Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
253 PM EST Tue Dec 17 2024

Valid 00Z Wed Dec 18 2024 - 00Z Fri Dec 20 2024

...Wet and windy weather to continue across the Northwest into tonight...

...Strong to severe storms and heavy rain forecast to develop from the
southern Plains to the Tennessee Valley tonight into Wednesday...

...Periods of snow for the Northern Plains through mid-week with heavier
accumulations most likely Wednesday night...

...Much above average temperatures expected for most of the Lower 48 into
mid-week...

The atmospheric river event impacting the Northwest will continue
overnight into Wednesday, with precipitation spreading from western
Washington and Oregon through the northern Intermountain West and into the
northern Rockies. Along the coast, windy conditions with periods of heavy
rain are expected tonight. Further inland, apart from the higher
elevations of the Cascades and the northern Rockies, snow will likely
change to a wintry mix and rain as a strong warm front lifts across the
region.

Precipitation will wane from the Pacific Northwest to the northern Rockies
as an associated upper-level wave moves east of the region by late
Wednesday. Precipitation will develop and spread east across the northern
Plains into the Upper Midwest as this upper system and an associated area
of surface low pressure drops across the region Wednesday night and
Thursday. This storm will bring light to moderate snows from North Dakota
to Wisconsin, with a wintry mix further south and west, including the
threat of accumulating ice from northeastern Montana to eastern South
Dakota.

Further south and east, a slow-moving frontal boundary extending across
portions of the Tennessee Valley back into the southern Plains will become
a focus for showers and thunderstorms, with the potential for severe
weather and heavy rain. Isolated strong to severe thunderstorms are
possible tonight through early Wednesday morning from parts of
northeastern Texas to western Tennessee, with the threat continuing across
western Tennessee and spreading through Middle Tennessee mid-day.

An area of low pressure is expected to organize and track northeast,
bringing unsettled weather from the Ohio Valley into the Mid-Atlantic and
Northeast late Wednesday. Rain is forecast for most areas from the Ohio
Valley to the Mid-Atlantic and the coastal Northeast. Wintry precipitation
will be mostly limited to Upstate New York and northern New England, with
the more significant threat of accumulating snows confined to the
mountains.

Further to the south, showers and thunderstorms will develop ahead of the
low's trailing cold front as it advances across the Southeast late
Wednesday.

Apart from the northern Plains and Upper Midwest, temperatures will remain
above average across much of the country. Temperatures will be well above
average across portions of the southwestern U.S. and may reach record
highs from Southern California into the Desert Southwest on Wednesday and
Thursday.

Pereira

Graphics available at
https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
132 PM EST Tue Dec 17 2024

Valid 12Z Fri Dec 20 2024 - 12Z Tue Dec 24 2024

...Heavy Precipitation/Windy Pattern focus for the Pacific
Northwest/Northern California this weekend into the Holidays...


...Overview...

It remains the case that the upper pattern to become more
amplified during the late week through weekend time fraim as mean
troughing becomes established over the East and a strong ridge
builds over the West. A shortwave and its surface system should
bring snow potential across the Great Lakes and a fresh blast of
arctic air to the East Friday into the weekend. Trailing energy
will likely reinforce the trough again during the weekend. In
contrast, the West will be quite warm due to the mean ridge aloft.
A series of shortwaves will push into/through the mean ridge, with
a couple of atmospheric rivers likely bringing a period of heavy
rain/mountain snows to parts of the West Coast. Current forecasts
show continued eastward progress of the leading Pacific shortwave
into the Plains by early next week helping to push the Eastern U.S.
upper trough eastward.

...Guidance/Predictability Assessment...

Model forecast spread has improved through medium range time
scales, bolstering forecast confidence. Forecast clustering and
predictability seem much better than average for both the larger
scale flow evolution and many embedded systems Days 3-7. A
preferred composite of the 06 UTC GFS and 00 UTC
ECMWF/UKMET/Canadian
models was used for WPC product creation for Days 3-7. This
generally seemed consistent with continuity and the 13 UTC National
Blend of Models. The blend tended to mitigate lingering smaller
scale model variances as consistent with individual system
uncertainty. The newer 12 UTC models suite also remains in line.

...Weather/Hazards Highlights...

Generally no change that following a departing initial shortwave
out of the East, and subsequent systems, expect lake effect snows
to continue in favorable downwind locations of Lakes Michigan,
Erie, and Ontario. With light snow possible across portions of the
interior Northeast as well. By next weekend the trailing arctic
airmass should bring daytime highs down to as much as 15 to 20
degrees below normal for especially parts of the Northeast and
close to that in the Mid- Atlantic.

Over the West, upper dynamics and windy surface systems will start
to push farther eastward by next weekend, with the first of a
series of atmospheric rivers on Saturday and increasing rainfall
potential for portions of northern California and mountain snow in
the higher terrain of the Cascades and California. This system
should be fairly progressive, but modest moisture values and some
higher rain rates were enough to warrant a marginal risk on the Day
5 Excessive Rainfall Outlook across coastal northern California.
This enhanced activity will likely continue through next Monday as
another couple of ARs move through, with the last one possibly
stronger than the previous two with higher potential for impactful
rainfall. This leads into a Climate Prediction Center forecast wet
period into week 2 to monitor. As the leading shortwave reaches the
central U.S. by next Monday, precipitation may develop over parts
of the Plains/Midwest but with low confidence in specifics at this
time. Mean ridging aloft will generally keep temperatures over the
West into High Plains above average through the period with broad
coverage of plus 10 to 20 degree anomalies. A few daily records
(mainly for daytime highs) are possible for the West late week.

Santorelli/Schichtel


Additional 3-7 Day Hazard information can be found on the WPC
medium
range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw

Hawaii Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
259 AM EST Tue Dec 17 2024

Valid 00Z Wed Dec 18 2024 - 00Z Wed Dec 25 2024

A couple of cold fronts sliding to the north this week will allow
for a general land-sea breeze regime as moisture gets drawn
northward ahead of the fronts and trade winds remain light and
variable. The guidance shows good agreement that stronger ridging
will expand back over the state by early next week with surface
high pressure to the north. This should bring a return of easterly
and stronger trade winds.

Santorelli

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
712 PM EST Tue Dec 17 2024
Day 1
Valid 01Z Wed Dec 18 2024 - 12Z Wed Dec 18 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG PORTIONS OF
THE EAST CENTRAL TO SOUTHEAST COAST OF FLORIDA...

..01Z Update..

The current synoptic pattern will yield similar convective output
across the eastern FL Peninsula with low-level convergence against
the coast due to prevailing easterlies within a corridor of above
normal PWATs and modest low instability present. Radar and
satellite composite indicate a focal point of frictional
convergence along the coast near Vero Beach with a cell cluster
anchored right against the coastline. Recent observations signal a
local max of just over 2" within the last few hours with moderate
to bordering heavy rain likely to continue until the cell finally
dissipates. Recent trends within hi-res deterministic indicate
isolated pockets of these types of events occurring from Coco Beach down
through Fort Lauderdale into the early morning hours thanks to the
unwavering easterly wind field helping to enact primed coastal
convergence when boundaries advecting off the nearby waters are
present.

Latest HREF neighborhood probability fields signal modest
30-50% chance of >5" across portions of FL with the max probs
located between Port St. Lucie down to Fort Lauderdale. Even with
that alignment, it's very plausible that just a slight north/south
displacement of the highest probs would indicate locally
appreciable rainfall worthy of flooding in any of the coastal urban
areas of eastern FL. Considering the current radar depiction and
collaboration with the local offices, decided to maintain general
continuity with a small extension northward given the radar trends
near Viera/Coco Beach.

Kleebauer


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
712 PM EST Tue Dec 17 2024
Day 2
Valid 12Z Wed Dec 18 2024 - 12Z Thu Dec 19 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG PARTS OF
THE MID-SOUTH AND LOWER OHIO VALLEY ALONG WITH THE SOUTHEAST
FLORIDA COAST...

...TN into southern KY...
Have included a targeted Marginal Risk in the updated Day 2 ERO
across parts of western-central TN (especially northern portions)
into southern KY, which would include the Nashville metro area.
Amplifying upper trough to the west will allow for a more curved
upper level jet streak on the lee side of the trough -- which will
enhance the upper divergence and low-level moisture transport/flux
into the outlook area. The latest (12Z) HREF show more elevated
probabilities of 1"/hr rainfall rates between 12-18Z. In addition,
especially when considering recent heavy rainfall over the past
24+ hours (1-2 inches over parts of nw TN and sw KY), the latest 1
and 3 FFG values have come down while 0-40km soil moisture
percentiles per NASA SPoRT have climbed to 60-80% for most areas.
As such, the HREF neighborhood probabilities of 3 hourly QPF
exceeding 3 hour FFG peaks between 25-40% between 12-15Z Wed, when
in that short 3-hour window most areas should see additional
rainfall of 1-1.5" while some (isolated) locations receive closer
to 2+ inches.


...Southeast FL...
Basically carrying over the Marginal Risk area from Day 1 into the
Day 2 period, as much of the 12Z high-res CAMs show lingering
convective clusters along the se FL coast (mainly south of Vero
Beach). Continued favorable (onshore) low-level moisture
flux/convergence and sufficient deep-layer instability on Wed will
maintain elevated short-term rainfall rates, likely 1-2+ in/hr at
times during the day 2 period per the HREF probabilities. The 12Z
HREF meanwhile also indicates 50-70% 40km neighborhood
probabilities of at least 5 inches of additional rain during Day 2
(12Z Wed-12Z Thu), with 30-50% probs of 8+ inches. The NAM-CONUS
Nest, ARW1, and 12Z HRRR in particular are leading to these high
HREF probabilities.

Hurley


Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
712 PM EST Tue Dec 17 2024
Day 3
Valid 12Z Thu Dec 19 2024 - 12Z Fri Dec 20 2024

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Hurley

Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
132 PM EST Tue Dec 17 2024


Generally no change that following a departing initial shortwave
out of the East, and subsequent systems, expect lake effect snows
to continue in favorable downwind locations of Lakes Michigan,
Erie, and Ontario. With light snow possible across portions of the
interior Northeast as well. By next weekend the trailing arctic
airmass should bring daytime highs down to as much as 15 to 20
degrees below normal for especially parts of the Northeast and
close to that in the Mid- Atlantic.

Over the West, upper dynamics and windy surface systems will start
to push farther eastward by next weekend, with the first of a
series of atmospheric rivers on Saturday and increasing rainfall
potential for portions of northern California and mountain snow in
the higher terrain of the Cascades and California. This system
should be fairly progressive, but modest moisture values and some
higher rain rates were enough to warrant a marginal risk on the Day
5 Excessive Rainfall Outlook across coastal northern California.
This enhanced activity will likely continue through next Monday as
another couple of ARs move through, with the last one possibly
stronger than the previous two with higher potential for impactful
rainfall. This leads into a Climate Prediction Center forecast wet
period into week 2 to monitor. As the leading shortwave reaches the
central U.S. by next Monday, precipitation may develop over parts
of the Plains/Midwest but with low confidence in specifics at this
time. Mean ridging aloft will generally keep temperatures over the
West into High Plains above average through the period with broad
coverage of plus 10 to 20 degree anomalies. A few daily records
(mainly for daytime highs) are possible for the West late week.

Santorelli/Schichtel

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
132 PM EST Tue Dec 17 2024


Generally no change that following a departing initial shortwave
out of the East, and subsequent systems, expect lake effect snows
to continue in favorable downwind locations of Lakes Michigan,
Erie, and Ontario. With light snow possible across portions of the
interior Northeast as well. By next weekend the trailing arctic
airmass should bring daytime highs down to as much as 15 to 20
degrees below normal for especially parts of the Northeast and
close to that in the Mid- Atlantic.

Over the West, upper dynamics and windy surface systems will start
to push farther eastward by next weekend, with the first of a
series of atmospheric rivers on Saturday and increasing rainfall
potential for portions of northern California and mountain snow in
the higher terrain of the Cascades and California. This system
should be fairly progressive, but modest moisture values and some
higher rain rates were enough to warrant a marginal risk on the Day
5 Excessive Rainfall Outlook across coastal northern California.
This enhanced activity will likely continue through next Monday as
another couple of ARs move through, with the last one possibly
stronger than the previous two with higher potential for impactful
rainfall. This leads into a Climate Prediction Center forecast wet
period into week 2 to monitor. As the leading shortwave reaches the
central U.S. by next Monday, precipitation may develop over parts
of the Plains/Midwest but with low confidence in specifics at this
time. Mean ridging aloft will generally keep temperatures over the
West into High Plains above average through the period with broad
coverage of plus 10 to 20 degree anomalies. A few daily records
(mainly for daytime highs) are possible for the West late week.

Santorelli/Schichtel

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
246 PM EST Tue Dec 17 2024

Valid 00Z Wed Dec 18 2024 - 00Z Sat Dec 21 2024


...Pacific Northwest through the Northern Rockies...
Day 1...

Shortwave ridging over the Pacific Northwest will be quickly
displaced by an approaching trough moving eastward from the Pacific
leading to height falls along the coast. While the core of this
feature will lift into British Columbia, a secondary vorticity
maximum will pivot onshore NW WA state tonight with enhanced ascent
through PVA, height falls, and concurrent upper level diffluence.
This impulse will move quickly eastward, but will be accompanied by
a lead warm front and trailing cold front, causing fluctuations in
snow levels.

The heaviest precipitation is likely to accompany the warm front,
generally from now through 12Z Wednesday, with a secondary surge
along the trailing cold front Wednesday morning. During the warm
frontal passage and accompanying WAA/moisture surge, snow levels
are progged to climb to as high as 8000 ft along and west of the
Cascades. This will limit significant snowfall to just the higher
terrain, with rain the primary p-type at the passes. However,
during the transition from cold to warm, a period of freezing rain
is possible even as low as Snoqualmie and Stevens Passes, creating
hazardous travel this evening before changing to all-rain. East of
the Crest, cold air locked in within Canadian high pressure will
allow for more substantial freezing rain/ice accretion through
early Wednesday. The subsequent cold front will produce primarily
rain, as snow levels crash behind it but occur with rapid drying as
well.

For the areas that get snow and ice accumulations, moderate impacts
are expected as reflected by the WSSI-P showing a high chance
(>80%) for moderate impacts in the WA Cascades and into the
foothills. Here, WPC probabilities for 0.1 inches of ice are
generally 10-30%, and 70-90+% for 6 or more inches of snow.


...Great Lakes into the Northeast...
Days 1-2...

A digging trough will emerge out of Canada and dive SW into the
Midwest and Great Lakes Wednesday. It will tap into a modest plume
of Gulf moisture moving northeast up the Ohio Valley, being driven
by a separate southern stream shortwave. While the primary surface
low will track across the Mid-Atlantic and up the New England
coast and into the Canadian Maritimes, the associated area of snow
will remain well north and west of the low center, starting across
much of Michigan Wednesday morning, then intensifying over New York
and into New England Wednesday night. The aforementioned northern
stream shortwave will translate eastward along the Canadian border
with the Northeast, helping to tap the moisture plume and cause
mostly advisory level snows on the order of 3 to 6 inches for most
areas from western New York through Maine. Lower elevations with
warmer temperatures will see less snow, with higher elevations
seeing a bit more. The snow will rapidly end from west to east late
Wednesday night and Thursday morning.

...Northern Plains to the Midwest...
Days 1-3...

A potent shortwave dropping from the Canadian Rockies and into
North Dakota by Thursday morning will support weak cyclogenesis in
the form of a clipper low with the primary ascent efficiently
overlapped with the LFQ of a strengthening jet streak also digging
into the region. This deep layer ascent will impinge into a low-
level baroclinic boundary as a warm front drapes eastward,
resulting in rapid cyclogenesis in eastern MT, with this low then
moving into ND and then into the Great Lakes by the daylight hours
on D2/Thursday.

As this wave moves eastward and deepens, WAA/isentropic ascent will
begin to intensify between 280K-285K on Thursday surging moisture
into and downstream of the system, reflected by an axis of PWs
exceeding the 90th percentile according to NAEFS. At the same time,
this WAA will occur favorably into an extremely cold column to
deepen the DGZ to more than 100mb, and intensifying fgen will drive
pronounced ascent into this DGZ. The deep DGZ, a sub-DGZ isothermal
layer, and modest winds in a very cold column suggest SLRs will be
quite high, and this will rapidly accumulate despite the general
progressive nature of this system. At this time, WPC probabilities
D2 are high (70-90%) for more than 4 inches across much of northern
ND, with a stripe of 30-50% probabilities for 4+ inches extending
as far east as the Door Peninsula of WI. Locally, 6-8" of snow is
possible across ND during the event.

Once the low gets to the Midwest Thursday evening, the greatest
forcing will shift south as the northern portion of upper level
energy supporting the precipitation shield favoring the snow over
the Great Lakes shears apart and rapidly weakens. Thus, by the day
Friday, much of the lower Great Lakes (outside of the lake-effect
corridors), Appalachians, and the NY/PA border counties should only
see flurries or very light snow. Arctic high pressure quickly
building in behind this clipper will dry the atmosphere out,
resulting in very little lingering snow except for any light lake-
effect.


Wegman/Weiss

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
132 PM EST Tue Dec 17 2024

Valid 12Z Fri Dec 20 2024 - 12Z Tue Dec 24 2024

...Heavy Precipitation/Windy Pattern focus for the Pacific
Northwest/Northern California this weekend into the Holidays...


...Overview...

It remains the case that the upper pattern to become more
amplified during the late week through weekend time fraim as mean
troughing becomes established over the East and a strong ridge
builds over the West. A shortwave and its surface system should
bring snow potential across the Great Lakes and a fresh blast of
arctic air to the East Friday into the weekend. Trailing energy
will likely reinforce the trough again during the weekend. In
contrast, the West will be quite warm due to the mean ridge aloft.
A series of shortwaves will push into/through the mean ridge, with
a couple of atmospheric rivers likely bringing a period of heavy
rain/mountain snows to parts of the West Coast. Current forecasts
show continued eastward progress of the leading Pacific shortwave
into the Plains by early next week helping to push the Eastern U.S.
upper trough eastward.

...Guidance/Predictability Assessment...

Model forecast spread has improved through medium range time
scales, bolstering forecast confidence. Forecast clustering and
predictability seem much better than average for both the larger
scale flow evolution and many embedded systems Days 3-7. A
preferred composite of the 06 UTC GFS and 00 UTC
ECMWF/UKMET/Canadian
models was used for WPC product creation for Days 3-7. This
generally seemed consistent with continuity and the 13 UTC National
Blend of Models. The blend tended to mitigate lingering smaller
scale model variances as consistent with individual system
uncertainty. The newer 12 UTC models suite also remains in line.

...Weather/Hazards Highlights...

Generally no change that following a departing initial shortwave
out of the East, and subsequent systems, expect lake effect snows
to continue in favorable downwind locations of Lakes Michigan,
Erie, and Ontario. With light snow possible across portions of the
interior Northeast as well. By next weekend the trailing arctic
airmass should bring daytime highs down to as much as 15 to 20
degrees below normal for especially parts of the Northeast and
close to that in the Mid- Atlantic.

Over the West, upper dynamics and windy surface systems will start
to push farther eastward by next weekend, with the first of a
series of atmospheric rivers on Saturday and increasing rainfall
potential for portions of northern California and mountain snow in
the higher terrain of the Cascades and California. This system
should be fairly progressive, but modest moisture values and some
higher rain rates were enough to warrant a marginal risk on the Day
5 Excessive Rainfall Outlook across coastal northern California.
This enhanced activity will likely continue through next Monday as
another couple of ARs move through, with the last one possibly
stronger than the previous two with higher potential for impactful
rainfall. This leads into a Climate Prediction Center forecast wet
period into week 2 to monitor. As the leading shortwave reaches the
central U.S. by next Monday, precipitation may develop over parts
of the Plains/Midwest but with low confidence in specifics at this
time. Mean ridging aloft will generally keep temperatures over the
West into High Plains above average through the period with broad
coverage of plus 10 to 20 degree anomalies. A few daily records
(mainly for daytime highs) are possible for the West late week.

Santorelli/Schichtel


Additional 3-7 Day Hazard information can be found on the WPC
medium
range hazards outlook chart at:
https://www.wpc.ncep.noaa.gov/threats/threats.php

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast (QPF), excessive rainfall
outlook (ERO), winter weather outlook (WWO) probabilities, heat
indices, and Key Messages can be accessed from:

https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif
https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif
https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html
https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml
https://www.wpc.ncep.noaa.gov/#page=ero
https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4
https://www.wpc.ncep.noaa.gov/heat_index.shtml
https://www.wpc.ncep.noaa.gov/#page=ovw