Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
156 AM EST Sun Dec 3 2023

Valid 12Z Wed Dec 6 2023 - 12Z Sun Dec 10 2023

...Pattern Overview...

An amplified upper ridge is expected to be in place across the
central U.S. for the middle of the week, along with a departing
upper trough along the East Coast.  The ridge axis is then
expected to flatten out with a quasi-zonal flow pattern commencing
for a brief time across the central and western U.S. on Thursday. 
Pacific shortwave energy reaching the West Coast Thursday night
will help build an upper trough across the western U.S. to close
out the work week, and this will eventually 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 with increasing rain and storm chances, and wintry
precipitation northwest of the surface low.


...Guidance Evaluation/Predictability Assessment...

The 00Z model guidance suite is in good overall agreement on
Wednesday, and a general model compromise works well as a starting
point in the forecast process.  This holds true going into
Thursday as well, with only some modest timing differences
regarding the arrival of frontal boundaries and shortwaves across
the central and western states, mainly a slower trough passage
with the CMC.  By the end of the forecast period next weekend, the
model guidance is struggling with the timing of the big western
U.S. emerging over the Plains, with the latest 00Z guidance
indicating a more progressive trend compared to the 12Z guidance. 
This is apparent when compared to the 12Z ECENS, which holds the
trough farther back to the west by Saturday night into Sunday. 
The GFS is the most progressive solution, although the CMC and
ECMWF have trended in that general direction also.  In any case,
there will likely be a sub-1000mb surface low tracking southwest
to northeast across the eastern U.S. on Sunday.  Ensemble means
accounted for up to 50% of the forecast by next Sunday.


...Weather/Hazards Highlights...

There is some good news to report across the Pacific Northwest
with an abatement in the atmospheric river and lesser rainfall
totals going into Wednesday and through the remainder of the week.
 There is still a Marginal Risk area of excessive rainfall from
extreme northwest California to the Oregon coastal ranges and the
western Oregon Cascades on day 4 (Wednesday) with locally 1 to 2
inches of rainfall expected, but this is a noteworthy improvement
compared to the much heavier rainfall expected during the short
term forecast period.  Even though rainfall totals are expected to
be lighter, the grounds will be quite saturated by this time with
elevated creeks and rivers, so there may still be some instances
of minor flooding for those areas.  Looking ahead to Day 5
(Thursday), further improvement is expected for this region and
therefore no risk areas are currently warranted at this time. 

The developing western U.S. trough by the end of the week is
expected to result in plenty of moisture advecting inland across
the Intermountain West and the Rockies, with widespread moderate
to heavy snow for the higher elevations.  Snowfall chances are
increasing for portions of the western High Plains by next
weekend, and the location of the main snow axis will be dependent
on the track of the developing surface low and the deformation
band to the northwest of it.  Farther south in the warm sector of
the 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.

Pleasantly mild conditions for early December are expected from
the central Plains to the Dakotas and Montana on Wednesday and
into Thursday, with temperatures running 20 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 and increased cloud cover and
precipitation.

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, excessive rainfall outlook,
winter weather outlook probabilities, heat indices and Key
Messages are at:

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

$$