Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 PM EST Sat Dec 7 2024

Valid 12Z Tue Dec 10 2024 - 12Z Sat Dec 14 2024

***Heavy rain threat For the East Coast Tuesday into Wednesday***

19Z Update: The 12Z models are in excellent synoptic scale
agreement across the Continental U.S. through midweek, especially
with the building deep trough over the Central U.S. and the strong
cold front exiting the East Coast. Similar to yesterday, the
greatest model differences emerge as the upstream upper ridge over
the Western U.S. breaks down as shortwave energy from the eastern
Pacific moves in. The ECMWF and UKMET solutions are stronger with
the shortwave crossing the northwestern U.S. on Thursday, and model
spread increases substantially going into next Saturday, with below
average confidence by that time. An examination of the ML guidance
is not in great alignment with the operational ECMWF with the
timing of the shortwave passages, and therefore a solution closer
to the CMC/GFS and the ensemble means was employed for the second
half of the forecast period. The ensemble means were gradually
increased to about 60% by Saturday, and the previous forecast
discussion is appended below. /Hamrick
---------------------------

...Overview...

Most guidance has been fairly consistent in showing the potential
for heavy rainfall rainfall along parts of the East Coast during
Tuesday-Wednesday, as central into eastern U.S. upper troughing
initially pushes along a wavy front and then by Wednesday onward
develops a stronger Mid-Atlantic into Northeast wave as the trough
takes on a negative tilt. Upper troughing over the East will likely
lift away after Thursday as mean flow from the eastern Pacific
into the lower 48 trends more zonal. Such a low amplitude regime
tends to decrease predictability, and indeed guidance shows a lot
of spread for individual details from the Pacific eastward late
week into next weekend. However the details turn out, there is at
least better agreement with the more general theme of increasing
rain and high elevation snow for the Northwest. The amplified
evolution over the East will favor a day or so of warm temperatures
in the followed by a couple chilly days, while above normal
readings will become established over the Plains and vicinity by
Friday-Saturday under the forecast zonal mean flow aloft.

...Guidance/Predictability Assessment...

Regarding the amplified evolution over the East during the first
half of the period, most guidance has maintained fairly good
continuity with the idea of frontal wave intensification from the
Mid-Atlantic into the Northeast and northward over Canada from
Wednesday into Thursday. Most trends are adding some confidence to
this being a fairly strong system, as the ensemble means have
trended better defined plus the 12Z machine learning (ML) models
have on average trended about 5-10 mb deeper by the time the
surface low reaches north of Maine by early Thursday. The UKMET is
the one misfit, as it hangs back the shortwave energy in the
southern part of the upper trough, resulting in a weaker/eastward
surface evolution. One ML model trait of note is that most track
this wave a little farther westward than most of the dynamical
models/means, so it will be worth watching to see if the dynamical
solutions eventually display any trend in that direction.

Model/ensemble spaghetti and other comparison plots show rapidly
increasing divergence in specifics behind this system. A weak
shortwave may reach the West Coast around midweek (with less
southern stream continuation than depicted by the 18Z GFS). Then ML
models cluster toward the idea of trailing energy arriving a day
later, and then progressing eastward as a defined shortwave into
the Plains and Great Lakes by Friday-Saturday (with an associated
surface system). The 18Z GFS/GEFS mean come closest to this idea.
Behind this feature, there is a signal for another feature or two,
with best defined upper troughing ultimately near the West Coast by
early next Saturday. Surface details will be very sensitive to the
timing and energy distribution within the upper trough. In varying
ways the 12Z ECMWF did not compare well to the majority guidance
cluster late, though the new 00Z run has at least trended somewhat
in the desired direction.

The aforementioned array of guidance recommended emphasis on the
18Z GFS and 12Z ECMWF/CMC for about the first half of the period,
followed by a transition toward a model/ensemble mean blend with
greater weight on the 18Z GFS/GEFS mean.

...Weather/Hazards Highlights...

Latest guidance continues to show a wavy cold front moving into the
East by Tuesday-Wednesday, with stronger wave development from the
Mid-Atlantic into the Northeast during Wednesday-Wednesday night as
the supporting upper trough takes on a negative tilt. Signals have
been fairly stable over the past day regarding a potential axis of
enhanced rainfall over parts of the Southeast on Tuesday within an
axis of anomalous moisture and some instability ahead of the front,
favoring maintenance of a Slight Risk area in the Day 4 Excessive
Rainfall Outlook, from central Georgia to the FL Panhandle. The
Storm Prediction Center is still monitoring for the possibility of
severe weather across this region. By Wednesday the majority of
guidance shows a broader area of heavy rainfall over the Northeast
in response to vigorous upper dynamics and surface development.
Snow cover or at least wet ground will cause higher sensitivity
across this region, so the Day 5 ERO maintains a Slight Risk area
covering about the eastern two-thirds of New England. A Marginal
Risk area extends from west of the Slight down into the southern
Mid-Atlantic where some instability could help to enhance totals
locally. The deepening wave 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 developing pattern along the West Coast should lead to a return
of rain and higher elevation snow across the Northwest from late
this week into the weekend. There will likely be multiple
shortwaves/frontal systems but there is still greater than average
uncertainty in the specifics of these features. Thus it will take
additional time to resolve the finer details of precipitation
coverage, intensity, and timing.

The East will see well above normal temperatures Tuesday into
Wednesday ahead of the cold front pushing into the region. Greatest
anomalies should be for morning lows, reaching plus 15-20F or more
at some locations. Then an area of below normal temperatures
(mostly negative 5-15F anomalies) will progress across the eastern
half of the country Wednesday-Thursday. The West and especially the
Plains will rebound to above normal from midweek onward, with
parts of the Plains seeing plus 10-15F anomalies by late next week.

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