Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/15/20
Area Forecast Discussion
National Weather Service Charleston SC
923 PM EST Mon Dec 14 2020
High pressure will build across the region tonight, then pass
to the north on Tuesday while a trough develops along the
coast. A storm system will bring impacts to the area Tuesday
night into Wednesday, followed by more high pressure during the
later half of the week.
.NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
No meaningful changes were made for the late evening update.
The cold front is now well offshore with high pressure starting
to wedge south into the Southeast States. Dry conditions will
prevail overnight, although isentropic assent will begin to
increase early Tuesday as winds above the boundary layer begin
to veer in response to low pressure organizing over the Texas
Caprock. This will result in a slow increase in clouds
overnight, first in the form of cirrus, followed by a 2500-4500
ft cloud deck forming by daybreak across mainly the coastal
counties. Lows from the upper 30s well inland to the upper
40s/lower 50s at the beaches look on track. Breezy conditions
will also redevelop along the beaches late in response to a
tightening pressure gradient.
Lake Winds: Winds at PNOS1 remain below Lake Wind Advisory
thresholds. However, the latest RAP data suggest another weak
surge will work south over the next few hours which could push
gusts into the 25 kt range. Per coordination with WFO Columbia,
the decision has been made to keep the advisory in place through
its current 4 AM expiration time.
.SHORT TERM /6 AM TUESDAY MORNING THROUGH THURSDAY/...
Tuesday: The area will remain situated between a ridge of high
pressure to the southeast and a trough of low pressure digging
across the Central United States. At the sfc, high pressure will
prevail across the area, especially inland and north, while a
coastal low develops off the Southeast coast late. The pattern
favors mainly dry conditions across land, especially away from the
coast for much of the day. Temps will be considerably cooler than
the previous day within a northerly sfc wind. In general, highs will
range in the mid 50s.
Tuesday night into Wednesday: Temps should dip into the low/mid 40s
Tuesday night as clouds spread across the area. A coastal low will
deepen along the Southeast coast overnight, then shift north of the
area early Wednesday as a low pressure system and associated cold
front approach from the west and shift offshore Wednesday afternoon.
Precip chances should initially develop along coastal locations
Tuesday night as the coastal low deepens, before the onset of
greater precip coverage associated with the arriving cold front late
Wednesday morning into the afternoon. Rainfall accumulations will be
limited somewhat given how quickly the front passes through the
region. However, PWATs peaking in the 1.25-1.50 inch range suggest
accumulations up to around 1/4 inch. High temps will remain below
normal Wednesday, peaking in the low/mid 50s inland to upper
50s/lower 60s along the immediate coast (warmest in Southeast
Thursday: Dry high pressure will build across the region behind a
departing cold front well offshore. Mostly sunny skies and a
downslope wind should result in slightly warmer temps (especially
inland), but temps should still remain a few degrees below normal
for this time of year. In general, highs will range in the mid/upper
.LONG TERM /THURSDAY NIGHT THROUGH MONDAY/...
High pressure will move over the Northeastern U.S. into the
weekend, leading to dry weather and below normal temperatures
into Saturday. Models then diverge early next week regarding
rainfall potential, so we generally went with chance POPs as a
placeholder until the models line up better.
.AVIATION /03Z TUESDAY THROUGH SATURDAY/...
VFR will prevail overnight. Cigs are expected to lower to MVFR
thresholds by mid-morning at KSAV and early afternoon at KCHS in
response to increasing isentropic assent atop the inland wedge
of high pressure. There are some indications that isolated-
scattered light showers could develop across the coastal
counties tomorrow in response to low pressure developing off the
coast. How close the low tracks will determine how much shower
coverage develops over land. Will keep any mention of shower
activity out the terminals for now given low confidence on both
timing and coverage. Gusty winds with gusts near 20 kt will
impact the terminals, especially from daybreak on.
Extended Aviation Outlook: Gusty winds near 20 kt are
likely into Tuesday evening, especially at KCHS. A low pressure
system and associated cold front should then bring flight
restrictions Wednesday before dry high pressure prevails across
the area late Wednesday night into Friday.
Tonight: High pressure will build in and winds will
increasingly become northeasterly. A solid surge is expected due
to the tightening pressure gradient and cold advection, mainly
20 to 25 kt with gusts to 30 kt. Seas will build back up to
3-7 feet, highest beyond 20 nm.
Tuesday and Wednesday: Strong northerly winds (25-30 kt) and seas up
to 6-8 ft will prevail across all waters Tuesday and Tuesday night
while a coastal low develops/deepens along the Southeast coast.
Small Craft Advisories will therefore remain in effect across all
waters through Tuesday night. The coastal low is then expected to
shift north of the local waters early Wednesday, resulting in a
decreasing pressure gradient across the area and improving wind/sea
conditions. Small Craft Advisories will begin to end across the CHS
Harbor and nearshore waters south of Charleston County by late
Wednesday morning, while lingering 6 ft seas result in Advisories
across northern South Carolina waters and offshore Georgia waters
into early Wednesday afternoon.
Wednesday Night through Friday: High pressure will build across the
area Wednesday night behind a departing cold front, then persist
through Friday. Winds/seas are expected to remain below small craft
advisory level conditions during the second half of the week.
Tide levels will remain elevated through the middle of the week due
to lingering astronomical influences and favorable winds. For the
Charleston Harbor tide gauge, guidance continues to indicate at
minor/moderate coastal flooding (7.0 ft MLLW/7.5 ft MLLW) with each
morning high tide cycle through Wednesday. At the Fort Pulaski tide
gauge, minor coastal flooding (9.5 ft MLLW) will be possible Tuesday
SC...Lake Wind Advisory until 4 AM EST Tuesday for SCZ045.
MARINE...Small Craft Advisory until 9 AM EST Wednesday for AMZ352.
Small Craft Advisory until 1 PM EST Wednesday for AMZ350.
Small Craft Advisory until 7 AM EST Wednesday for AMZ330-354.
Small Craft Advisory until 4 PM EST Wednesday for AMZ374.
Area Forecast Discussion
National Weather Service Detroit/Pontiac MI
556 PM EST Mon Dec 14 2020
There has already been a downward trend in wind speeds. This will
continue during the evening with the loss in diurnal mixing and a
relaxing of the cold air advection. The moisture flux off Lake Mi is
still sustaining a healthy VFR based strato cu field across Lower
Mi. Area observations and recent RAP soundings suggest these clouds
will prevail during the evening. Persistent mid level subsidence,
the advection of dry air across the lake and a more northerly and
divergent component to the low level flow will all contribute to the
eventual erosion of these clouds overnight.
For DTW...Based on the RAP and with clouds still extending all the
way back to Lake Mi, the 4-5kt strato cu field may hold over metro
until at least 06Z. Otherwise, wind directions will hold around 300
to 310 degrees but speeds will continue to drop markedly through the
.DTW THRESHOLD PROBABILITIES...
* Moderate in ceilings at or below 5000 feet this evening.
Issued at 300 PM EST Mon Dec 14 2020
Low level cold advection under the northwest flow this afternoon is
just about exhausted. With 850 MB temps around -13 C, steep low
level lapse rates have been able to generate scattered flurries, but
aggressive subsidence/drying is now beginning to taking place, as
850 MB dews pt depressions of 30-40 C take hold this evening through
tomorrow. A ridge of high pressure will build in tonight, with winds
likely decoupling by morning. Fortunately, lack of snow cover around
should prevent temps from really cratering, but still a cold night
with mins down into the teens, as there also may be some lingering
low clouds but more likely some increase in high clouds to keep
The strong upper level northwest confluent flow in place over the
Great Lakes Region will be important, as the upper level energy/wave
ejects out of the Four Corners region. None-the-less, still enough
model consensus of the lingering mid level trough/500 MB low
reaching the southern Michigan border by 00z Thursday as the
transfer of energy/height falls occur along the East Coast Wednesday
night. Although the airmass is very dry initially over southern
Lower Michigan Wednesday morning, the 12z NAM does show the remnant
850-700 MB circulation tracking along the southern Michigan border
during the afternoon, with specific humidity rising to 2-2.25 g/kg
in that layer. Thus, light snow is now appearing likely along and
south of I-94, as the low level easterly winds off off Lake
Erie/Lake St. Clair also help pre-condition/moisten up the levels.
Thermal profiles are not bad, with 850 MB temps in the negative mid-
high single numbers, with 700 MB temps in the negative lower teens.
Certainly can see us grind out up to an inch of snow accumulation
Wednesday afternoon-early evening along and south of the I-94
corridor, with snow chances quickly tapering off as one heads north.
The forecast for Thursday is a bit murky for light snow/flurry
chances, as the pace of the 500 MB shortwave/trough exiting east is
in question, with another upper wave diving south through the Western
Great Lakes during Thursday. Inverted surface trough arching through
western Lake Erie and into southern Lake Huron, coupled with the
favorable 850 mb temps of -7 to -10 C argues to maintain slight
chance pops/flurries across far eastern areas with the low level
convergence/northeast winds off the lakes.
Good warm advection on Friday as amplified upper level ridge
translates east through Lower Michigan. A strong upper level wave
will track through Ontario Friday evening/night, but will be
tracking off to the northeast. Thus, the cold front moving through
the Great Lakes region will be weakening, and losing its identity on
Saturday. Per 12z Euro, another series of fast moving Pacific waves
to track across the northern Great Lakes late in the Weekend into
early next week, but 850 MB temps look marginal enough not to
illicit a whole lot of help from the Great Lakes to support snow
showers. Thus, only slight chance pops appear warranted, especially
when you factor in the timing uncertainty to the equation.
Cold advection continues across the region this afternoon in the
wake of a cold front. West-northwest gusts to marginal gales will
continue through the afternoon across northern Lake Huron where the
Gale Warning remains in effect through this evening. The gradient
and wind speed weaken tonight as arctic high pressure builds in from
the north and west. This high will dominate the weather on Tuesday
with lower impact marine conditions in store as waves diminish. Wind
then reorients from the east on Wednesday as a low pressure system
passes through the Tennessee Valley into the eastern seaboard. Some
light snow will be possible across western Lake Erie and Lake St.
Clair, otherwise the greater impacts will remain to the south and
east of our area.
Lake Huron...Gale Warning until 10 PM EST this evening for LHZ361-362.
Small Craft Advisory until 4 AM EST Tuesday for LHZ421-441-442.
Lake St Clair...NONE.
Michigan waters of Lake Erie...NONE.
You can obtain your latest National Weather Service forecasts online
Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
855 PM CST Mon Dec 14 2020
855 PM CST
The forecast through the rest of tonight and into the day Tuesday
is in fine shape. Also keeping an eye on 00Z guidance for
Wednesday morning, which thus far in RAP and NAM solutions
continue the trend of inching light snow northward with the
increasing possibility of light accumulation for mainly, but
maybe not entirely, south of I-80 and east of I-55.
High clouds have been a little slow to thicken this evening over
far northern Illinois and northwest Indiana, but immediate
upstream satellite and model solutions support a greater coverage
and thickness by after midnight. The day shift had already
accounted for this in the low temperature forecasts. Without the
clouds, single digits would be realized in the far north and west.
Clouds may thin temporarily Tuesday morning before thickening
again Tuesday afternoon.
Water vapor imagery early this evening depicts the potent upper
trough (mentioned in the prior issued discussion) directly over
the Four Corners region. It is potent in that it is still digging
yet moving eastward at a good clip. The 00Z Flagstaff AZ sounding
indicated a 500 mb temperature of -27C, in the lowest percentile
for this time of year, just an indicator of the anomalous nature.
The 00Z RAP and NAM continue to trend a wee bit slower, deeper,
and further north with this as it migrates into the Lower Ohio
Valley region late Tuesday night into Wednesday morning. The
850-700 mb flow is now more southeasterly with better moisture
transport and sufficient saturation into the southeast third of
the forecast area to support light snow. The overnight shift will
fully analyze the suite of guidance to address probable higher
PoPs in this area, and with the timing near daybreak into the
morning after a couple cold days, to potentially mention light
snow accumulation. Conceivably this could even be over an inch in
the far southeast CWA (Benton, Iroquois Counties area) looking at
the incoming guidance and upper/mid-level pattern and vorticity
maximum path. Further north into the heart of the Chicago metro,
the atmosphere has less deep saturation persistence, but the
northeast flow off the lake does look to possibly support lake-
induced instability with the anticipated lake effect clouds for a
regular flurry, maybe even at times light snow, on Wednesday.
Again overnight shift will assess in full and update if needed.
1224 PM CST
Through Tuesday night...
Currently temperatures are in the upper 20s to lower 30s with mostly
sunny skies. A weak cold front passed through this morning, giving
way to breezy northwest winds (the air does have a bite today). All
things considered it`s a pleasant December day.
Tranquil conditions will prevail through the next 48 hours.
Tonight, clouds will increase in accordance with a developing
storm system well southwest of our area. As a surface high
pressure system slides over the northern Great lakes, winds will
gradually become easterly. Lows overnight should fall into the
upper teens to lower 20s areawide, with the coldest temperatures
along the IL/WI stateline where a snowpack (seen nicely in
visible satellite imagery today) resides. Tomorrow will be rather
drab with mostly cloudy skies and highs similar to today - near
the freezing mark. With flow off Lake Michigan, a few flurries
will be possible along the lakeshore.
150 PM CST
Wednesday through Monday...
Overall quiescent weather conditions are expected through much of
the extended portion of the forecast, although our tranquil
periods may be punctuated by some occasional precipitation chances
as an amplified pattern across the Pacific continues.
The first such disturbance we`ll be keeping an eye on will be at
the start of the extended very late Tuesday night and into
Wednesday morning. This feature is currently dropping
southeastward across the Great Basin within a positively-tilted
trough axis which stretches northeast into the Upper Midwest.
Deterministic guidance remains in pretty good agreement depicting
this wave`s eastward translation across the Central Great Plains
tomorrow and tomorrow night as it gradually acquires more of a
neutral tilt with time. The ensemble envelope--which began a shift
to the northwest yesterday--is now being reflected a bit more in
the current iterations of the ECMWF, GFS, and UKMET which are now
squeezing out some (light) precipitation south and east of the
I-57 corridor into early Wednesday afternoon.
Large scale forcing for ascent, at least for a brief window, is
actually relatively robust as the combination of warm advection,
DCVA, and some degree of upper jet divergence overlap before the
trough axis pushes east of our longitude. This forcing is now
showing up well on the 300-305 K theta surfaces in today`s
guidance. Fairly cool temperatures aloft (-28 C at 500 mb) will
support at least some steepening of the mid-upper lapse rate plume
with this feature, so will be monitoring this system for any
additional northwestward jogs given the potential for embedded
convective elements. Also seeing some signs of a weak lake-
enhancement and f-gen circulation in some of the guidance trying
to establish farther northwest closer to the I-88 corridor,
although prolonged dry easterly trajectories will reinforce a
rather dry low-level airmass with north and westward extent across
our CWA. For now, have continued to expand some low PoPs farther
northwest than the offered blend today with some light snow
accumulations for locales roughly south and east of I-57 with dry
conditions elsewhere, although could envision some seeder-feeder
type scenario developing across northern Illinois which could
bring some low snow chances farther north than currently depicted
in the forecast grids.
We`ll get stuck in a col/saddle point in the low-level flow into
Thursday afternoon. The surface pressure gradient begins to
tighten as we head into Friday which should allow our temperatures
to begin to climb back above the 40-degree mark with the return of
breezy south to southwesterly winds. The associated upper vort/jet
streak pass overhead then on Saturday. The main corridor of large
scale forcing looks to remain just north of the region with a
somewhat less amplified look to things than earlier runs today.
Moisture is sufficient, however, to warrant continued low PoPs
with thermal profiles quickly transitioning to one favoring rain
as surface temperatures warm to either side of 40 degrees or so.
The active upper pattern will persist into next week although the
belt of faster flow is presently forecast to remain immediately to
For the 00Z TAFs...
547 PM CST
Minor concerns include:
* MVFR CIGs likely developing on Tuesday over Chicago area
* East winds of 10+ kt Tuesday afternoon-evening, gusts possible
Another mostly quiet aviation period is in store. The only
forecast items of note are the potential for MVFR CIGs moving
west and southwest off the lake on Tuesday and the east winds.
High pressure will provide light winds into Tuesday morning, with
high VFR CIGS streaming overhead. Very marginal lake effect
parameters are then expected to result in BKN 2000-3500 ft CIGs
developing over Lake Michigan and then pushing inland as easterly
winds increase. GYY being on the south tip of the lake would
probably have temporary CIGs into early afternoon, with a medium
confidence forecast of prevailing MVFR CIGs by mid afternoon for
ORD, MDW, and DPA. RFD may see MVFR CIGs after the current TAF
period. Winds will pick up out of the east as the pressure
gradient strengthens Tuesday PM, with possibility of sporadic
gusts in the mid to high teens kts at ORD and MDW.
LM...Small Craft Advisory...Gary to Michigan City IN until midnight Tuesday.
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Area Forecast Discussion
National Weather Service Riverton WY
353 PM MST Mon Dec 14 2020
.SHORT TERM...(This evening through Tuesday night)
Issued at 115 PM MST Mon Dec 14 2020
Elongated slightly positively-tilted trough is currently moving
through the forecast area. Moderate snow fell overnight across
parts of western WY; light snow showers will continue through
midnight with little to no accumulation. As the trough moves
eastward, snow showers have been occurring east of the divide and
will also continue through about midnight, with areas around
Casper Mountain seeing light showers through sunrise. Weak upslope
is possibly this evening along the SE Absarokas and Lander
Foothills as moist NNE flow strengthens somewhat. Southern Johnson
county into Natrona county and over to east-central Fremont
county will also see snow through midnight or so with about an
inch accumulation. Another area to watch out for tonight according
to the HRRR is Rock Springs-Green River area along the western
edge of the northerly flow where weak convergence zones like to
set up. Some clearing is already occurring across NW and western
WY as the system progresses. Likely to see some very cold
temperatures again overnight with the fresh snow and partly cloudy
to mostly clear skies. Tuesday morning as mentioned will see the
storm mostly cleared of the forecast area with moist northwest
flow poised to push into NW WY as weak high pressure builds over
the Great Basin. Thus, clouds will increase over NW WY by noon,
with snow showers starting over the Tetons and SW YNP by mid
afternoon. The snow should intensify by early evening from YNP
south to the northern Salts/Wyoming Range with snow falling in
Jackson and other western valleys. The snowfall rate does not
appear to be very strong through, with the mountains perhaps
picking up another 2-4 inches by Wednesday morning. The surface
pressure gradient will keep breezy to windy flow with gusts over
25 mph from southern into central WY Tuesday afternoon and
evening; the breezy flow is likely through the overnight hours as
well. Have increased the western mountain winds with the NW flow
pattern since models tend to weaken them as compared with the
Summit and Mt Coffin observations.
.LONG TERM...(Wednesday through Monday)
Issued at 1234 PM MST Mon Dec 14 2020
Model agreement has northwest flow set up across the entire area
allowing for upslope snow showers to occur across the western
mountains through Thursday. As the next trough approaches, brief
southwest flow ahead of the trough is possible for Thursday morning.
Snow looks to start around early Thursday morning for the western
mountains and slowly spread west throughout the day Thursday and
into Friday. The bulk of the snow will remain in the west but some
accumulating snow looks possible East of the Divide through late
Friday morning. Will start to begin looking at snow totals for this
event in the next coming shifts. Winds will increase across the Wind
Corridor with this system especially from Jeffrey City to Casper.
Northwest flow regime sticks around after the departing low; keeping
snow in the mountains through the weekend. Brief zonal flow sets up
late Sunday night ahead of the next potential mountain snow for
.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 348 PM MST Mon Dec 14 2020
East of the Divide...KCOD/KCPR/KLND/KRIW/KWRL Terminals
Snow will continue to impact KRIW and KLND through 07Z Tuesday which
could cause some IFR/MVFR visibilities at those two sites. KCPR will
begin to see snow after 04Z which could create some IFR/MVFR
visibilities at that terminal through 09Z. KCOD and KWRL will stay
out of the snow but winds will begin to impact KCOD and KCPR after
10Z where winds will gust to 25 knots. Mountaintop obscuration may
be possible with this most recent snow event.
West of the Divide...KBPI/KJAC/KPNA/KRKS Terminals
Snow will continue to taper off from west to east for the first part
of the TAF period. Isolated snow showers could develop over KRKS
after 02Z and cause IFR/MVFR visibilities before diminishing after
07Z. Winds will pick up at KRKS after 08Z with wind gusts up to 30
knots by the end of the TAF period. KPNA will also see increased
winds but will decrease around 03Z and become light and variable
along with KBPI and KJAC. Mountaintop obscuration may be possible.
Please see the Aviation Weather Center and/or CWSU ZDV and ZLC for
the latest information on icing and turbulence forecasts.
Issued AT 130 PM MST Mon Dec 14 2020
Storm system currently moving through western and central Wyoming
today, with some clearing already occurring across western WY.
Isolated to scattered snow showers will continue through midnight
east of the divide and into SW WY. Lingering snow is likely around
Casper Mountain through sunrise Tuesday. Tuesday morning will
again see temps below zero in the western valleys. Due to moist
northwest flow, light snow showers will return to northwest WY
Tuesday afternoon and spread across western WY Tuesday night,
bringing another 2-4 inches to the mountains and up to an inch in
the western valleys. Tuesday will also see west-southwest wind
increase from SW into central WY with gusts over 25-30 mph. Breezy
flow will continue Tuesday night. Weak high pressure moves over
the area Wednesday, with a strong storm on tap for Thursday
afternoon into Friday. Smoke dispersal through the week will only
be poor to fair with low mixing heights, though a bit better in
areas where the winds increase.