795
FXUS02 KWBC 151958
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
258 PM 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...

20Z Update: The 12Z model guidance suite maintains good overall
agreement and continuity with the previous WPC forecast, with a
general deterministic model blend working well as a starting point
for fronts and pressures. In terms of adjustments to the NBM, QPF
was raised some across the Great Lakes and more so around Lake Erie
where lake effect snow is expected in the wake of the departing
storm system. Temperatures were raised several degrees across
portions of the Northeast U.S. to account for stronger warm air
advection ahead of the front, closer to NBM 90 percentile. Across
the West, snow-to- liquid ratios were slightly reduced from NBM
across the Cascades and the Wasatch Range to account for higher
moisture content in conjunction with the atmospheric river as it
moves inland. Winds were also increased some across the Front Range
of the Rockies for late this week. Otherwise the NBM appeared to
be okay for most aspects of the forecast, and the previous forecast
discussion is appended below for reference. /Hamrick
---------------------

...General 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 and
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, although it has been adjusted a little to the south to
account for trends in the 12Z guidance. 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 and southwestern
Oregon, where a Slight Risk area has been introduced. 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. A Marginal Risk of excessive rainfall has been introduced
for portions of New England on Friday with a band of heavier
rainfall moving in ahead of the front, and in combination with snow
covered and frozen grounds, this makes the region more susceptible
to greater run-off issues. Some lake effect snow is likely 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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