Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
157 PM EST Fri Dec 6 2024

Valid 12Z Mon Dec 09 2024 - 12Z Fri Dec 13 2024

***Heavy rain threat across the central Gulf Coast and Southeast
 early next week***

19Z Update: The 12Z models are in excellent synoptic scale
agreement with the amplifying deep trough over the Central U.S.
through the middle of the week, and the evolving storm system and
cold front that will affect the East Coast. There are more
questions that remain regarding Pacific shortwave energy reaching
the West Coast and the eventual pattern evolution across the
Western U.S. for the end of next week, and how the ridge axis over
the West decays and becomes more zonal. The ECMWF and GFS are
seemingly at odds by Friday, with the GFS maintaining the ridge
axis longer and the ECMWF bringing in a well defined trough. An
examination of the ML guidance at the time of forecast preparation
did not strongly support the operational ECMWF with regards to
trough placement, and therefore a solution closer to the CMC/GFS
was preferred going into Thursday and Friday, with below average
forecast confidence by this time. Ensemble means were gradually
increased to about half by Friday amid growing uncertainty. The
previous forecast discussion is appended below. /Hamrick
-----------------------

...Overview...

The South and East will be the focus for potentially significant
rainfall during the early-middle part of next week as a northern
Plains into western U.S. upper trough as of early Monday heads into
the east-central U.S. The leading wavy cold front will likely reach
the East Coast around Wednesday with potential for stronger wave
development over/near the Northeast later Wednesday into early
Thursday. Upper troughing should linger over the East into next
Friday but with a weakening trend by then. While most of the East
dries out behind this system (aside from snow to the lee of the
Great Lakes), expect the Pacific Northwest to see increasing
precipitation after midweek as a mean trough aloft approaches.
System progression will lead to an area of well above normal
temperatures in the warm sector ahead of the central-eastern U.S.
cold front, followed by an area of below normal readings and then a
rebound to above normal over the Plains late in the week.

...Guidance/Predictability Assessment...

Very early in the forecast, 12Z/18Z guidance as well as the new 00Z
runs have trended stronger/faster with low pressure now forecast to
reach the Canadian Maritimes by early Monday. As a result the
trailing front pushes farther south along the Northeast coast with
more high pressure wedging into New England. Latest consensus
suggests the GFS could be a little overdone with its southward
extent of the front though.

For the upper trough progressing into the East and associated
surface front/wave development, guidance agrees well in terms of
the large scale evolution but shortwave differences lead to some
variability for wave details and frontal timing. Still, there is
decent model clustering toward expected Mid-Atlantic through New
England wave development by Wednesday into early Thursday, with the
majority of 12Z machine learning (ML) models also showing this low
reaching just north or northeast of Maine by early Thursday. Some
ensemble means are not as defined with this low yet due to spread
among individual members. Within the upper trough, one notable
questionable detail is the 12Z ECMWF's upper low that drops into
the northern Plains/Midwest on Wednesday, with most other guidance
and the new 00Z ECMWF run showing a more sheared depiction of
energy digging south from Canada.

Guidance spread increases noticeably over the eastern Pacific and
western U.S. during the latter half of the week. The most common
theme among dynamical and ML guidance at the moment is for a
possible weak system or two to arrive from the Pacific and then for
a better defined eastern Pacific upper trough to approach the West
Coast by next Friday, with a surface front and anchoring surface
low of very uncertain latitude. The ensemble means, 12Z and 00Z
GFS, and 00Z/05 ECMWF plus a relative majority of ML models fit
into this majority cluster. Other model runs and some ensemble
members show different ideas so confidence in specifics is below
average for now.

Guidance comparisons among the 12Z/18Z runs led to the updated
forecast starting out with an operational model blend followed by a
transition toward some ensemble mean input (18Z GEFS/12Z ECens) as
well as a late-period shift of the ECMWF component more toward the
00Z/05 run and GFS toward the 12Z run due to better comparisons to
the ensemble means/ML guidance along the West Coast.

...Weather/Hazards Highlights...

Early next week, upper trough dynamics reaching the West will
support a brief period of enhanced snow over the central Rockies,
primarily in Colorado. This energy will swing eastward within a
larger scale upper trough eventually reaching the East, supporting
a wavy cold front and eventual stronger Northeast surface
development by around late Wednesday into early Thursday. Leading
moist flow from the Gulf of Mexico will interact with the upper
trough/front to generate an area of potentially heavy rainfall from
the central Gulf Coast into Southeast/southern Appalachians
Monday-Tuesday and continuing into parts of the Mid-Atlantic and
Northeast on Wednesday. During the Monday-Tuesday night period
covered by the Days 4-5 Excessive Rainfall Outlooks, locations
along and a little inland from the central/east-central Gulf Coast
are within a Slight Risk area for both days, given a favorable
combination of anomalous moisture and some instability plus a
brief period of slower frontal progression as western U.S. dynamics
approach--thus leading to the possibility of some training/repeat
convection. The Day 5 Sight Risk area also extends a little farther
northeast into north-central Georgia. The updated Day 5 outlook has
the Slight Risk area adjusted about a row of counties eastward
compared to the initial overnight issuance to account for latest
trends in the 12z guidance. The Storm Prediction Center is
monitoring the potential for severe weather across this region.

As the midweek wave develops near the East Coast, the far
northwestern part of the moisture shield may contain some snow.
Then expect the cold air arriving behind this system to produce
another episode of lake effect snow during Wednesday-Thursday.
Otherwise expect mostly dry conditions over the East late in the
week.

Rain and higher elevation snow should return to the Pacific
Northwest and perhaps into the northern Rockies later in the week
as one or more system affect the region. There is a lot of guidance
spread for specifics at the moment, so it will take some time to
get better resolution of details in terms of precipitation coverage
and amounts.

An area of well above normal temperatures will spread across the
eastern half of the country early next week in the warm sector of
the wavy cold front likely to reach the East Coast around
Wednesday. Greatest anomalies should be for morning lows, reaching
plus 15-20F or more at some locations. Then an area of below normal
temperatures (generally negative 5-15F anomalies) will progress
from the central-southern Rockies into the East. Then the West and
Plains will rebound to above normal, with parts of the Plains
seeing plus 10-15F anomalies by late next week.

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