047
FXUS02 KWBC 041858
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 PM EST Wed Dec 4 2024

Valid 12Z Sat Dec 07 2024 - 12Z Wed Dec 11 2024

...Return flow to fuel emerging rainfall from the west-central
Gulf Coast to the Appalachians this weekend to early next week...

19Z Update: The models and ensemble means are well clustered
through next Monday with a relatively amplified flow pattern in
place. The 00Z UKMET was deemed an outlier solution across the
southern tier of the U.S. and became out of phase with the ridge
axis near the Southeast Coast, so a non-UKMET blend was
incorporated for the first half of the forecast period. However,
the 12Z UKMET trended more in line with the model consensus.
Looking ahead to early-mid week, there remain some timing and
surface low evolution differences with the cold front approaching
the Eastern U.S., with the GFS and CMC portraying a slower passage
with a secondary low forming along the front, whereas the ECMWF
features a more progressive passage. The ensemble means were
gradually increased to about 40% by next Wednesday to account for
these mesoscale differences. The previous forecast discussion is
appended below for reference. /Hamrick
----------------------

...Guidance/Predictability Assessment...

The WPC medium range product suite was mainly derived from a
composite of reasonably clustered guidance from the latest
guidance GFS/ECMWF/UKMET/Canadian models that have good ensemble
support and continuity. Pivoted to the most compatible solutions of
the ECMWF/ECMWF ensemble mean and Canadian/Canadian ensemble mean
at longer time frame to maintain best continuity amid otherwise growing
forecast spread.

...Weather/Hazards Highlights...

Arctic air that has persisted over the east-central U.S. will
moderate by this weekend as amplified upper troughing finally
lifts out after a prolonged time period. Nevertheless, lake effect
snow is likely to be ongoing into early Saturday in the wake of a
strong clipper-type system. By early next week, temperatures should
be near to a little above average across much of the Eastern U.S..

Meanwhile, emerging moist return flow from the Gulf of Mexico as
ambient high pressure over the South retreats to the east will
support increasing coverage of showers and thunderstorms, some of
which could be heavy, from the west-central Gulf Coast states up
through the southern Appalachians this weekend through early next
week. In terms of the WPC Excessive rainfall outlook, we maintained
a Day 4/Saturday Marginal Risk area centered over Southeast Texas
to cover event genesis, with a small expansion to the northwest
compared to the earlier outlook area. The existing Marginal and
Slight Risk areas have been maintained with very little in the way
of changes for Day 5/Sunday into Monday morning centered over the
ArkLaMiss region as convection erupts in a favored right entrance
region of the upper jet pattern to support multiple rounds of
convection as strong moisture flux from the central Gulf continues
ahead of an approaching cold front. There may also be some severe
weather that develops in the warm sector of this low pressure
system, and this is something that the Storm Prediction Center is
monitoring in the days ahead.

Upstream, moderately heavy terrain enhanced precipitation is
expected to develop this weekend over the Pacific Northwest with
energetic upper trough passage and long lead moisture feed.
Terrain enhanced snows will then spread increasingly inland across
the Northwest to the north-central Rockies with cold frontal
passage over the West into early next week. Expect a nice warming
trend in advance of this front out from the West through over the
central U.S. this weekend/early next week prior to cooling
passage.

Schichtel/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


















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