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