Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
144 AM EST Thu Dec 5 2024

Valid 12Z Sun Dec 08 2024 - 12Z Thu Dec 12 2024

...Return Gulf flow to fuel a heavy rainfall and runoff threat
from the west-central Gulf Coast states to the southern
Appalachians/Southeast this weekend into early next week...


...Guidance/Predictability Assessment...

The WPC medium range product suite valid for Sunday and Monday was
primarily derived from a composite of best clustered guidance of
the 18 UTC GFS and 12 UTC ECMWF/UKMET. These have good ensemble
support and continuity in an active pattern with above normal
overall predictability despite lingering embedded shortwave timing
issues mitigated by the blending process. Pivoted at longer time
frames to the most compatible solutions of the 18 UTC GEFS mean and
12 UTC ECMWF/ECMWF ensemble mean to maintain best continuity as
forecast spread grows to average levels. The 01 UTC National Blend
of Models and latest guidance from the 00 UTC guidance cycle mainly
seems in line with the forecast plan.

...Weather/Hazards Highlights...

It remains well heralded in guidance that 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
enhanced to heavy showers and thunderstorms from the west-central
Gulf Coast states to the southern Appalachians and parts of the
Southeast this weekend through early-mid next week. In terms of the
WPC Excessive rainfall outlook, Marginal and Slight Risk areas
have been maintained with very little in the way of changes for Day
4/Sunday centered over the ArkLaMiss region and as introduced for
Day 5/Monday from the Central Gulf Coast states to the southern
Appalachians 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 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 being monitored by the Storm Prediction Center. Expect the
moderate to heavy rain/convective focus will also shift to the
Southeast and up across the east-central U.S./Eastern Seaboard with
system translation through next midweek. Colder air on the northern
periphery of this broad system and addiitonal northern stream
energies should also support some snow chances from the Great
Lakes through the Appalachians to northern areas of the Northeast.

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 and eventually spread
lighter winter precipitation over the Northern Plains/Upper
Midwest with
cold frontal passage over/out from the West into early-mid next
week. Expect a warming trend ahead of the front out from the West
this weekend/early next week prior to cold frontal and secondary
seasonal surges across most of the central to eastern U.S..

Schichtel


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