916
FXUS02 KWBC 211843
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
143 PM EST Thu Nov 21 2024

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


...Overview...

It generally remains the case that 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...

Model and ensemble forecast spread and predictability seem better
than normal Sunday into Tuesday and a composite 06 UTC GFS and 00
UTC ECMWF/Canadian/UKMET solution offers a solid forecast basis
along with the National Blend of Models and WPC continuity.
Forecast spread and cycle to cycle continuity remain less than
stellar for next week, as mainly attributable from the extent of
system energy digging over the eastern Pacific and uncertaincies of
subsequent inland progressions into the West and downstream over
the east-central U.S. over time. The WPC product suite was
primarily derived with a concept of maintaining a middle of the
road of the full envelope of timing solutions given lingering
uncertainties and with an eye toward maintaining WPC product
continuity. Latest 12 UTC guidance soltuions seem to be
increaisngly clustering toward this middle of the road soluiton,
possibly bolstering forecast confidence closer to average level.

...Weather/Hazards Highlights...

It is still expected that 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-northern Rockies 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 an associated frontal system, may result in
emergence of a round of organized to heavy rainfall around next
Wednesday/Thursday from the Mid-South to the Appalachians.

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/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















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