Thursday
morning/early aft high tide. Highest probability of widespread
moderate coastal flood impacts (2 to locally 3 ft inundation)
is along vulnerable south shore bays of LI and Queens with
limited tidal drainage, with widespread minor to locally
moderate impacts (1 to locally 2ft inundation) elsewhere. Have
stayed close to deterministic surge guidance and NYHOPS weighted
mean, with expectation of wind shifting to a less conducive N
flow for surge. Will have to monitor guidance trends.
CT...Winter Storm Watch from Wednesday afternoon through Thursday
afternoon for CTZ011-012.
Winter Storm Warning from 2 PM Wednesday to 1 PM EST Thursday
Coastal Flood Advisory from 11 PM Wednesday to 2 AM EST
Thursday for CTZ009.
NY...Winter Storm Watch from Wednesday afternoon through Thursday
afternoon for NYZ079>081-179.
Winter Storm Warning from 2 PM Wednesday to 1 PM EST Thursday
Coastal Flood Advisory from 11 PM Wednesday to 2 AM EST
Thursday for NYZ071-073-078-176-177.
Coastal Flood Watch from Thursday morning through Thursday
afternoon for NYZ080-178-179.
Coastal Flood Advisory from 8 PM Wednesday to 1 AM EST
Thursday for NYZ179.
NJ...Winter Storm Warning from 2 PM Wednesday to 1 PM EST Thursday
MARINE...Gale Warning from 4 PM Wednesday to 1 PM EST Thursday for
Gale Warning from 4 PM Wednesday to 6 AM EST Thursday for
Gale Warning from 4 PM Wednesday to 6 PM EST Thursday for