859
FXUS02 KWNH 150757
PREEPD

Preliminary Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
256 AM EST Mon Dec 15 2025

Valid 12Z Thu Dec 18 2025 - 12Z Mon Dec 22 2025


...Another significant atmospheric river will impact the Pacific
Northwest, particularly on Thursday...


...Overview...

As the medium range period begins Thursday, an upper trough will
be present atop the north-central U.S. accompanied by a surface low
pressure system, spreading some precipitation to the eastern third
of the lower 48. After that trough pivots east into Friday, quasi-
zonal flow will again set up across the country with generally
fast-moving and shallow shortwaves. Upstream, strong westerly flow
will force another atmospheric river (AR) into the
Northwest/northern California on Thursday. Additional flooding is
possible especially in areas that have seen excessive rainfall
recently, and heavy snow is likely in the higher terrain. This AR
should drift southward on Friday but additional rounds of
precipitation are possible in the Northwest into the weekend and
early next week. Temperature-wise, the flat flow will allow for
above average temperatures for most of the CONUS as astronomical
winter begins, though the northern tier can expect colder periods
with shortwaves moving through.


...Guidance/Predictability Assessment...

Model guidance shows minor spread with the initial trough in the
north-central U.S. on Thursday moving eastward into late week and
its accompanying surface low. The 12Z and new 00Z ECMWF are a tad
slow compared to consensus with the low initially, and UKMET runs
become slow to pivot the trough axis eastward. Still, a multi-model
blend including the ECMWF/GFS/CMC runs worked well for this
feature. Deterministic and AI models also show reasonable agreement
regarding Thursday`s AR moving into the Pacific Northwest,
focusing in Oregon in particular. Since even relatively small model
differences in the orientation and placement could be impactful,
will continue to monitor.

Behind this AR, the flat flow with shallow troughs/ridges makes it
hard to determine model outliers. For the 12/18Z cycle, some
differences did arise regarding potential weak troughing across the
northern tier over the weekend. The ECMWF and GFS (especially the
12Z) were supporting slightly deeper troughing across the northern
Rockies/Plains Saturday moving into the Great Lakes by Sunday, but
their AI counterparts and the CMC were more zonal. The CMC even
ended up out of phase with minor ridging over the Great Lakes on
Sunday. Fortunately it appears the 00Z guidance is converging
better, showing a shallower solution for this trough with more
agreeable timing across the models. Then, northeast Pacific energy
may approach the Northwest once again early next week. The 00Z GFS
seems to be faster/deeper with this energy entering the Northwest
compared to other guidance including the AI-GFS. The WPC forecast
increased the proportion of ensemble means as the period progressed
to account for these model differences, with means comprising half
the blend Day 6 and more Day 7 as model spread increased for the
details of the low amplitude pattern.


...Weather/Hazards Highlights...

The Pacific Northwest can expect heavy precipitation on Thursday
as a moderate to strong AR impacts the region. Forecast guidance is
in reasonably good agreement that the AR will be directed
primarily at Oregon, with global models indicating 5-7+" of QPF
given the strong IVT. Thus a Slight Risk remains in place for the
Day 4/Thursday ERO across western Oregon into far southern
Washington. The exact positioning of the AR will make a difference
in terms of how much overlap there is with the previous couple of
extreme AR/rainfall events and thus the sensitivity of the
antecedent conditions; the farther north the AR ends up, the more
overlap. On Friday, the AR is forecast to gradually move southward
into northern California with the IVT starting to weaken. A
Marginal Risk is in place across portions of western Oregon and
northern California for Day 5/Friday. Meanwhile, heavy snow is
likely into the Cascades and into the northern Rockies with the
ample moisture combining with sufficiently cold temperatures.
Lingering precipitation is possible into Saturday across the West
Coast states and interior West, though with lesser amounts. But
moisture surges are once again possible on Sunday for additional
heavy precipitation, though it will take additional time to refine
the details.

The upper trough moving from the north-central U.S. into the Great
Lakes and through the Northeast Thursday and Friday will provide
forcing for a Midwest to southeast Canada surface low
pressure/frontal system. Some moisture will be pulled north from
the Gulf ahead of the cold front, allowing for broadening
precipitation chances across the eastern third of the U.S.
Thursday. Most of this precipitation is likely to be rain except in
the Great Lakes to interior Northeast. Most precipitation should
be pulling away on Friday, but with some lake effect precipitation
possible behind the main system. Light to moderate precipitation
(mostly rain) may get renewed across the east-central U.S. over the
weekend, though with the details uncertain.

Warmer than average temperatures are forecast across the western
and central U.S. through much of the period. The highest anomalies
will likely be across the Plains most days, with temperatures
generally 15-25 degrees above normal, locally even higher. On
Friday highs in the 60s should expand into western Nebraska and
South Dakota, and by Saturday temperatures in much of Texas should
reach the upper 70s to low 80s. The eastern U.S. can expect more
transient above average temperatures around Thursday and perhaps
again into next weekend, but with periods of near to below normal
temperatures in between. Meanwhile the northern tier is forecast to
see shots of cold air with shortwaves and cold surface highs
behind cold fronts.


Tate


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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