716
FXUS02 KWBC 291930
PMDEPD

Extended Forecast Discussion
NWS Weather Prediction Center College Park MD
230 PM EST Mon Dec 29 2025

Valid 12Z Thu Jan 01 2026 - 12Z Mon Jan 05 2026


...General Overview...

Broad cyclonic flow aloft will be in place across the eastern half
of the country for the end of this week, with a deep low pressure
system centered over James Bay, Canada keeping a cold airmass in
place. Meanwhile, an upper ridge will initially be in place across
the West, but this will flatten out as it gets shunted to the east
across the Plains by the weekend. An unsettled weather pattern
over the eastern Pacific will start affecting the West Coast by
Thursday and even more so by Friday with a potentially well
organized storm system reaching central and northern California
into Oregon Friday into early Saturday. Across the southern tier
states, a wave of low pressure develops along a frontal boundary
and brings higher rainfall chances from the Deep South to the
Southeast Coast on Friday.


...Guidance/Predictability Assessment...

Model guidance has remained in good agreement with the general
synoptic pattern across the CONUS through the end of the week,
featuring broad upper-troughing across the central to eastern U.S.
and an upper-ridge over the western U.S. The guidance has also
continued to converge on a solution featuring an upper-wave
interacting with a lingering frontal boundary from the south-
central to southeastern U.S., with a surface low passing through
the region and moist Gulf return flow supporting an expanding area
of precipitation east towards the Atlantic Coast Friday and into
the early weekend. A composite blend of the 00Z ECMWF/CMC and 06Z
GFS is used for the first part of the updated WPC forecast and
follows continuity well. The NBM QPF is a reasonable compromise
solution as well for coverage and amount differences amongst the
guidance across the Southeast.

Uncertainty rapidly increases across the western U.S. as
the flow is expected to flatten and upper-wave(s)/trough(s)
approach the West Coast and pass through the region. A contribution
from the GEFS/ECens means is used up to 50% of the blend by the end
of the forecast period as differences in the deterministic
guidance increase with respect to the phasing of these features.
While the CMC has trended closer to the other guidance it still
remains more of an outlier particularly in the evolution of the
flow from the West Coast and upstream into the northeastern
Pacific, developing a much stronger system off the coast, so it was
removed from the blend. While these phasing differences exist, all
guidance does agree on widespread precipitation across much of the
West, with the heaviest amounts closer to the coast and some
moderate amounts for the interior mountains. Amounts in the NBM QPF
have trended up over the last couple of days and the updated WPC
forecast was increased a bit more based on higher amounts in the
deterministic and ensemble mean guidance for the first part of the
period, and the NBM QPF is a good compromise after as differences
in coverage and amounts in the other guidance increase.

Finally, much of the guidance is suggestive of another clipper-
like system dropping east-southeastward across the Great Lakes and
vicinity late this weekend and into early next week. The blend of
the GFS/ECMWF and ensemble means represented timing differences
well and the WPC QPF forecast was increased across this region as
the NBM remained rather dry.


...Weather/Hazards Highlights...

Lake effect snow is expected from Michigan to Upstate New York
with strong cold air advection across the Great Lakes through
Friday. Some heavy accumulations are likely over the course of
this multi-day event, particularly across portions of Upstate New
York where 1-2 feet is well within the realm of possibility
downwind of Lake Erie and Lake Ontario. Some upslope snows across
the central Appalachians are also likely on Thursday with strong
northwest flow.

In terms of heavy rainfall prospects, a weakening low pressure
system from the subtropics will head in the general direction of
southern California, bringing a modest surge of moisture from the
Pacific. This will bring another round of moderate rainfall to the
Transverse Ranges of southern California and also the Peninsular
Range for Thursday, and a Marginal Risk is valid for the Day
4/Thursday ERO as this area remains sensitive given recent heavy
rains. The risk for heavy rainfall will expand northward to include
northwestern California and the eastern slopes of the central and
northern Sierra by Thursday into Friday where Marginal Risk areas
are valid on both days, and heavy snowfall is also expected for
the higher terrain of the Sierra Nevada as precipitation becomes
heavier and more widespread as a stronger Pacific storm system
approaches the coast. Rainfall chances look to trend downward for
southern California especially after Saturday while remaining
potentially heavy for northern California through the weekend and
into early next week. Farther inland, some moderate snows will be
possible for the interior mountains of the Intermountain West.
Elsewhere, low pressure developing across the Deep South Friday
into Saturday will likely result in increasing rainfall coverage
and some thunderstorms to close out the week and start the weekend.

Temperatures will continue to be mild for this time of year across
much of the western half of the country through this forecast
period as a general upper ridge limits arctic airmass intrusions.
The warmest anomalies should be across the western High Plains and
western Texas, where highs could be 10 to 20+ degrees above
average. The opposite will hold true from the Upper Midwest to the
Northeast where the upper trough will tend to keeps readings about
5 to 15 degrees below normal, with perhaps some moderation going
into Sunday and Monday.

Putnam/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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