Weather Prediction Center

National Oceanic and Atmospheric Administration

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

Valid 12Z Sun Dec 08 2024 - 12Z Tue Dec 10 2024

...Heavy rain threat emerges across the Deep South late Sunday into
Monday...

...Periods of mixed rain and snow linger from the Great Lakes to northern
New England through Monday night...

...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....

A major reversal of the weather pattern is forecast to occur across
mainland U.S. during the next couple of days. The remaining cold air
across the eastern U.S. originated from the recent arctic outbreak will be
dispelled by an eastward expanding mild air mass from the central U.S.
Meanwhile, the mild air across the western U.S. in recent days will be
dispelled by a surge of cold air currently advancing through the
northwestern U.S. The upper-level trough responsible for this cold surge
into the western U.S. will first spread mixed rain and snow through the
northern Rockies today behind a cold front along with windy conditions.
Daytime heating will keep the precipitation mainly in the form of rain as
it moves quickly across the northern Plains today. By tonight, colder air
from Canada is forecast to filter south from Canada behind a low pressure
system. This will bring periods of snow into the northern Plains and
areas farther west to the foothills of the northern Rockies through
tonight and into Monday, along with windy conditions. The higher elevations
will receive higher amounts of snow, with the highest snowfall likely near
the northern slopes of the Black Hills where more than a foot of new snow
is possible. Portions of the central Rockies into the Front Range of
Colorado and down into northeastern New Mexico can expect to receive a
period of snow from this system through Monday night into Tuesday morning.

Ahead of the cold front trailing south from the aforementioned low
pressure system, a jet stream sliding across northern Mexico will interact
with a coastal front and a low pressure wave near the western Gulf Coast
to bring an increasing threat of heavy rain farther inland across the
lower Mississippi Valley and then into the Deep South through the next
couple of days. Warmer and more unstable air arriving from the Gulf will
help trigger heavier showers and thunderstorms for the eastern portion of
these areas. The overlapping elements coupled with some recent rainfall
will result in the possibility of scattered instances of flash flooding.
The Weather Prediction Center maintains a Marginal Risk of Excessive
Rainfall across the central Gulf Coast region for Monday into early
Tuesday, with a Sight Risk area from southeastern Louisiana to southern
Mississippi.

Across New England, widespread snowfall is in progress today ahead of a
clipper system moving quickly eastward along a warm front lifting north
across the region. Widespread snowfall of up to 6 inches can be expected
from the Adirondacks eastward through Maine before the snow tapers off
tonight. More snow will then skirt the upper Great Lakes ahead of the
warm front associated with the northern Plains low pressure system, with
6-8 inches expected over the Upper Peninsula of Michigan and the Arrowhead
of Minnesota. A period of rain will sweep across the Ohio Valley Sunday
night, then the Appalachians and the northern Mid-Atlantic on Monday ahead
of the trailing front, followed by the next round of snow across northern
New England Monday night. Mild Atlantic air will keep the precipitation
as rain for southern New England into Tuesday morning.

Kong


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

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
213 AM 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***


...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.
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


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
305 AM EST Sun Dec 8 2024
Day 1
Valid 12Z Sun Dec 08 2024 - 12Z Mon Dec 09 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE
LOWER MISSISSIPPI VALLEY...

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
305 AM 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
EASTERN LOUISIANA AND COASTAL MISSISSIPPI...

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
305 AM 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 PORTIONS OF
SOUTHEAST AND CENTRAL GULF COAST...

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
312 AM EST Sun Dec 8 2024

Valid 12Z Sun Dec 08 2024 - 12Z Wed Dec 11 2024

...Northeast...
Day 1...

Quick-moving system crossing from Ontario into Quebec before 12Z
will continue eastward across northern Maine this morning and early
afternoon before moving into Atlantic Canada. Light snow is
forecast after 12Z this morning for the Adirondacks and northern
New England, with a bit higher amounts over eastern Maine where
snow will eventually taper off later this afternoon. WPC
probabilities for at least 4 inches of snow today after 12Z are
20-40% over far eastern Maine.

Day 2...

The next winter storm to affect the region comes from a
combination of a shortwave over the Ohio Valley and a northern
stream upper low over southern Canada, both acting in concert to
coax a moisture plume northward from the Gulf of Mexico on WAA. A
mix of some freezing rain on the southern side (northeast PA
northward through NYS and also over parts of western New England)
and snow on the northern side (north of I-90) may cause some minor
impacts on Monday. WPC probabilities for at least 4 inches of snow
are not higher than 10 percent, but are moderate (40-70%) for at
least 2 inches over northern NH into western ME.

Day 3... Yet another system will start to affect the Northeast as
the pattern becomes more amplified in the East. Through 12Z Thu,
strong WAA should scour out most of the cold air over the region,
but interior Maine may see some light icing very early Wednesday
with much more precipitation into D4.


...Pacific Northwest/Northern Rockies...
Days 1-2...

Second part of the two-part system will affect the NW today,
pushing moisture into a much colder environment. Moderate to heavy
snow at the higher passes may continue to cause moderate impacts
this morning before the snow starts to taper off from west to east
this afternoon. WPC probabilities depict high chances (>70%) for at
least 8 inches of snow above about 3,500ft while the more remote
areas above 5,000ft likely receive an additional 12"+ of snowfall.

Moisture will continue east of the Cascades across to the Rockies
as the cold front has moved well past the Divide. WPC probabilities
for >8" of snow are over the higher terrain including the
Bitterroots, Lewis Range, Blue Mountains, and into the Little Belt
Mountains in central Montana, the Absarokas in southern Montana,
and the Tetons of western Wyoming. Snow will continue into D2
farther east over the Black Hills where probabilities of at least 4
inches reach 60-90%.


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

Low pressure over southern Saskatchewan this morning will move ESE
toward northern ND/MN this evening and overnight, lifting its warm
front through the region. While precipitation may start out as
snow along the Canadian border, mid/low-level WAA may promote an
area of freezing rain for portions of eastern North Dakota into
Minnesota this morning, with several hundredths of an inch of ice
possible after 12Z. Closer to the Canadian border, a colder column
supports light to modest snowfall, with the heaviest snow occurring
with the initial WAA over northern Minnesota (esp the Arrowhead,
which will be enhanced by easterly flow off Lake Superior). WPC
probabilities for at least 4 inches of snow are >50% across much of
the northern part of MN, with higher probabilities over the
Arrowhead (>80%). There, probabilities for at least 8 inches of
snow are moderate (40-70%).


...CO Rockies/Sangre de Cristos...
Day 2...

Height falls at the southwest side of a large positively-tilted
longwave trough over the middle of North America will swing through
the central Rockies on Monday, brining 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 some
weak 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 light-to-
moderate snow across the region are expected as the system moves
quickly through the region and heights build in by Tuesday. WPC
probabilities show >50% probabilities for at least 4 inches mainly
limited to the Sangre de Cristos in southern Colorado and northern
New Mexico, which does include the Raton Mesa. The highest and more
remote elevations could see localized amounts approach 8" by the
time the snow comes to an end Tuesday morning.


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


Fracasso/Mullinax

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
213 AM 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***


...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.
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


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