Forecast Discussions mentioning any of
"HRRR" "RAP" "RUC13" "RUC" "RR" received at GSD on 12/18/24
Area Forecast Discussion
National Weather Service Columbia SC
716 PM EST Tue Dec 17 2024
.SYNOPSIS...
Warm and dry conditions continue this afternoon, courtesy of
departing upper ridging. A cold front approaches the region on
Wednesday with a chance for showers and isolated thunderstorms
during the late afternoon and evening hours. Cold and dry
conditions then dominate for the remainder of the week and into
early next week.
&&
.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
Key Message(s):
- Patchy dense fog possible late tonight.
A deep upper level trough to our west will promote southwest
flow over the Southeast. As the upper trough amplifies,
moisture and cloud cover will increase. There will also be a
short wave which moves across the area early Wednesday morning.
We can`t rule out isolated light showers especially along the
I-95 corridor early Wednesday morning. Guidance is suggesting at
least patchy fog overnight, mainly in the central and southeast
Midlands/CSRA where upper levels remain mainly clear. Overnight
lows will be a little warmer than the previous night, low to
mid 50s.
&&
.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH THURSDAY NIGHT/...
Key Message(s):
- Wednesday is warm with a chance for late day showers and
isolated thunderstorms ahead of an approaching cold front.
- Thursday is cooler and cloudy behind the cold front.
Wednesday and Wednesday Night: The upper flow will be
southwesterly ahead of an approaching trough, which is slated
to move into the forecast area around daybreak Thursday. At the
surface, the attendant cold front will approach during the day
on Wednesday, crossing the forecast area at night. This will
allow for moisture advection out of the GOMEX with PWATs
generally in the 1 to 1.25 inch range. Early morning low clouds
and fog should give way to scattered cumulus or stratocumulus by
midday. In terms of convective potential, model guidance shows
a variety of solutions with the 12Z HRRR producing isolated
development during the late afternoon and early evening while
the 12Z NAMNest shows nothing at all. Modeled SBCAPE in the 500
to 1000 J/kg range and 30 to 40 knots of 0-6km shear would be
sufficient for a few stronger thunderstorms, however the overall
lack of a trigger and cloud cover could limit any development
until the cold front moves through at night. Will continue to
monitor the potential for stronger thunderstorms but for now
agree with the general thunder outlooked on the latest SPC Day 2
SWO. A warm day is expected despite the abundant cloudiness
with forecast highs in the upper 60s to lower 70s. Temperatures
fall into the mid-40s to mid-50s at night.
Thursday and Thursday Night: Shortwave trough moves through the
FA at the start of the day while a second trough approaches
from the north and west. PWATs decrease as a cold and drier air
mass begins to filter into the region. However, latest guidance
shows clouds lingering through the day as moisture becomes
trapped in the lower levels of the atmosphere. Temperatures will
be about 5 to 10 degrees colder than Wednesday with highs in
the upper 50s to upper 60s and overnight lows in the mid-30s to
lower 40s.
&&
.LONG TERM /FRIDAY THROUGH TUESDAY/...
Key Message(s):
- The extended is cold and dry as temperatures decline through
the period.
- Several nights of subfreezing temperatures are expected.
Second trough moves into the eastern CONUS on Friday and remains
in place through the weekend, before ridging builds in near the
end of the long term. At the surface, a weak clipper system will
pass to our north, but it appears it will be too dry for any
precipitation from it. Skies finally clear out on Friday once
this system passes to our east and we come under the influence
of Canadian high pressure. This will keep the region in a cold,
dry air mass but with the anticyclone never passing overhead,
conditions will not be ideal for radiational cooling at night.
Regardless, well below normal temperatures are expected with
Sunday and Monday being the coldest days. Overnight temperatures
are likely to be below freezing each night but the lack of a
surface high passing overhead calls into question how cold
temperatures get. Will need to monitor for a potential Cold
Weather Advisory, especially on Sunday night. Regardless, cold
weather precautions should be taken, especially if you plan to
be away from your home this weekend into early next week.
&&
.AVIATION /00Z WEDNESDAY THROUGH SUNDAY/...
VFR conditions expected through over the next several hours.
Restrictions develop after midnight.
Mid level clouds passing over the terminals expected to diminish
over the next couple hours. Low level moisture has increased
further today which will lead to higher probability for
restrictions developing tonight with more consistency among
model guidance in widespread IFR/LIFR restrictions developing
tonight. Highest probability in LIFR restrictions is at OGB but
there remains moderately high chances at the other terminals.
Forecast soundings are more favorable for fog development
tonight with lighter winds in the low levels. Improvement is
likely by mid to late morning tomorrow with light southwesterly
winds. Some light showers are possible late in the TAF period
but uncertainty as to their coverage so have left them out of
the TAF for now.
EXTENDED AVIATION OUTLOOK...Lingering showers possible into
Wednesday evening. Patchy fog and low ceilings possible
Thursday and Friday morning.
&&
.CAE WATCHES/WARNINGS/ADVISORIES...
SC...None.
GA...None.
&&
$$
Area Forecast Discussion
National Weather Service Indianapolis IN
1044 PM EST Tue Dec 17 2024
.KEY MESSAGES...
- Dry this evening; Rain arrives overnight, especially southern
Central Indiana
- Rain ends Wednesday morning, leading to a cloudy and cooler
afternoon
- Cooler than normal temperatures will be around later in the week
through the weekend
- Light snow possible Thursday night into Friday; low potential for
some light freezing drizzle - this may impact the Friday morning
commute
&&
.FORECAST UPDATE...
Issued at 1044 PM EST Tue Dec 17 2024
Based on radar and nearby ob trends, across east central Illinois,
sped up rain chances to late evening, over the Wabash Valley.
Otherwise, an upper level system, from the upper Midwest to the High
Plains, will continue to push eastward. 290K Isentropic data was
showing decent upglide overnight and 30 millibar and below
condensation pressure deficits will reach the Wabash Valley shortly
before Midnight signaling top down moistening. Still, there is some
concern that rain will have a hard time getting going, especially
north of I-70, with all the dry air in place and the Hi-Res
soundings capture this nicely. Still, confidence is medium in
widespread showers, south of I-70 during the overnight with best
chances over the Metro, after 3 AM. Looking at the Hi-Res soundings,
would not rule out light snow mixing in in any shower, near and
north of Lafayette and Tipton toward 7 AM. That said, temperatures
should hold to near or just above freezing which should keep impacts
limited to wet roads.
&&
.SHORT TERM (This evening through Wednesday)...
Issued at 304 PM EST Tue Dec 17 2024
Surface analysis this afternoon shows high pressure in place over
western KY. This high was providing westerly surface flow to Central
Indiana. The high was also providing subsidence and mostly clear
skies across Central Indiana. Looking aloft, a mainly zonal flow was
in place across Central Indiana, extending west to the high plains.
An upper level disturbance was pushing across the plains providing
cloud cover across KS/NB. IA and MN. This wave has shown signs of
deepening over the past few hours.
Tonight -
The surface high pressure system to the south is expected to slowly
exit east this evening. This will lead to dry weather through the
evening hours as forecast soundings show a dry column continuing.
However the effects of the subsidence will be lessening through the
evening. The approaching upper level wave over the plains is
expected to deepen and advance toward Indiana. This will allow for a
gradual increase in cloud cover through the late evening and into
the overnight hours.
Overnight, forecast soundings trend toward a saturated column after
09Z. The 290K GFS isentropic surface shows isentropic lift arriving
after 09Z with favorable specific humidities. Best forcing will be
found across the southern half of Central Indiana, and we will use
highest pops at that location. Lesser, probably only chance pops
will be found to the north. Forecast soundings are in good
agreement, showing saturation within the low and mid levels by 12Z,
in a warm air advection type pattern. So confidence is high for some
light precipitation across the southern parts of the forecast area,
with less confidence for places like LAF-OKK-MIE.
As the associated surface low passes well to the south across TN and
KY, cold air advection will begin on northerly winds late tonight.
This will allow for temperatures to fall to the lower to middle 30s.
Wednesday -
Lingering rain showers from the departing upper wave and surface
low are expected to be present for Wednesday morning, through the
morning rush hours, before tapering off by midday. Best isentropic
lift departs to the southeast by early Wednesday morning. HRRR also
trends toward a dry forecast in the 15Z-17Z window and forecast
soundings show a dry column as subsidence returns. Lower levels
remain saturated although, showing trapped cold air advection
stratocu. Thus will keep chances for morning rain, especially
across the southeastern parts of the forecast area and trend toward
a dry forecast everywhere by mid day. The expected clouds and cold
air advection in the wake of the low and trough will lead to minimal
rises in temperatures. Look for cooler high temperatures in the
upper 30s and lower 40s.
&&
.LONG TERM (Wednesday night through Tuesday)...
Issued at 304 PM EST Tue Dec 17 2024
Primary focus during the long term period is on a weak clipper
system that pushes through the region Thursday night into Friday,
followed by a brief weekend cool spell.
High pressure will keep the area dry Wednesday night into Thursday.
Thursday night into Friday a weak clipper system will drop southeast
across the area, with solid potential for wintry precipitation.
Light measurable snow appears possible, with anywhere from a few
tenths to perhaps an inch or two possible, assuming deep enough
saturation.
Some forecast soundings show an airmass that is fairly dry in the
mid and upper levels, including well into the dendritic growth zone,
though the most recent runs have improved slightly in this respect.
That said, this does lend to at least low concern that shallow
moisture could lead to freezing drizzle or a mix of snow and
freezing drizzle, which would be poorly timed with respect to the
Friday morning commute. This will require monitoring over the next
few days. Regardless, light snow and/or freezing drizzle
accumulation will be a concern late Thursday night into Friday
morning.
The GFS has consistently shown a second weak wave on Saturday that
could produce a few additional light snowflakes, but it appears to
be a bit of an outlier for the time being.
The weekend will be cool, with highs a few degrees either side of
freezing and lows a few degrees either side of 20 degrees, but this
will be brief, with heights rising early next week but another weak
wave necessitating low precipitation chances Monday night into
Tuesday. Temperatures will moderate a good bit, but may be cool
enough overnight for a mix or a few snowflakes.
&&
.AVIATION (00Z TAF Issuance)...
Issued at 603 PM EST Tue Dec 17 2024
Impacts:
- MVFR through at least 08z
- MVFR with rain showers possible after 09Z, continuing through
14z-15z
Discussion:
VFR conditions will continue through the early overnight under the
influence of a ridge of surface high pressure. There is only
expected to be an increase in cirrus and AC ahead of a Missouri
Valley to central Plains upper trough
Isentropic lift and the approaching trough combined with moisture
building down in the column will then lead to some MVFR ceilings and
rain showers. In addition, some snow could mix in at KLAF 12z-15z
before the precipitation ends.
Cold air advection in the wake of the departing system will lead to
more MVFR ceilings for a time Wednesday afternoon.
Winds will be light and variable to calm tonight and north and
northwest to around 10 knots Wednesday.
&&
.IND WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
UPDATE...MK
SHORT TERM...Puma
LONG TERM...Nield
AVIATION...MK
Area Forecast Discussion
National Weather Service Twin Cities/Chanhassen MN
549 PM CST Tue Dec 17 2024
.KEY MESSAGES...
- Light snow continues to spread across the area from west to
east this afternoon & evening. The heaviest amounts of up to
2" are expected across southern Minnesota.
- A swath of 3-6" of snow continues to look likely Thursday,
mainly along and north of Interstate 94.
&&
.DISCUSSION...
Issued at 356 PM CST Tue Dec 17 2024
Tonight... Afternoon radar, surface observation stations, and
MnDOT cameras all show widespread snow sliding across the
MN/IA border, and lighter snow extending all the way towards
northern MN. Totals are still expected to be around 1 to 2
inches across southern MN, and generally lighter totals up to an
inch elsewhere. Visibilities will be reduced during snowfall
and roads could become covered with snow, especially for areas
further south. Light snow is expected to taper off late this
evening.
Wednesday through Thursday... Wednesday will feel cooler, as
breezy northwesterly winds increase and usher in colder air on
the backside of our exiting system. The increased CAA won`t
last long though, as high pressure settles in by Wednesday
evening. Both winds and cloud cover will look to diminish,
making for a decent Wednesday evening for anyone going out on
the town.
The forecast takes a quick turn as our anticipated Clipper
system tracks through the Upper Mississippi River Valley,
bringing a relatively broad swath of 3"+ across the US/Canada
border down through the Great Lakes region. We are starting to
get more into the range of hi-res models and unfortunately they
are resulting in more uncertainty than confidence... the 15z
RAP and 18z HRRR push the low track along the I-94 corridor,
keeping the bulk of accumulation over northern MN. If this
solution pans out, a dry slot of air could then push in across
southern and central MN, resulting in two main periods of snow:
during the AM commute, and during the afternoon/early evening.
Of the two, the AM commute is likely when the bulk of
accumulation would occur, with hourly rates approaching 0.5 to
0.75 inches per hour. On the flip side, the global models
continue to favor a track further south, bringing some of the
heavier totals close to the I-94 corridor. With this southern
solution, the heaviest rates would also be favored during the
initial push during the morning... forecast soundings show deep
saturation, with roughly a 4 to 6 kft layer within the DGZ.
To summarize, onset is most likely around or just after midnight
Wednesday into Thursday. There is high confidence in at least a
3 to 5 hour window of snowfall rates up to 0.5 to 0.75
inches/hour. There is low confidence in how long the snowfall
persists into the late morning, with hi-res models favoring a
dry period before light snow moves in again during the afternoon
and global models favoring light snow persisting through the
entire day. Light snow will taper off during the evening.
Friday through Tuesday... Another Canadian high pressure will
slide in behind the Clipper, bringing a return to below
average temperatures heading into the weekend. Lows will drop
into the single digits Friday morning through Sunday morning,
and highs in the teens for Friday and Saturday. Thankfully winds
are relatively tame (5 to 10 mph), keeping wind chills from
dropping as much as they did during our last cold snap. A weak
disturbance could bring a quick 1 to 3 inches Friday night into
Saturday, however this will be a narrow swatch and there is too
much uncertainty to pin down a location as models drastically
change from run to run. The active stretch continues into next
week with ensembles suggesting another potential system some
time Monday/Tuesday. Temperatures will be about trending up, so
if there is precip moving through, precipitation type will be
key to watch again.
&&
.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 535 PM CST Tue Dec 17 2024
Light snow continues across much of the region. The heaviest
areas have solid IFR vsbys, but broad MVFR vsbys are likely to
start the period. Conditions begin to improve overnight as -SN
exits to our east. Ceilings will quickly turn to MVFR and remain
there through mid to late Wednesday morning. Winds start off
light & nearly variable before turning out of the WNW by
sunrise. W/NW winds at 10kts with sporadic gusts up to 20kts is
likely through Wednesday aft. A layer of high-mid level clouds
will begin to move in from the Dakotas by the end of the period
ahead of an approaching system.
KMSP... Light snow will begin at or shortly after TAF start
time. MVFR vsby possible for the first few hours before vsby
returns to VFR. Cigs will start out at VFR before turning to
MVFR overnight. Cigs improve late Wednesday morning with NW
winds gusting up to 20kts possible. Another round of snow awaits
us just beyond the end of the period for MSP.
/OUTLOOK FOR KMSP/
THU...IFR/SN. Chc LIFR w/ SN. Wind SE 5-10 kts bcmg N 15G25 kts.
FRI...Chc MVFR cigs. Wind NNW 5-10kts.
SAT...Chc MVFR. Wind NE 5-10kts.
&&
.MPX WATCHES/WARNINGS/ADVISORIES...
MN...None.
WI...None.
&&
$$
DISCUSSION...BED
AVIATION...BPH
Area Forecast Discussion
National Weather Service Marquette MI
528 PM EST Tue Dec 17 2024
.KEY MESSAGES...
- Lake effect snow picks up and could become moderate at times
tonight into Wednesday night over the west-northwest, then
northwest wind snow belts. Winter Weather Advisories are in
effect over portions of the western and eastern U.P. for this.
- Eyes are on a fast-hitting Clipper system that could bring
some fluffy lake enhanced snowfall over the U.P. late Thursday
into Thursday night.
&&
.SHORT TERM /THROUGH TONIGHT/...
Issued at 216 PM EST Tue Dec 17 2024
Afternoon water vapor imagery and RAP analyses show midlevel
troughing extending from Quebec to the Lower Great Lakes, with
another shortwave moving through the Plains and a Clipper over
Manitoba. Behind the cold front associated with the system to our
east, 850mb temperatures are falling to around -10C already. This
has been enough to kick up some WNW lake effect snow bands -
prominent on radar across the north-central and eastern UP and
somewhat obscured on satellite as high cloud cover associated with
the Plains system streams into the area. Still, at least some spotty
LES can be expected to the west, with occasional surface
observations and glimpses of snow showers on webcams. Some light
accumulations mainly below an inch are possible by 00Z in the
eastern UP, with mainly trace amounts to the west.
Otherwise, WNW winds remain breezy throughout the Keweenaw, gusting
to 20-30mph. Winds should continue to fall back the rest of the day,
however, while temperatures hover in the 20s (west) to lower 30s)
east) under cloudy skies.
West-northwest lake effect snow will ramp up later tonight as the
Clipper shortwave trough moving through the area provides deeper
moisture and modest lift for enhancement. The combination of weak
lift, deeper moisture and CAA with 850 mb temps dropping to -12C to -
13C by 12Z Wed should lead to 1-3 inches of LES accumulation tonight
over the wnw snow belts, greatest over the higher terrain of the
Copper Country, particularly from Ironwood northeast to Painesdale.
LES continues the rest of Wednesday into Wednesday night, with
chances for heavier snowfall rates around/in excess of an inch per
hour. With a potential for several inches of fluffy snow
accumulations tonight into Wednesday night, a Winter Weather
Advisory has been issued for portions of the western and eastern UP.
Expect lows tonight ranging from the upper teens to mid 20s, coolest
over the western interior and warmest south central and east.
&&
.LONG TERM /WEDNESDAY THROUGH MONDAY/...
Issued at 256 PM EST Tue Dec 17 2024
The light to moderate at times lake effect snowfall continues over
the northwest wind snow belts Wednesday as cold air advection
continues to move over the area with marginal cyclonic flow across
the Great Lakes basin as a shortwave low tries to develop over it.
With some enhancement from the developing shortwave Wednesday
morning, expect the strongest snowfall rates to be during the first
half of the day, with the transition to pure lake effect by the
afternoon hours as the developing shortwave moves east and high
pressure ridging builds in from our west. With delta-Ts increasing
to around 20 C by Wednesday morning and increasing throughout the
day, the thermodynamic profile will be more than sufficient to
support lake effect snowfall, with preconditioning from Lake Nipigon
also lending a hand in adding moisture flux to the eastern lake
effect bands too. Overall, expect slick roads and quickly
accumulating snowfall at times; slow down and be sure to take your
time. By the time the lake effect dies out late Wednesday night,
expect to see around 3 to 7 inches of fluffy snow over the eastern
northwest wind snow belts, with some patchy blowing snow
occasionally developing from time to time along the lakeshores as
winds pick up to around 35 mph Wednesday afternoon. As for the
western northwest wind snow belts, the area can expect to see around
3 to 6 inches of fluffy snow, with isolated higher amounts around 8
inches near the Porkies. Therefore, given the snowfall amounts and
impacts to travel conditions late tonight into Wednesday night, a
Winter Weather Advisory has been issued from 1 AM EST tonight to 7
PM EST Wednesday for Gogebic, Ontonagon, and northern Houghton
counties, and from 1 AM EST tonight to 1 AM EST Thursday for Alger,
northern Schoolcraft, and Luce counties.
Our attention then goes to a weakening Clipper system that drops
towards the Chicago area Wednesday night through Thursday. While we
look to receive a quick reprieve from snowfall Thursday morning, the
approaching Clipper looks to help develop inverted secondary
shortwave troughs over both northern Lake Michigan and southern Lake
Superior. While transient high pressure moving from northern Ontario
into northern Quebec will likely keep system snowfall out of our
area too far to the south, with lake enhancement developing off of
Lake Michigan and Lake Superior, we could see some moderate to
occasionally heavy lake enhanced snowfall off of Green Bay in
Menominee and southwestern Delta counties late Thursday through
Thursday evening, and light to occasionally moderate lake enhanced
snowfall over the east to north-northeast wind snow belts off of
Lake Superior late Thursday to Friday morning. While Winter Storm
Warnings are not expected for this event, we may need to issue
Winter Weather Advisories for Menominee and possibly Marquette
counties in the near future for this event as slick roads and
quickly accumulating snowfall at times could be seen. As high
pressure builds in from the Northern Plains Friday, expect the
incoming subsidence to greatly diminish the snowfall across the
area. However, remnant troughing over the Great Lakes basin may be
just strong enough to allow weak lake effect snowfall to continue
over the north wind snow belts into early this weekend as the
thermodynamic profile will be very supportive of lake effect snow
despite the ridging pushing into the area (delta-Ts increasing to
the lower to mid 20s C). The lake effect snow looks to eventually
end this weekend as the high pressure ridging finally fights its way
into the Great Lakes basin.
Expect a general warming trend for Christmas week as warm air from
more tropical areas of the Pacific moves over the Rockies and
downslopes into the Canadian and U.S. Plains. That being said, it
does look like we will hold on to cooler temperatures a little
longer early next week, at least enough to possibly give us another
snowfall event as another Clipper dips into the Northern Plains
before bringing warm frontogenesis over the U.P. around next Monday.
While there is surprisingly a good amount of medium range model
convergence on this Clipper low`s track early next week, uncertainty
on the snow chances early next week still remains fairly high as the
forecast could significantly change that far out (system energy has
not been picked up by radiosondes over the CONUS yet). That being
said, it does seem to improve confidence a little for a `White
Christmas` over the U.P., before we likely see more snow melt
Christmas Day and beyond as surface high temperatures are currently
projected to rise above freezing.
&&
.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 528 PM EST Tue Dec 17 2024
MVFR conditions will dominate for this forecast period at all sites.
Some brief periods of IFR conditions are possible in snow showers
overnight as a cold front passes over the UP, then IFR settles in
during the early morning hours at CMX in lingering lake effect snow
showers.
&&
.MARINE...
Issued at 256 PM EST Tue Dec 17 2024
Northwest winds of 20 to 30 knots over the east half of the lake
this afternoon weaken to 20 knots or less this evening as weak
ridging builds into the area. However, as a weak shortwave builds
over the Great Lakes basin Wednesday before moving east, expect
winds to build back to 20 to 25 knots over the west half of Lake
Superior Wednesday morning, and 20 to 30 knots over the east half of
Lake Superior Wednesday morning and afternoon; some freezing spray
could be seen across the lake Wednesday as colder air drops down
from Canada. The freezing spray ends Wednesday night as winds weaken
to 20 knots or less again due to a high pressure ridge moving
through the Upper Great Lakes. As a Clipper drops down towards the
Chicago area Wednesday night and Thursday, secondary inverted
shortwave troughs over northern Lake Michigan and southern Lake
Superior associated with the Clipper may be strong enough to create
northeast to northerly winds of 20 to 25 knots over western Lake
Superior Thursday night before weakening again by Friday. The weak
winds look to continue through the weekend as high pressure slowly
fights its way into the Great Lakes basin.
&&
.MQT WATCHES/WARNINGS/ADVISORIES...
Upper Michigan...
Winter Weather Advisory from 1 AM EST /midnight CST/ to 7 PM
EST /6 PM CST/ Wednesday for MIZ002-003-009.
Winter Weather Advisory from 1 AM Wednesday to 1 AM EST
Thursday for MIZ006-007-085.
Lake Superior...
None.
Lake Michigan...
None.
&&
$$
SHORT TERM...LC
LONG TERM...TAP
AVIATION...07
MARINE...TAP
Area Forecast Discussion
National Weather Service Morristown TN
1041 PM EST Tue Dec 17 2024
...New UPDATE...
.UPDATE...
Issued at 1020 PM EST Tue Dec 17 2024
Key messages remain in good shape this evening. The main updates
to the forecast have focused on fog and PoPs. For fog, have
introduced an SPS across the forecast area highlighting areas of
fog with localized dense fog and visibilities of 1/4mi or less.
While some hi-resolution guidance suggest the coverage of dense
fog may briefly increase overnight, said guidance also suggest a
light wind developing as early as 4-6am. This would be unfavorable
for increased coverage of dense fog and is the reason why an SPS
was opted for rather than a Dense Fog Advisory at this time. For
precipitation, trimmed back/slowed down the onset of slight
chance/chance precipitation into the overnight as isentropic
ascent has proven insufficient at this point. Have also limited
the relatively higher probabilities to the northern Plateau.
Updated zone forecast has been published.
&&
.SHORT TERM...
(This evening through Wednesday)
Issued at 315 PM EST Tue Dec 17 2024
Key Messages:
1. Areas of fog tonight with poor visibility.
2. Near-record temperatures for overnight high minimums and for
daytime highs on Wednesday.
3. Rain and storms on Wednesday. An isolated damaging wind gust and
brief tornado are possible, though low probability.
Discussion:
Tonight:
Low clouds and fog are expected across the area. There is
moderate to high confidence that 1 mile or less visibilities occur,
as numerous guidance sources show this. There is low confidence on
how widespread it is. Some guidance keeps the lowest visibilities
isolated while other sources show it to be more widespread. The
evening and overnight shift will have to monitor trends and
observations to see if a Dense Fog Advisory will be needed.
Tomorrow:
In the morning, a warm front will be lifting north
through the area. During the afternoon, a cold front will move
through and bring a round of showers and storms across the area.
There continues to be some minor model differences on timing. Most
of the CAMs suggest the line of showers and storms should be moving
onto the northern Cumberland Plateau around 16z. The NAM nest shows
a later arrival of 21Z. Storms then progress east southeast through
the rest of our area through the afternoon.
Rain:
The higher QPF amounts will likely occur west of the I-75 and
I-81 corridor. QPF amounts will generally be around 0.5 inches.
Amounts closer to 0.25 inches are expected east of the I-75 and I-81
corridor.
Temperatures:
Southwest flow ahead of the cold front will allow
temperatures to remain very mild overnight. Near-record high minimum
temperatures are expected with temps in the low to mid 50s.
Tomorrow, southwest flow continues and high temps will be in the low
to upper 60s, which will be near-record breaking across the southern
TN Valley.
Wind:
A 30 to 40kt southwest 850mb jet will be in place through the day on
Wednesday. This will allow for wind gusts from 30 to 40 mph across
the highest elevations of the east TN, southwest VA, and southwest
NC mountains.
Storms:
High Shear Low Cape setup for tomorrow. This means that isolated
damaging wind gusts up to 60 mph along with an brief, isolated,
tornado are possible. The probability of occurrence is low, but
there are enough ingredients in place to warrant mention of an
isolated tornado. The main threat window will be from roughly noon
through 6 PM EST. A few things that stand out. 12Z HRRR shows weak
helicity tracks across our area. NAM soundings indicate 50 to 100
J/kg 0-3km CAPE, low LCL heights, and 0-1km shear of 20 to 30kts.
HREF MUCAPE indicates a few hundred Joules. SPC currently has most
of our area within the marginal risk for severe storms.
&&
.LONG TERM...
(Wednesday night through next Tuesday)
Issued at 315 PM EST Tue Dec 17 2024
Key Messages:
1. Cooler behind the front for the end of the week into the weekend.
2. Light snow chances for Southern Appalachians and SW Virginia
Friday into Saturday.
3. Dry later Saturday and the days leading up to Christmas Eve.
Discussion:
Early Thursday at the start of the period, the cold front and drier
air will be filtering into the region. Showers and isolated
thunderstorms will be ending overnight from west to east. Much of
Thursday into Friday will be dry for many, but a SE diving shortwave
across the Ohio Valley will form a weak frontal system with little
moisture later Friday into Saturday. With cold air in place Saturday
morning, rain may change over to light snow for far northeastern
parts of the forecast area. QPF amounts for this forecast package
look light with totals no more than a tenth of an inch. Which
translates to under an inch of snow for parts of the Southern
Appalachians from the Smokys northward and higher elevations of
Southwest Virginia.
Once the northwest flow enhanced precipitation passes, flow aloft
remains NWly while surface high pressure from middle Canada moves to
the Mid-Atlantic and northeast, where it will really strengthen. We
will be locked under the influence of the high with dry weather
expected later Saturday through the end of the term on Christmas
Eve. Will see if that persists into Christmas Day, but it isn`t part
of the long term just yet. Temperature trends from the cold front on
the front end to the weak frontal system Fri into Sat, will be
downward trending. Sat into Sun look steady, but very gradual
warming possible Monday and Christmas Eve. Still, the temperatures
expected will be below climatological norms for this part of
December.
&&
.AVIATION...
(00Z TAFS)
Issued at 637 PM EST Tue Dec 17 2024
Unsettled weather with poor flight conditions are expected to
continue this TAF cycle. Low-level stratus has started to build
into the region bringing MVFR to IFR cigs. Potential for reduced
visibility with a mist/fog is also present into the morning but
lower confidence exists with how hard vis may drop due to the
IFR/MVFR cigs reducing radiational cooling. A cold front will
bring widespread showers with the potential for embedded
thunderstorms tomorrow afternoon. Southwesterly to westerly winds
will also become gusty as the front progresses through the area.
&&
.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN 56 70 40 55 / 20 90 20 0
Knoxville McGhee Tyson Airport, TN 55 67 39 52 / 20 90 30 0
Oak Ridge, TN 54 65 36 52 / 30 100 20 0
Tri Cities Airport, TN 51 65 36 50 / 10 90 50 0
&&
.MRX WATCHES/WARNINGS/ADVISORIES...
NC...NONE.
TN...NONE.
VA...NONE.
&&
$$
SHORT TERM...
LONG TERM....KS
AVIATION...KRS
Area Forecast Discussion
National Weather Service Pocatello ID
235 PM MST Tue Dec 17 2024
.SHORT TERM...THIS AFTERNOON THROUGH WEDNESDAY NIGHT...
A weak area of high pressure is building over the region this
afternoon in the wake of a trough that brought widespread snow to
the area. Afternoon satellite imagery still shows widespread cloud
cover across much of the Snake Plain and into the eastern highlands
and this will likely continue into the evening and much of the
overnight. Will likely see some additional very light snowfall out
of this across the eastern highlands but generally not expecting
much more than a few additional tenths of accumulation out of this
before the precip looks to shut off later this evening. As the
aforementioned ridge continues to build over the area, we will have
to watch out for some overnight stratus/fog across the upper Snake
Plain, upper Snake highlands and into the Arco Desert and have added
freezing fog into the forecast accordingly. Confidence isn`t the
highest with this but given ample low level moisture in place and
building high pressure, it certainly seems plausible.
After daybreak tomorrow, skies look to slowly clear throughout the
day across the southern half of the region. Across the central
mountains and around the Montana Divide, clouds will likely linger
as the weak ridge already will begin to erode as a weak shortwave
dips into the central and northern portion of the Gem State. This
will bring additional clouds and some light snow to those areas but
the majority of the region should remain dry throughout the day.
Snowfall accumulation where it does fall will remain around an inch,
maybe as high as 2 near Island Park. Valley temperatures should
climb into the lower to mid 40s with mid to upper 30s the norm
across the higher terrain. After this weak Wednesday system, looks
like we`ll see a few dry and relatively mild days. More on that
below. McKaughan
.LONG TERM...THURSDAY THROUGH NEXT TUESDAY...
A ridge of high pressure will bring dry, quiet weather with light
winds for Thursday, Friday, and Saturday, with high temperatures
running about 10 degrees above climatological normals for mid-
December. The ridge will break down starting Saturday night and
surrender to an active/progressive pattern again, with the first
shortwave trough bringing elevation-dependent rain and snow Saturday
night and Sunday, the second Sunday night and Monday, and a third
Tuesday night and Wednesday. Glancing at NBM 50th-percentile
snowfall totals and the ECMWF Extreme Forecast Index (EFI), none of
these systems look particularly strong as far as winds or precip at
this juncture (nor are they particularly long-duration), with the
third (Tuesday night and Wednesday) system perhaps wetter and
windier than the other two. So far temperatures look to hold fairly
steady throughout this period too with no significant cooldowns,
which should keep any snow accumulations in places like the eastern
Magic Valley, Raft River region, Shoshone area, Snake Plain, and
southern highland valleys quite light with rain during daytime
periods. There are hints that this more active/progressive pattern
may continue though beyond next week, so...multiple opportunities
where we will need to watch for slick travel in a few spots like
mountain passes as well as potential to build some mountain
snowpack.
&&
.AVIATION...
Although precipitation has ended, abundant low-level moisture
remains in place and will make for a continued impactful, somewhat
low-confidence forecast for aviation. Low stratus is lingering today
outside of KSUN and KBYI, and a combination of fog and low stratus
is expected to continue (and perhaps expand a bit) tonight. KPIH may
reside right on the southern edge of this activity per NBM and HRRR
guidance, and while impacts are not currently included in the TAF,
we can`t give an "all-clear" with any level of confidence. NBM,
HRRR, and MOS guidance all support IFR conditions at KIDA and at
least MVFR conditions at KDIJ, with timing adjusted slightly in the
18z TAF package. All affected terminals should trend back into VFR
conditions by Wednesday afternoon.
&&
.PIH WATCHES/WARNINGS/ADVISORIES...
None.
&&
$$
Area Forecast Discussion
National Weather Service Seattle WA
730 PM PST Tue Dec 17 2024
.SYNOPSIS...Active weather will remain for the next several days
across western Washington. An atmospheric river will bring heavier
rain, gusty winds, and mountain winter weather into early Wednesday.
Additional systems will move across the area at times into the
beginning of next week.
&&
.SHORT TERM /TONIGHT THROUGH FRIDAY/...Latest radar imagery shows
the back end of warm frontal precip continuing to creep northward,
allowing for activity over the southern half-ish of the CWA to
become more scattered as well as easing rainfall rates as well. The
northern two-thirds of the Olympic Peninsula, the central Sound area
/including the northern half of the Seattle metro area/ and the
northwest interior still on tap for consistent rainfall this evening
and into early tonight. It is worth noting that there is a little
bit of a rain shadow in place over portions of Skagit and Snohomish
counties at the time of this writing /710 PM PST/, however latest
trends suggest the resulting hole in the echoes is starting to fill
in, so any respite from the rainfall in those locations may be
drawing to a close. While only starting to emerge on the outer
periphery of the radar mosaic, the quick-following cold front is
better visible on satellite. This feature will bring another slug of
rainfall to the area as well as wind speeds as advertised in current
headlines for the interior locations within the advisory area.
Coastal locations under the High Wind Warning have been meeting
criteria this evening and that threat will persist as the incoming
cold front moves in overnight.
All of that being said, inherited forecast in on track and will not
need any updates at this time. For additional forecast details, and
there are many, please refer to the Previous Discussion section
below.
18
From Previous Discussion...We`re well into a period of active
weather in the PNW. An occluded low center just west of Haida Gwaii
is steering a weak atmospheric river into western Washington with
PWAT values up to 1.20-1.30" given recent RAP analysis. This feature
is expected to persist into Wednesday morning with up to 2.00-4.00"
inches of rainfall for the Cascades and 3.00-6.00" for the Olympics.
1.00-3.00" is at play for the coast with lower amounts (0.50-2.00")
for the interior lowlands with rain shadowing helping to trim down
those totals. Hydrologic flooding along the Skokomish river is
happening with more information provided below in the hydro section.
Along with rainfall, wintry precipitation is ongoing in the
mountains with snow fall up of 4.00-8.00" expected for the passes
this afternoon. However, a warm front currently along the coast
will spread inland this evening. The PSL Wind Profiler at Forks
shows snow levels near 8,000 ft on the western fringe of the
Olympics. This warm column of air extending towards the Cascades
will act in concert with cold, shallow air at the passes provided
by easterly winds. This set up will bring a period of freezing
rain this evening into tonight up on the mountains. An onshore
switch will bring warmer surface temperatures to these parts,
allowing warmer rain to return. Ice accumulations up to 0.10-0.20"
of an inch is possible for tonight at the passes.
Wind will also be of concern this afternoon into early Wednesday
morning. The sub 990mb surface low just west of Haida Gwaii has
helped to tighten gradients as it interacts with surface high
pressure over the Inland Northwest. As a result, a Wind Advisory
has been issued for all of western Washington lowlands beginning
this evening into early Wednesday morning. HREF guidance indicates
a large swath of the probability for wind gusts exceeding 45 mph
around 70-80%. Gusty winds could blow around unsecured objects. A
High Wind warning is in effect along the coast as HREF
probabilities of gusts exceeding 55 mph are pinging around 70-80%.
Pretty significant. Damaging winds can blow down trees and power
lines with widespread power outages expected for areas in the High
Wind Warning from Cape Elizabeth to La Push, and along the
immediate coastline. Winds should subside into Wednesday morning.
If those impacts aren`t enough, coastal flooding is an issue as an
advisory is in effect for the coast for minor flooding through
this evening. Additional minor flooding in possible tomorrow for
San Juan County and the Northern Interior around high tide.
As mentioned, rainfall and wind is expected to taper off into
Wednesday as a cold front pushes east of the Cascades. However, a
convergence zone may linger across Snohomish and northern King
Counties Wednesday morning for a brief period of heavier rainfall
after the frontal passage. Upper-ridging will allow for a brief
spell of drier weather Wednesday evening. But of course, it won`t
be long lasting as deterministic guidance has a weak front
entering on Thursday with light rain. We`ll rinse and repeat
Thursday night into Friday with a similar pattern. High
temperatures will top out in the 50s with lows generally in the
40s with some upper 30s in the short term forecast.
McMillian
.LONG TERM /SATURDAY THROUGH TUESDAY/...From Previous
Discussion...Ridging builds further across the intermountain west
into Saturday. However, western WA will be on the western fringe of
the upper ridge, allowing for PoPs later in the week. A frontal
system moves towards the Pacific Northwest later Saturday into
Sunday, increasing the risk for lowland rain and higher elevation
mountain snowfall. The active pattern will then continue into next
week as highlighted by the CPC Week-2 hazard outlook for a moderate
risk in heavy precipitation, high winds, and heavy mountain
snowfall.
McMillian
&&
.AVIATION...Southwesterly to west flow aloft as a front moves
through W WA this afternoon. Steady rain, heavy at times, has
begun moving throughout the area terminals, with conditions
varying between MVFR/IFR. Localized spots of LIFR observed at
times with lower visibilities due to heavier rain passing over
terminals. This will continue to be the trend throughout the
evening and into the overnight as rain will likely reduce
visibilities at times between 2-5SM, keeping conditions MVFR/IFR
into Wednesday morning. Rain will quickly cease following the
frontal passage Wednesday morning by around 15Z for the interior.
Conditions look to raise to VFR by Wednesday afternoon.
Very strong winds persist through the coastal terminals (gusts up
to 50 kt) and the Southwest Interior (OLM, gusts up to 25 to 30)
this evening, with winds slow to increase through the remainder of
the area. Southerly winds will increase through the interior
later tonight (by around 06-09Z) with winds near the interior
terminals ranging around 15 to 25 kts and gusts up to 35 to 40
kts. Winds will peak overnight into early Wednesday morning. Gusty
winds should begin to decrease Wednesday afternoon.
KSEA...MVFR conditions at the terminal this evening with rain.
Both ceilings and visibility are impacted, with visibility ranging
from 3-5 SM, potentially lower in the heaviest bands. A bounce
around to MVFR/IFR is possible through Wednesday morning as rain
persists. Southerly winds will increase late tonight into
Wednesday morning, with sustained winds 15 to 25 kts and gusts
possible up to 30 to 40 kts. The highest winds will peak around
05z-09z, with wind speeds gradually decreasing by 12z Wednesday.
Rain will end steadily Wednesday morning after around 15Z, with a
return to VFR conditions by Wednesday afternoon.
Mazurkiewicz/62
&&
.MARINE...A front will continue to move through the area waters
tonight into Wednesday morning, bringing elevated winds and seas.
Southerly winds up to gale force will continue across the coastal
waters and most of the interior waters (with the exception of the
Central Strait of Juan de Fuca, which remains in a Small Craft
Advisory) These southerly winds will transition westerly once the
front passes Wednesday morning, with additional headlines likely
needed after the initial southerly wind direction with a push of
westerlies down the strait Wednesday mid-day.
Active weather will only continue as another system on Thursday
looks to deliver another round of gusty winds over the coastal
waters, with perhaps multiple impactful systems this weekend and
into early next week.
Coastal seas will build this afternoon upwards to around 13 to 16
feet, decreasing into Wednesday but remaining above 10 feet.
Latest guidance seems to be hinting at seas reaching 20 feet early
next week as additional systems impact the area.
Mazurkiewicz/62
&&
.HYDROLOGY...An atmospheric river impacting the region will bring
about 3 to 5 inches of rain to the Olympic Peninsula and portions
of the North Cascades tonight into Wednesday as snow levels rise
to 7000 to 8000 ft. At this time, only the Skokomish River is
expected to rise into flood stage. However, expect other area
rivers along the central and North Cascades to experience rises
and approach Action Stage.
Additional incoming precipitation from wet systems later in the
week and into next week will maintain the threat of river flooding
across western Washington, but models continue to show large
discrepancies and the forecast remains uncertain at this time.
14
&&
.SEW WATCHES/WARNINGS/ADVISORIES...
WA...Wind Advisory until 4 AM PST Wednesday for San Juan County-
Western Skagit County-Western Whatcom County.
Coastal Flood Advisory from 6 AM to noon PST Wednesday for San
Juan County-Western Skagit County-Western Whatcom County.
Wind Advisory until 4 AM PST Wednesday for Bellevue and Vicinity-
Bremerton and Vicinity-East Puget Sound Lowlands-Everett
and Vicinity-Hood Canal Area-Lower Chehalis Valley Area-
Seattle and Vicinity-Southwest Interior-Tacoma Area.
Wind Advisory until 7 AM PST Wednesday for Admiralty Inlet Area.
High Wind Warning until 4 AM PST Wednesday for Central Coast-
North Coast.
Winter Weather Advisory until 4 AM PST Wednesday for West Slopes
North Cascades and Passes-West Slopes North Central
Cascades and Passes-West Slopes South Central Cascades and
Passes.
PZ...Small Craft Advisory until 10 AM PST Wednesday for Grays Harbor
Bar.
Gale Warning until 10 AM PST Wednesday for Admiralty Inlet-
Coastal Waters From Cape Flattery To James Island 10 To 60
Nm-Coastal Waters From Cape Flattery To James Island Out 10
Nm-Coastal Waters From James Island To Point Grenville 10
To 60 Nm-Coastal Waters From James Island To Point
Grenville Out 10 Nm-Coastal Waters From Point Grenville To
Cape Shoalwater 10 To 60 Nm-Coastal Waters From Point
Grenville To Cape Shoalwater Out 10 Nm-East Entrance U.S.
Waters Strait Of Juan De Fuca-West Entrance U.S. Waters
Strait Of Juan De Fuca.
Small Craft Advisory until 10 PM PST this evening for Central
U.S. Waters Strait Of Juan De Fuca.
Gale Warning until 10 AM PST Wednesday for Central U.S. Waters
Strait Of Juan De Fuca.
Gale Warning until 7 AM PST Wednesday for Northern Inland Waters
Including The San Juan Islands-Puget Sound and Hood Canal.
&&
$$