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Extended Forecast Discussion
(Caution: Version displayed is not the latest version. - Issued 1902Z Dec 04, 2023)
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Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
202 PM EST Mon Dec 04 2023

Valid 12Z Thu Dec 07 2023 - 12Z Mon Dec 11 2023

...Pattern Overview...

The ridge axis that will be in place across the north-central U.S.
mid-week is expected to flatten out by Thursday while a ridge then
builds across the eastern U.S. to close out the week. Pacific
shortwave energy reaching the West Coast Thursday night will help
build an upper trough across the western U.S. Friday and into
Saturday, and this will then eject eastward across the Plains and
spur surface cyclogenesis over the weekend across the central
Plains. This will likely result in a well organized storm system
tracking northeast towards the Ohio Valley and Great Lakes, with
increasing rain and storm chances ahead of the cold front, and
wintry precipitation northwest of the surface low.

...Guidance Evaluation/Predictability Assessment...

The latest model guidance continues to be in good agreement with
the large scale pattern, allowing for a general model compromise
of the deterministic solutions as a starting point for the days
3-5 forecast. After Day 5/Saturday, there are some minor timing
and strength differences in the trough as it progresses through
the Midwest and East. The ECMWF has been most consistent in
wanting to close off an upper low over the Great Lakes by next
Monday. There are some differences with individual shortwaves
rouding the upper ridge as well, and the 00z CMC is faster and
becomes out of phase with the GFS/ECMWF concerning a shortwave
into the West on Saturday. To mitigate these differences,
incorporated 40 percent of the ensemble means into the blend along
with the GFS/ECMWF days 6-7. Overall, this maintains good
continuity with the previous WPC forecast as well.

...Weather/Hazards Highlights...

The next round of rainfall for the Pacific Northwest is expected
to be considerably lighter on Thursday compared to the ongoing
atmospheric river event in the short range forecast period, and
although some portions of the Oregon coastal ranges may get an
additional inch of rain, it does not appear the intensity nor the
overall totals are currently high enough to merit any excessive
rainfall concerns, although local rivers may still be running
high. Even less precipitation is expected here on Friday, before
the next round of Pacific moisture advects inland next weekend. 

The developing western U.S. trough by the end of the week is
expected to result in moisture advecting inland across the
Intermountain West and the Rockies, with widespread moderate to
locally heavy snow for the higher elevations. There may also be
some snow for adjacent portions of the western High Plains as the
low exits the Rockies. Farther south in the warm sector of the
developing surface low, increasing coverage of showers and
thunderstorms can be expected from the central Gulf Coast to the
Ohio Valley, along with breezy to windy conditions. Heavy rainfall
is looking increasingly likely particularly across parts of the
mid-South on Saturday, with the Storm Prediction Center also
highlighting parts of this region for a severe thunderstorm
threat. This storm system should then reach the East Coast by late
Sunday into Monday with widespread rain from the Mid-Atlantic to
New England, with some snow behind the system across the central
and northern Appalachians.

Pleasantly mild conditions for early December are expected from
the central Plains to the Dakotas and Montana on Thursday, with
temperatures running 15 to 30 degrees above average in many cases.
The mild conditions then reach the Midwest by Friday and much of
the eastern U.S. for the weekend with widespread 10-20 degree
above average temperatures likely, with gradually cooling
temperatures across the Plains. Colder weather envelops much of
the interior West and Rockies by the weekend in response to the
upper trough/low.


Additional 3-7 Day Hazard information can be found on the WPC
medium range hazards outlook chart at:

WPC medium range 500mb heights, surface systems, weather grids,
quantitative precipitation forecast, excessive rainfall outlook,
winter weather outlook probabilities, heat indices and Key
Messages are at: