Weather Prediction Center

National Oceanic and Atmospheric Administration

Short Range Forecast Discussion
NWS Weather Prediction Center College Park MD
216 PM EST Sun Dec 08 2024

Valid 00Z Mon Dec 09 2024 - 00Z Wed Dec 11 2024

...Heavy rain threat emerges across the South late Sunday into Tuesday...

...Periods of mixed rain and snow linger across Interior New England
through tonight...

...Unsettled and windy weather spreading across the Pacific Northwest this
weekend will reach into the northern Plains as snow/wintry mix by Monday...

...Well above average temperatures will overspread the central and eastern
U.S. as cold air surges into the western U.S....

..Strong Santa Ana Winds result in Critical Fire Weather day starting
Monday across Southern California...


The last vestiges of cold air are being dispelled out of New England for
the remainder of today into tonight with some lingering snow showers still
possible across far Interior New England. Clearing skies and return flow
ahead of the Pacific System sliding through the Rockies, combining with
warmer/moist air returning out of the Western Gulf ahead of an average
upper-level trough; will result in well above temperatures today across
the Northern Plains and Midwest, shifting to the eastern half of the
country on Monday, and finally east of the Mississippi into Tuesday ahead
of the Pacific cold front. Today, 20-30 degree departures will exist in
the Plains and temper a bit to 15-25 degrees above normal overnight
through Tuesday morning. Increased moisture through the lower Mississippi
River Valley will have some stronger showers and possible thunderstorms
closer to the Gulf of Mexico. Rain-rates may result in a few widely
scattered incidents of flooding across the Delta Region into SW LA today,
shunting south to the Central Gulf Coast Monday into Tuesday. The Weather
Prediction Center has issued a Marginal Risk for today; and upgrades to a
Slight Risk (level 2 of 4) on Tuesday across south-central LA to Mobile
Bay mainly for risks near susceptible urban locations along I-10/12.

The aforementioned Pacific airmass/frontal zone has cross much of the
northern Rockies and has already developed a stronger surface low just
north of the U.S. Canada border. Associated strong winds and snows from
overnight is resulting in dangerous travel conditions across E Montana
with a Blizzard Warning still in affect across far east-central MT until 6
pm MST today. Downstream warm air advection will over-top some remaining
colder air across northern MN into the Western Great Lakes tonight.
Winter Storm Warnings and Advisories are in effect across the norther Red
River Valley into northern MN where 4-7" of snow are expected. Strong
onshore flow off Lake Superior will further enhance local snow totals
across the Arrowhead of MN range with 9-12"+ totals expected.

Behind the cold front a combination of cold air off the Pacific, as well
as from eastern AK, Yukon will continue to filter southward directed
through the Intermountain Region and Great Basin over the next few days.
Below average temperatures will also bleed out in the Plains by late
Monday into Tuesday with a classic 'Blue Norther' cold front driving well
through the Southern Plains into northern Mexico by Tuesday. High
temperatures will be about 5-10 degrees below normal across much of the
Rockies, Southwest and eventually into the Plains.

Perhaps most impactful, is the surge directed through the Great Basin into
the Lower Colorado River Valley will result in well above average high
pressure that should allow for the front to cross the Peninsular Range
Monday. This will bring very strong Santa Ana winds across southern
California on Monday and given dry/low humidity conditions already in
place; Red Flag Warnings have been issued for the eastern Transverse and
all Peninsular Ranges from Monday into mid-week; as well as a Critical
Fire (level 2 of 3) from the Storm Prediction Center. Winds of 35 to 45
mph in the valleys and gusts of 50 to 65 mph in the mountains will help
expand any fires that do develop quickly and may reduce visibility with
blowing smoke and dust adding to travel concerns in the region.

Gallina


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
143 PM EST Sun Dec 8 2024

Valid 12Z Wed Dec 11 2024 - 12Z Sun Dec 15 2024

***Heavy rain threat for the East Coast continuing into
Wednesday***

19Z Update: The guidance continues to agree well on most aspects
of the forecast going into the middle of the week, and therefore a
general model compromise works out well as a starting point in the
forecast process. However, the NBM likely remains too low with QPF
from lake effect snow, so this was blended with some GFS/ECMWF and
then raised about another 40%. Going forward into the end of the
week, similar to yesterday, the guidance continues to differ on
incoming Pacific shortwave energy as the upper ridge across the
West starts to break down. The AI guidance from the ECMWF is in
better agreement with the operational ECMWF compared to yesterday,
and this helps to bolster more confidence in the ECMWF and CMC
solutions for days 5 through 7, since there is overall better
agreement among those models compared to the GFS. Given the degree
of ensemble spread going into Sunday especially, the ensemble means
accounted for a majority of the forecast blend by that time.
However, there is higher confidence that a moderation trend in
temperatures can be expected going into next weekend after a very
cold end to the week across the central and eastern parts of the
country. The previous forecast discussion is appended below for
reference. /Hamrick
-----------------------------

...Overview...

With typical localized adjustments, latest guidance continues to
show the potential for heavy rainfall from the Mid-Atlantic into
especially the Northeast on Wednesday, as a dynamic upper trough
supports rapidly deepening low pressure that tracks from the Mid-
Atlantic northward into Canada. Mean flow across the lower 48 still
looks to trend more zonal behind this system, so the amplified
upper trough over the East should lift out after Thursday while
predictability of system specifics should decrease late week
through the weekend. The pattern will favor at least a couple
episodes of rain and higher elevation snow along parts of the West
Coast, with latest guidance starting to show a stronger eastern
Pacific system around Friday-Saturday which would support
relatively greater precipitation totals in that time frame. Below
normal temperatures will spread across the East late this week,
replacing above normal readings near the East Coast on Wednesday.
Then expect near to above normal readings over most of the country
by next weekend, with warmest anomalies over the Plains and
vicinity.

...Guidance/Predictability Assessment...

Latest guidance continues to refine details near the East Coast
Wednesday into Thursday, with improving consensus as the past two
UKMET runs have finally joined the majority cluster for strongly
deepening Mid-Atlantic into eastern Canada low pressure in response
to approaching vigorous upper trough dynamics. Dynamical/machine
learning (ML) guidance on average shows the surface low depth
reaching at least as low as the 970s mb by early Thursday. There
has been a slight westward nudge since yesterday, a nod to earlier
ML runs that were westward. 12Z ML runs were a little more varied
for longitude.

Upstream features continue to be difficult to resolve. ML models
show a bit more coherence with a Thursday West Coast shortwave
continuing eastward thereafter, approaching the Northeast or
vicinity by next Sunday with varied degrees of surface reflection,
and then a better defined eastern Pacific system by Saturday moving
into the West/High Plains by Sunday. Most dynamical guidance is
showing less of a surface reflection than the ML models (or at
least slower, holding onto stronger eastern U.S. high pressure)
with the leading shortwave. The dynamical and ML models are
gradually showing an improved signal for what could be a fairly
strong eastern Pacific system around Friday-Saturday. ML models are
generally more agreeable with bringing a moderately amplified upper
trough into the West by next Sunday, while dynamical guidance
varies--albeit with the means at least suggesting some troughing.

The updated forecast started with a composite of 12Z/18Z
operational models, and then transitioned toward a blend of the 18Z
GFS, past two ECMWF runs, and 18Z GEFS/12Z ECens-CMCens means as
those solutions provided the most coherent evolution when compared
to each other and ML models.

...Weather/Hazards Highlights...

Expect elongated low pressure to reach the Mid-Atlantic/New York
state region by early Wednesday. Strong upper dynamics becoming
negatively tilted will support deepening/consolidation of this
system into the Northeast by late Wednesday followed by continued
strengthening as it continues north into Canada into early
Thursday. Accompanying this strong development, guidance still
shows a pronounced surge of moisture into the Northeast, with
precipitable water values reaching 3+ standard deviations above
normal for a time, supporting a broad area of heavy rainfall. Snow
cover or wet ground will lead to increased sensitivity over the
Northeast while persistence of model guidance and strengthening
signals from first-guess fields add to the confidence in the
potential for runoff issues. Thus the Day 4 Excessive Rainfall
Outlook maintains a Slight Risk area covering much of New England
while a surrounding Marginal Risk area extends southward to the
southern Mid-Atlantic where some instability could enhance rainfall
totals. The first-guess fields actually suggest potential for some
embedded probabilities at least in the higher half of the Slight
range over New England, so this will merit continued monitoring as
the event enters the short range period. For the Day 4 updated
ERO, both risk areas have been expanded a little westward from last
night's continuity to correspond with latest guidance trends.
Meanwhile, the western edge of the precipitation shield may contain
some snow and the deepening storm may produce a period of brisk to
strong winds in its wake. Also expect the cold air arriving behind
this system to produce another episode of lake effect snow during
Wednesday- Thursday.

The forecast pattern from the Pacific into the West will favor at
least a couple episodes of rain and higher elevation snow over the
Pacific Northwest, possibly extending into parts of California. The
progressive nature of the mean flow continues to temper confidence
in some details, though guidance is starting to cluster around the
idea of a fairly strong eastern Pacific system helping to produce
the highest totals of the period and greatest southward extent of
precipitation around Friday-Saturday. This storm could produce
areas of brisk to strong winds depending on its depth and track.
Some moisture may extend into the Rockies, while leading shortwave
energy/surface feature(s) may produce areas of light precipitation
over parts of the central/east-central U.S.

The amplified pattern over the East will lead to above normal
temperatures near the Atlantic coast on Wednesday while trailing
cold from the Upper Midwest into the South pushes eastward into
late week. Coldest anomalies should be from the Upper Midwest into
Great Lakes/Ohio Valley Wednesday-Thursday, at least 10-20F below
normal. The transition toward flatter mean flow aloft will
lead to a warming trend over the Rockies and Plains late this
week with above normal temperatures (some pockets of plus 10F or
greater anomalies) persisting through the weekend. The East will
trend closer to normal by next weekend.

Rausch/Hamrick


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
258 AM EST Sun Dec 08 2024

Valid 00Z Mon Dec 09 2024 - 00Z Mon Dec 16 2024

Guidance has been consistent and agreeable with the overall low
level pattern for the coming week. An initial surface trough over
the area and a front approaching from the northwest but not quite
reaching the islands will favor light to moderate trades through
Sunday, leading to a mixed focus for showers. Then stronger trades
will prevail from Monday onward as multiple surface highs pass by
to the north, with relatively higher wind speeds tending to be in
the Wednesday-Thursday time frame but possibly extending into
Friday. This pattern will resume a more pronounced
windward/mountain shower focus.

However, guidance continues to vary somewhat for specifics aloft
along with corresponding effects on moisture. Dynamical models
still pull off weak energy aloft to form an upper low to the
southwest/south of the islands near 10N latitude by mid-late week,
but with continued east-west spread. The CMC/CMCens mean hold the
feature farthest east while the ECMWF/ECens mean show more
retrogression. Machine learning (ML) guidance has shown multi-day
oscillations, reflecting an intermediate timing a couple days ago,
hanging on to the eastern side of the spread 24 hours ago, and now
returning to a solution similar to or just a tad slower than the
00Z ECMWF. It is still a close call as to whether the northern
periphery of enhanced moisture reaches the Big Island and vicinity
around late week, with the ECMWF/ECens mean showing more moisture
than the GEFS mean. At the very least, the dynamical and ML
models offer minimal support for the 00Z GFS solution of an open
trough aloft just east of the state by Saturday and persistence of
fairly high PWATs through the weekend. Thus would recommend an
average of the ECMWF/ECens mean and GEFS mean for the upper level
and moisture forecasts.

Rausch

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
301 PM EST Sun Dec 8 2024
Day 1
Valid 16Z Sun Dec 08 2024 - 12Z Mon Dec 09 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
LOWER MISSISSIPPI AND TENNESSEE VALLEYS...

...16Z Update...

No changes were made to the previous Marginal Risk area, while the
threat is certainly on the lower end of the Marginal Risk threat,
previous days' rain in Louisiana and concern about training led to
keeping the threat as is. Meanwhile the guidance is in good
agreement that the heaviest rains will be focused over much of
northern Mississippi, where conditions are much drier, but more
rainfall could lead to isolated flash flooding. The previous
discussion remains valid.

Wegman

...Previous Discussion...

The inherited Marginal Risk area was maintained for this update,
suggesting that any flash flooding should remain localized today,
as rainfall rates will generally remain capped below 1"/hr (with
lacking instability being the primary limiting factor). The
heaviest rains for the period are expected in the middle of the
Marginal Risk area over northern Mississippi, where an interaction
with a front in that area will lead to greater forcing than areas
farther south along the Gulf Coast. Where the front adds additional
forcing, localized convective bursts may be embedded within the
broader rainfall shield.

Hydrologically, most if not all of the soils in the are are at or
below average for moisture. Low/dry creek beds should easily
handle most of the light to briefly moderate rainfall rates. Any
isolated flooding will be in any urban areas, as well as other low-
lying or more flood-prone areas.

Churchill/Wegman


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
301 PM EST Sun Dec 8 2024
Day 2
Valid 12Z Mon Dec 09 2024 - 12Z Tue Dec 10 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE CENTRAL GULF COAST...

...2030Z Update...

In coordination with LIX/Slidell, LA forecast office, the inherited
Slight Risk was expanded south to include the rest of the
Mississippi Delta as well as the towns along Bayous Lafourche and
Terrebonne. The focus for the heaviest rain on Monday is likely to
include the New Orleans Metro and points south. The storms will
track east-northeastward from Louisiana through the western Florida
Panhandle. Since many of these areas are urban and therefore more
prone to flooding, the potential for some convection, even if
instability is limited, is still enough to forecast widely
scattered instances of flash flooding. The surrounding Marginal
Risk was left largely unchanged.

Wegman

...Previous Discussion...

The shortwave trough responsible for heavy rainfall on Sunday over
much of the Lower MS Valley looks to eject northeastward into the
central Appalachians and Mid-Atlantic on Monday, bringing largely
beneficial rainfall (with totals generally well under 1") to much
of the Northeast. Farther south along the central Gulf Coast, a
weaker shortwave within the southern stream may locally enhance
heavy rainfall along a trailing cold front. The inherited Slight
Risk in this area was maintained, mainly from the I-10 & I-12
corridors from Baton Rouge east to Biloxi, where the flooding
potential appears greatest in urban and more flood-prone areas
(given relatively dry antecedent conditions and streamflows normal
to below normal). There is still some question as to how far east
the heavy rainfall threat will extend (as the ECMWF and ECENS have
been persistent outliers suggesting an eastward extension into the
FL Panhandle and surrounding southeast AL and southwest GA), but
the signal for localized totals of 2"+ are maximized near the
Slight Risk (and confined to where instability and forcing are
greatest).

Churchill


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

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
301 PM EST Sun Dec 8 2024
Day 3
Valid 12Z Tue Dec 10 2024 - 12Z Wed Dec 11 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF
THE SOUTHEAST FROM SOUTHEASTERN ALABAMA TO WESTERN NORTH CAROLINA...

...2030Z Update...

The Slight Risk area inherited from the previous forecast was
nudged a bit to the north with this update to now include portions
of northeastern Georgia, western South Carolina and southwestern
North Carolina with this update. Meanwhile the Slight was removed
out of the Florida Panhandle.

A strong upper level trough and attendant cold front will swing
eastward across the Southeast on Tuesday and especially Tuesday
night. The greatest forcing with the front and southerly flow ahead
of it will likely be focused into Georgia and the Carolinas...with
less forcing further south along the Florida Panhandle. While
convection will still be present along the Gulf Coast, it will
become better organized as it moves inland into northern Georgia
and the Carolinas, hence the expansion of the Slight Risk into
those areas. Despite the long stretch of very dry weather into the
Carolinas, 1-3 inches of rain in a short period of time could still
cause flooding issues, particularly in urbanized areas and small
creeks and streams which fill up quickly.

Wegman

...Previous Discussion...

Large scale upper trough will be digging into the central US by
Tuesday, setting the stage for more organized heavy rainfall into
portions of the Southeast. Initially positively tilted, the trough
looks to quickly take on a more neutral tilt over the course of
the course of the day, as an associated 250 mb jet streak
strengthens from ~110 kts to ~170 kts over 24 hours (with our area
of interest for the Slight Risk located near the right-entrance
region). As with prior days, instability once again looks to be the
primary limiting factor, and this is particularly the case farther
north into the Carolinas. While the GFS/GEFS depict 2-4" localized
totals into northern GA and western SC, the ECMWF/ECENS largely
limit this threat (for 2" and 3" exceedance) a bit farther south
into central GA, southeast AL, and the FL Panhandle (where the
inherited Slight Risk was maintained and adjusted a bit based on
the new model data). Given relatively dry antecedent conditions and
associated 3-6 hr FFGs generally ranging from 3.0-4.0", the Slight
is on the lower-end of the 15-40% probability distribution,
suggesting widely scattered instances of flooding (at best).

Churchill

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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
213 AM EST Sun Dec 8 2024


Expect elongated low pressure to reach the Mid-Atlantic/New York
state region by early Wednesday. Strong upper dynamics becoming
negatively tilted will support deepening/consolidation of this
system into the Northeast by late Wednesday followed by continued
strengthening as it continues north into Canada into early
Thursday. Accompanying this strong development, guidance still
shows a pronounced surge of moisture into the Northeast, with
precipitable water values reaching 3+ standard deviations above
normal for a time, supporting a broad area of heavy rainfall. Snow
cover or wet ground will lead to increased sensitivity over the
Northeast while persistence of model guidance and strengthening
signals from first-guess fields add to the confidence in the
potential for runoff issues. Thus the Day 4 Excessive Rainfall
Outlook maintains a Slight Risk area covering much of New England
while a surrounding Marginal Risk area extends southward to the
southern Mid-Atlantic where some instability could enhance rainfall
totals. The first-guess fields actually suggest potential for some
embedded probabilities at least in the higher half of the Slight
range over New England, so this will merit continued monitoring.
Both risk areas have been expanded a little westward from
continuity to correspond with latest guidance trends. Meanwhile,
the western edge of the precipitation shield may contain some snow
and the deepening storm may produce a period of brisk to strong
winds in its wake. Also expect the cold air arriving behind this
system to produce another episode of lake effect snow during
Wednesday-Thursday.

The forecast pattern from the Pacific into the West will favor at
least a couple episodes of rain and higher elevation snow over the
Pacific Northwest, possibly extending into parts of California. The
progressive nature of the mean flow continues to temper confidence
in some details, though guidance is starting to cluster around the
idea of a fairly strong eastern Pacific system helping to produce
the highest totals of the period and greatest southward extent of
precipitation around Friday-Saturday. This storm could produce
areas of brisk to strong winds depending on its depth and track.
Some moisture may extend into the Rockies, while leading shortwave
energy/surface feature(s) may produce areas of light precipitation
over parts of the central/east-central U.S.

The amplified pattern over the East will lead to above normal
temperatures near the Atlantic coast on Wednesday while trailing
cold from the Upper Midwest into the South pushes eastward into
late week. Coldest anomalies should be from the Upper Midwest into
Great Lakes/Ohio Valley Wednesday-Thursday, at least 10-20F below
normal. The transition toward flatter mean flow aloft will
lead to a warming trend over the Rockies and Plains late this
week with above normal temperatures (some pockets of plus 10F or
greater anomalies) persisting through the weekend. The East will
trend closer to normal by next weekend.

Rausch

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
213 AM EST Sun Dec 8 2024


Expect elongated low pressure to reach the Mid-Atlantic/New York
state region by early Wednesday. Strong upper dynamics becoming
negatively tilted will support deepening/consolidation of this
system into the Northeast by late Wednesday followed by continued
strengthening as it continues north into Canada into early
Thursday. Accompanying this strong development, guidance still
shows a pronounced surge of moisture into the Northeast, with
precipitable water values reaching 3+ standard deviations above
normal for a time, supporting a broad area of heavy rainfall. Snow
cover or wet ground will lead to increased sensitivity over the
Northeast while persistence of model guidance and strengthening
signals from first-guess fields add to the confidence in the
potential for runoff issues. Thus the Day 4 Excessive Rainfall
Outlook maintains a Slight Risk area covering much of New England
while a surrounding Marginal Risk area extends southward to the
southern Mid-Atlantic where some instability could enhance rainfall
totals. The first-guess fields actually suggest potential for some
embedded probabilities at least in the higher half of the Slight
range over New England, so this will merit continued monitoring.
Both risk areas have been expanded a little westward from
continuity to correspond with latest guidance trends. Meanwhile,
the western edge of the precipitation shield may contain some snow
and the deepening storm may produce a period of brisk to strong
winds in its wake. Also expect the cold air arriving behind this
system to produce another episode of lake effect snow during
Wednesday-Thursday.

The forecast pattern from the Pacific into the West will favor at
least a couple episodes of rain and higher elevation snow over the
Pacific Northwest, possibly extending into parts of California. The
progressive nature of the mean flow continues to temper confidence
in some details, though guidance is starting to cluster around the
idea of a fairly strong eastern Pacific system helping to produce
the highest totals of the period and greatest southward extent of
precipitation around Friday-Saturday. This storm could produce
areas of brisk to strong winds depending on its depth and track.
Some moisture may extend into the Rockies, while leading shortwave
energy/surface feature(s) may produce areas of light precipitation
over parts of the central/east-central U.S.

The amplified pattern over the East will lead to above normal
temperatures near the Atlantic coast on Wednesday while trailing
cold from the Upper Midwest into the South pushes eastward into
late week. Coldest anomalies should be from the Upper Midwest into
Great Lakes/Ohio Valley Wednesday-Thursday, at least 10-20F below
normal. The transition toward flatter mean flow aloft will
lead to a warming trend over the Rockies and Plains late this
week with above normal temperatures (some pockets of plus 10F or
greater anomalies) persisting through the weekend. The East will
trend closer to normal by next weekend.

Rausch

Probabilistic Heavy Snow and Icing Discussion
NWS Weather Prediction Center College Park MD
300 PM EST Sun Dec 8 2024

Valid 00Z Mon Dec 09 2024 - 00Z Thu Dec 12 2024

...Northeast...
Days 1-3...

As the snow today in parts of the Northeast winds down, focus
shifts to the next wintry setup that to affect the region beginning
midday Monday, peaking in intensity Monday evening, and concluding
Tuesday morning. An upper level disturbance in the Ohio Valley
will direct a plume of rich Gulf of Mexico moisture towards the
region Monday afternoon. NAEFS shows IVT values of 300-400 kg/m/s
approaching the Northeast at 18Z. These IVT values are above the
90th climatological percentile according to NAEFS, highlighting the
anomalous nature of this moisture plume and advection. Meanwhile,
a storm system in the northern Great Lakes will work in tandem with
the Ohio Valley disturbance to produce brisk 700-300mb WSWrly
winds aloft. There is weak high pressure located over Quebec that
will help to lock in some low-level cold air and allow for
precipitation to fall in the form of snow Monday afternoon over
northern New England. However, the strong WSW flow aloft will force
a >0C warm nose to ensue at low levels and cause precipitation to
transition to an icy wintry mix for most areas.

By Monday evening, a weak coastal low near the tip of Long Island
that will then track towards the Massachusetts Capes. This low is
expected to introduce additional low-level moisture into Maine late
Monday night and into Tuesday. This could make for a more prolonged
period of snow or icy wintry mix, with the latter increasingly
more likely early Tuesday morning due to the dryslot aloft making
it increasingly difficult to favor dendrites. By the time the
storm departs for Nova Scotia around midday and precipitation
tapers off, snowfall amounts will generally range in the 1-3" range
for the Adirondacks, Green and White Mountains above 2,000ft.
Minor ice accumulations (generally topping out around 0.1") are
likely in many of these mountain ranges, as well as near the Maine
coast and as far south as the Poconos and Catskills.

By Wednesday morning a far more amplified jet stream pattern will
bring about the next chance for measurable snow, but there remains
a good deal of uncertainty in its evolution. NAEFS and ECMWF are in
good agreement on an anomalous upper level trough over the
Mississippi Valley 12Z Wednesday, but depth/tilt of the primary
500mb shortwave at the base of the upper level trough differs by
guidance members. The ECMWF ENS is a bit deeper with this southern
disturbance while the GEFS is stronger with the upper low
traversing southern Canada. This can have implications on the
transition from rain to snow throughout the Northeast and how
quickly rain can changeover to snow in the central Appalachians.
There are some aspects ensembles do agree upon; a lack of
sufficiently colder air (albeit enough to where parts of northern
Maine could witness up to a tenth of an inch of ice), a storm track
over the I-95 corridor, and lower SLRs that would support a
heavy/wet snow type. WPC PWPF does show the potential for some ice
accumulations This kind of setup typically favors the Appalachians
for potential heavy snowfall, but when this transition to snow
occurs will be key in determining amounts and impacts in subsequent
forecast cycles.


...Northern Rockies & Northern High Plains...
Day 1...

Snow will taper off later this evening in the Olympics and Cascades
as the upper level trough responsible for the disruptive snowfall
amounts heads east. Pacific moisture associated with the upper
trough will continue east of the Cascades across to the Rockies as
the cold front advances through the High Plains and Central
Rockies. WPC probabilities show moderate-to-high chances (50-70%)
for
snowfall >4" of snow are over the higher terrain including the
Bitterroots, Lewis Range, Blue Mountains, and into the Little Belt
and Big Snowy Mountains in central Montana, the Absarokas in
southern Montana, and the Big Horns in northern Wyoming.

As a storm system tracks east along the US/Canada border, a
deformation band of snow will pivot over eastern Montana and
western North Dakota this evening. The storm's progressive movement
and strong winds fracturing dendrites will put a cap on snowfall
totals, but those same winds are likely to cause blizzard
conditions in parts of eastern Montana. The WSSI does suggest
Moderate Impacts that are primarily driven by wind gusts that could
approach 55 mph, leading to whiteout conditions and snow drifts.
Strong NWrly winds also favor upsloping flow into the Black Hills
where WPC PWPF shows moderate-to-high chances for snow totals >4"
above 6,000ft. The strong upper level ascent and steep lapse rates
could support possible snow squalls tonight and into Monday
morning as well across the western Dakotas. Snow should taper off
throughout the region by Monday afternoon.


...Northern Minnesota and northeastern North Dakota...
Day 1...

Low pressure over southern Saskatchewan will move toward northern
ND/MN tonight, lifting its warm front through the region and acting
as the primary source of lift for precipitation throughout the
region. While precipitation will start out as snow along the
Canadian border, persistent mid/low-level WAA may cause snow to
changeover to freezing rain for portions of eastern North Dakota
and north-central Minnesota this morning, with several hundredths
of an inch of ice possible in parts of north-central Minnesota and
as far east as northern Wisconsin. The heaviest snow is set to
occur along the Minnesota Arrowhead where the initial snowfall via
WAA aloft will stay snow the longest, and snowfall rates tonight
will be enhanced by easterly flow off Lake Superior. WPC
probabilities for at least 6 inches of snow are >50% along the
northern coast of the Minnesota Arrowhead. In fact, the very tip of
the Arrowhead sports moderate chances (40-60%) for snowfall totals
over 12" through Monday afternoon.


...CO Rockies/Sangre de Cristos...
Days 1-2...

Height falls at the southwest side of a large positively-tilted
longwave trough over the Heartland will swing through the central
Rockies on Monday, bringing with it a plume of residual Pacific
moisture. High pressure building in from the north in the wake of a
cold frontal passage Monday morning will result in modest upslope
flow into Colorado's Front Range during the day. This upslope
component will extend as far south as the Sangre de Cristos in
northern New Mexico by Monday night. Periods of snow across the
region are expected as the system moves quickly through the region
and heights build in by Tuesday. WPC probabilities have increased
over the past 24 hours with high chance probabilities (>70%) for
snowfall >4" that now stretches from parts of the Palmer Divide
all the way to Raton Pass. The highest and more remote elevations
could see localized amounts approach 8" by the time the snow comes
to an end Tuesday morning, and this does include the Raton Pass
where snow could cause hazardous travel conditions.

...Great Lakes...
Day 3...

As a robust 500mb low dives south Wednesday morning, an Arctic
front and blustery NWrly winds will race over the Great Lakes
starting Wednesday morning over Lake Superior, then over Lake
Michigan by Wednesday evening. NAEFS and ECMWF SATs agree that
850mb temps rushing over Lake Superior and Michigan will be below
the 10th climatological percentile. The sharp contrast between the
Lakes water temperatures and frigid low-level temperatures advected
by brisk NW winds will kick-start the lake-effect snow (LES)
machine into high gear yet again by Wednesday night. This same cold
air-mass will race over the eastern Great Lakes Wednesday night
and lead to more heavy snowfall into Thursday. For the scope of
this discussion (ending 00Z Thurs), WPC PWPF shows high chance
probabilities (>70%) for snowfall totals >6" along the northern
coasts of the Michigan U.P. and along the western coast of
Michigan's Mitten. Look for some snowfall rates within the more
robust bands to top >2"/hr Wednesday night in these areas.


The probability of significant ice >0.25" across the CONUS is less
than 10 percent.


Mullinax

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
143 PM EST Sun Dec 8 2024

Valid 12Z Wed Dec 11 2024 - 12Z Sun Dec 15 2024

***Heavy rain threat for the East Coast continuing into
Wednesday***

19Z Update: The guidance continues to agree well on most aspects
of the forecast going into the middle of the week, and therefore a
general model compromise works out well as a starting point in the
forecast process. However, the NBM likely remains too low with QPF
from lake effect snow, so this was blended with some GFS/ECMWF and
then raised about another 40%. Going forward into the end of the
week, similar to yesterday, the guidance continues to differ on
incoming Pacific shortwave energy as the upper ridge across the
West starts to break down. The AI guidance from the ECMWF is in
better agreement with the operational ECMWF compared to yesterday,
and this helps to bolster more confidence in the ECMWF and CMC
solutions for days 5 through 7, since there is overall better
agreement among those models compared to the GFS. Given the degree
of ensemble spread going into Sunday especially, the ensemble means
accounted for a majority of the forecast blend by that time.
However, there is higher confidence that a moderation trend in
temperatures can be expected going into next weekend after a very
cold end to the week across the central and eastern parts of the
country. The previous forecast discussion is appended below for
reference. /Hamrick
-----------------------------

...Overview...

With typical localized adjustments, latest guidance continues to
show the potential for heavy rainfall from the Mid-Atlantic into
especially the Northeast on Wednesday, as a dynamic upper trough
supports rapidly deepening low pressure that tracks from the Mid-
Atlantic northward into Canada. Mean flow across the lower 48 still
looks to trend more zonal behind this system, so the amplified
upper trough over the East should lift out after Thursday while
predictability of system specifics should decrease late week
through the weekend. The pattern will favor at least a couple
episodes of rain and higher elevation snow along parts of the West
Coast, with latest guidance starting to show a stronger eastern
Pacific system around Friday-Saturday which would support
relatively greater precipitation totals in that time frame. Below
normal temperatures will spread across the East late this week,
replacing above normal readings near the East Coast on Wednesday.
Then expect near to above normal readings over most of the country
by next weekend, with warmest anomalies over the Plains and
vicinity.

...Guidance/Predictability Assessment...

Latest guidance continues to refine details near the East Coast
Wednesday into Thursday, with improving consensus as the past two
UKMET runs have finally joined the majority cluster for strongly
deepening Mid-Atlantic into eastern Canada low pressure in response
to approaching vigorous upper trough dynamics. Dynamical/machine
learning (ML) guidance on average shows the surface low depth
reaching at least as low as the 970s mb by early Thursday. There
has been a slight westward nudge since yesterday, a nod to earlier
ML runs that were westward. 12Z ML runs were a little more varied
for longitude.

Upstream features continue to be difficult to resolve. ML models
show a bit more coherence with a Thursday West Coast shortwave
continuing eastward thereafter, approaching the Northeast or
vicinity by next Sunday with varied degrees of surface reflection,
and then a better defined eastern Pacific system by Saturday moving
into the West/High Plains by Sunday. Most dynamical guidance is
showing less of a surface reflection than the ML models (or at
least slower, holding onto stronger eastern U.S. high pressure)
with the leading shortwave. The dynamical and ML models are
gradually showing an improved signal for what could be a fairly
strong eastern Pacific system around Friday-Saturday. ML models are
generally more agreeable with bringing a moderately amplified upper
trough into the West by next Sunday, while dynamical guidance
varies--albeit with the means at least suggesting some troughing.

The updated forecast started with a composite of 12Z/18Z
operational models, and then transitioned toward a blend of the 18Z
GFS, past two ECMWF runs, and 18Z GEFS/12Z ECens-CMCens means as
those solutions provided the most coherent evolution when compared
to each other and ML models.

...Weather/Hazards Highlights...

Expect elongated low pressure to reach the Mid-Atlantic/New York
state region by early Wednesday. Strong upper dynamics becoming
negatively tilted will support deepening/consolidation of this
system into the Northeast by late Wednesday followed by continued
strengthening as it continues north into Canada into early
Thursday. Accompanying this strong development, guidance still
shows a pronounced surge of moisture into the Northeast, with
precipitable water values reaching 3+ standard deviations above
normal for a time, supporting a broad area of heavy rainfall. Snow
cover or wet ground will lead to increased sensitivity over the
Northeast while persistence of model guidance and strengthening
signals from first-guess fields add to the confidence in the
potential for runoff issues. Thus the Day 4 Excessive Rainfall
Outlook maintains a Slight Risk area covering much of New England
while a surrounding Marginal Risk area extends southward to the
southern Mid-Atlantic where some instability could enhance rainfall
totals. The first-guess fields actually suggest potential for some
embedded probabilities at least in the higher half of the Slight
range over New England, so this will merit continued monitoring as
the event enters the short range period. For the Day 4 updated
ERO, both risk areas have been expanded a little westward from last
night's continuity to correspond with latest guidance trends.
Meanwhile, the western edge of the precipitation shield may contain
some snow and the deepening storm may produce a period of brisk to
strong winds in its wake. Also expect the cold air arriving behind
this system to produce another episode of lake effect snow during
Wednesday- Thursday.

The forecast pattern from the Pacific into the West will favor at
least a couple episodes of rain and higher elevation snow over the
Pacific Northwest, possibly extending into parts of California. The
progressive nature of the mean flow continues to temper confidence
in some details, though guidance is starting to cluster around the
idea of a fairly strong eastern Pacific system helping to produce
the highest totals of the period and greatest southward extent of
precipitation around Friday-Saturday. This storm could produce
areas of brisk to strong winds depending on its depth and track.
Some moisture may extend into the Rockies, while leading shortwave
energy/surface feature(s) may produce areas of light precipitation
over parts of the central/east-central U.S.

The amplified pattern over the East will lead to above normal
temperatures near the Atlantic coast on Wednesday while trailing
cold from the Upper Midwest into the South pushes eastward into
late week. Coldest anomalies should be from the Upper Midwest into
Great Lakes/Ohio Valley Wednesday-Thursday, at least 10-20F below
normal. The transition toward flatter mean flow aloft will
lead to a warming trend over the Rockies and Plains late this
week with above normal temperatures (some pockets of plus 10F or
greater anomalies) persisting through the weekend. The East will
trend closer to normal by next weekend.

Rausch/Hamrick


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