577
FXUS02 KWNH 210659
PREEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
158 AM EST Thu Nov 21 2024

Valid 12Z Sun Nov 24 2024 - 12Z Thu Nov 28 2024


...Overview...

A quieter overall weather pattern can be expected going into
Sunday
after the prolonged atmospheric river event in the short range
period, and improving conditions for the Northeast after the low
pressure system departs. The upper level flow pattern then becomes
more quasi-zonal for the Sunday to Monday time period with the
Western U.S. trough moving quickly inland and becoming less
amplified. However, there will still be enough moisture to produce
moderate to locally heavy snow from the Sierra to the central and
northern Rockies, and lingering light to moderate rain for the
lower elevations of the West Coast. A surface low is forecast to
develop across the Plains with a much colder airmass moving in
across the Dakotas and Montana early to mid next week, and an upper
level trough likely builds back in across the north-central U.S.
by Wednesday.

...Guidance/Predictability Assessment...

The 00Z models remain in decent synoptic scale agreement with the
pattern, including the exiting large upper/surface low pressure
system across New England and into southeastern Canada. A blend of
the latest 12Z/18Z deterministic models worked well as a starting
point for Sunday into Monday, with slightly more weighting to the
GFS/ECMWF. The main difference noted for the second half of the
forecast period is that the CMC lingers the trough near the Pacific
Northwest much longer than the more progressive ECMWF/GFS
locations by midweek, and therefore the CMC is also out of phase
across the Rockies and western High Plains by midweek, so it was
not used beyond Tuesday. Overall model spread increases even more
by Thanksgiving with the overall trough evolution across the Plains
and the downstream flow across the Eastern U.S., so forecast
confidence is below average for Days 6 and 7. The ensemble means
accounted for about 50-70% of the forecast blend for the Wednesday-
Thanksgiving time period amid growing model spread.

...Weather/Hazards Highlights...

There will be some abatement in the atmospheric river across
California going into the weekend as the attendant cold front
moves inland and the moisture flux anomalies decrease some,
although there may be some resurgence of moisture directed towards
the northwestern California Coast on Monday. Right now the plan is
to not have any risk areas in the Excessive Rainfall Outlook given
the model differences regarding placement of highest QPF, but a
Marginal Risk area could be needed later.  Snow levels will be
dropping in the wake of the front, and this is expected to result
in heavy snow for the higher elevations of the Sierra Nevada, where
1-3 feet of snow is possible for the higher elevations, and
moderate snow for the Intermountain West ranges and eventually the
central and northern Rockies going into early next week.

Improving conditions are also forecast for the Eastern U.S. as the
strong low pressure system moves away from the region. However, the
developing low pressure system over the south-central U.S., in
combination with a frontal boundary, may result in another round of
organized rain going into Wednesday across the Mid-South and into
Kentucky. There has been an upward trend in QPF compared to the
previous forecast.

Temperatures are expected to slowly moderate to near average across
the Eastern U.S. over the weekend compared to the rather chilly
conditions late this week. There is still the potential for Arctic
air to reach portions of Montana and into North Dakota next week
with highs in the 10s and low 20s, and overnight lows near zero
close to the Canadian border based on the latest NBM guidance.
There is still some uncertainty on how far south this arctic
airmass gets, so this will be refined in future forecasts. Warm
conditions should continue from southern Texas to Florida, with
highs in the 70s and 80s.

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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