559
FXUS02 KWNH 070646
PREEPD

Preliminary Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
145 AM EST Sun Dec 7 2025

Valid 12Z Wed Dec 10 2025 - 12Z Sun Dec 14 2025


...Multi-day heavy precipitation threat for the Pacific Northwest
into mid next week with a potent/persistent Atmospheric River...


...Overview...
A broad upper level trough with reinforcing shortwaves will
continue to dominate across the eastern two-thirds of the lower 48
during the medium range period, with broad ridging generally
preferred near the West Coast. Strong westerly flow over the broad
ridge will bring an atmospheric river into the Northwest with
heavy rain and high elevation snow next week, along with potential
for high winds. Cold weather and clipper systems will be favored
in this pattern east of the Rockies along with enhanced northern
tier wind/snow chances to include to the lee of the Great Lakes.


...Guidance/Predictability Assessment...
Guidance continues to show general agreement with mid-larger scale
systems and the overall pattern evolution early on. From 13/12z
onward, both the 12z Canadian and 12z ECMWF flattened the Western
Ridge and forced downstream forward propagation of the Eastern U.S.
trough, which differed from the other deterministic and ensemble
guidance. While a general model compromise works until that time,
there was weighting towards the 18z GFS with most grids thereafter.
QPF-wise, increased were made from the 01z NBM across portions of
the East, but particularly the Great Lakes. Day 5 QPF across the
Northwest was a special challenge due to significant variability in
the guidance there, including consecutive GFS runs; 18z run was
rather dry and 00z GFS rather wet. Trended continuity and the 01z
NBM downward modestly as a precaution.


...Weather/Hazards Highlights...
The incoming atmospheric river across the Northwest looks strong
and relatively persistent from the 8th possibly into the 11th, with
some wavering in the axis over time.  Precipitable water values
approaching 1.5" not far offshore with 50+ kts of low-level inflow
from the Pacific promises days of heavy rains. Significant multi-
day precipitation totals are possible both with coastal rain and
higher elevation snows across the Cascades. The Slight Risk was
maintained for day 4/10-11th with some slight southward shift based
on the new guidance. A day 5/11-12th Marginal Risk was introduced
as a precaution for western portions of WA to account for the
wetter 00z GFS. Meanwhile, dynamic energies working progressively
inland to drive enhanced periods of snow with max focus into
favored Northern Continental Divide.

Elsewhere, clipper systems will bring rounds of precipitation,
mainly snow, over a cold northern Plains and Midwest, Great
Lakes/Appalachians, and the Northeast next week. Aforementioned
surface cyclo/fronto genesis will act to enhance wrapping activity
and especially Lake Effect with system passages. Wavy and stalling
trailing fronts lingering down off southern Florida and into the
Gulf may also focus uncertain rain chances over/offshore next week.

While a general thaw is noted across the East on Wednesday, the
beginning of a colder air mass invades the northern Plains with 10F
cold anomalies noted, before intensifying and expanding from the
northern Plains southeast across the Midwest and towards the
Southern Appalachians with 20F+ cold anomalies across a broader
swath east of the Rockies Friday into next Sunday.  Locations
across the West -- and portions of the High Plains Wednesday into
Thursday -- will experience 15-25F warm anomalies next week
underneath upper ridging.


Roth/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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