613
FXUS21 KWNC 152017
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST December 15 2025

SYNOPSIS: A persistent pattern featuring strong mid-level low pressure over the
North Pacific favors an elevated risk of Atmospheric River (AR) activity to
promote multiple weather related hazards, including high winds, heavy
precipitation, and high-elevation snow across the western Contiguous U.S.
(CONUS) throughout week-2. The strongest AR potential is expected over the
lower half of the West Coast before this risk retreats northward to the Pacific
Northwest later in the week-2 period. Surface low pressure over the Great
Plains along with surface high pressure over the Southeast U.S. are expected to
induce potentially hazardous winds over much of the Plains, Mississippi Valley,
and western Great Lakes. Farther north, strong surface high pressure over the
Yukon and interior Alaska is favored to entrench extremely cold air into
southeastern Alaska early in the period.

HAZARDS

High risk of heavy precipitation for coastal California and southwestern
Oregon, Tue-Thu, Dec 23-25.

High risk of heavy snow for the Sierra Nevada and Klamath mountain ranges,
Tue-Fri, Dec 23-26.

Moderate risk of heavy precipitation for southwestern Oregon, western Nevada,
and California, Tue-Fri, Dec 23-26.

Moderate risk of heavy snow for the Klamath Range, and Sierra Nevada, Tue-Sat,
Dec 23-27.

Moderate risk of heavy precipitation for the Pacific Northwest, Fri-Mon, Dec
26-29.

Moderate risk of heavy snow for the Cascades, Thu-Mon, Dec 25-29.

Moderate risk of high winds for the West Coast and the Interior West, Tue-Mon,
Dec 23-29.

Moderate risk of heavy snow for the Northern Rockies and Northern
Intermountain, Tue-Mon, Dec 23-29.

Slight risk of heavy snow for the Cascades, Klamath, and Sierra Nevada,
Tue-Mon, Dec 23-29.

Slight risk of heavy snow for the Central and Northern Rockies, parts of the
Great Basin and Northern Intermountain, Tue-Mon, Dec 23-29.

Slight risk of heavy snow for the lower Four Corners, Wed-Fri, Dec 24-26.

Slight risk of heavy precipitation for the West Coast and portions of the
Desert Southwest, Tue-Mon, Dec 23-29.

Slight risk of high winds for much of the central and western Contiguous United
States (CONUS), Tue-Mon, Dec 23-29.

Moderate risk of much below-normal temperatures for southeastern Alaska, Tue,
Dec 23.

Slight risk of much below-normal temperatures for much of southern Mainland and
Southeast Alaska, Tue-Mon, Dec 23-29.

Slight risk of high winds for much of southern Alaska, including the Alaska
Range, Tue-Mon, Dec 23-29.

Flooding possible along much of the West Coast.

DETAILED SUMMARY

FOR THURSDAY DECEMBER 18 - MONDAY DECEMBER 22:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR TUESDAY DECEMBER 23 - MONDAY DECEMBER 29: Heading into early next week,
there continues to be good agreement in the dynamical models featuring a highly
amplified 500-hPa pattern over North America, consisting of a potent ridge over
the north-central Pacific, with a strong mean trough downstream that extends
from the higher latitudes of Canada into the eastern North Pacific. Both of
these mid-level features remain the primary focus for multiple weather hazards
during week-2, namely in regards to an elevated risk for well below-normal,
bitter cold temperatures over much of Alaska, Atmospheric River (AR) activity
affecting the West Coast, along with unsettled conditions throughout much of
the Interior West. As the period progresses, model guidance shows some
variations in the height pattern, however much this appears to be associated
with additional shortwave energy propagating along these longwave features.
Overall, the pattern looks to remain pretty stable, implying the persistence of
potentially hazardous weather for much of western North America through the end
of December.



Over the past few days, the GEFS, ECMWF, and Canadian ensembles feature a more
amplified and deeper 500-hPa trough off the West Coast which is expected to
bring a period of enhanced onshore flow further south into portions of Southern
California. While the GEFS remains comparably the weakest with the trough,
there is good agreement in Integrated Vapor Transport (IVT) tools depicting the
southward solution of the AR potential, with enhanced probabilities for IVT
values exceeding 250 kg/m/s reaching as far south as 30N though the middle of
the period before retreating northward. The precipitation responses are also
more pronounced in the Probabilistic Extreme Tools (PETS), where both the GEFS
and ECMWF calibrated solutions show at least 50% chances for 3-day amounts
exceeding the 85th percentile throughout California, with the strongest signals
focused from the Bay Area southward. Based on this wet trend, an expansion of
the high risk of heavy precipitation is warranted, with its coverage extended
to include southern California and valid for Dec 23-25 where the uncalibrated
ECMWF indicates at least 50% chances for 3-day amounts exceeding 2 inches early
in the period.  Based on raw snow tools indicating 50-60% chances for 3-day
amounts exceeding a foot through the middle of week-2, an accompanying high
risk of heavy snow remains posted over the higher elevations of the Sierra
Nevada and Klamath ranges though Dec 26. Surrounding these high risk areas,
moderate risk of heavy precipitation and high elevation snow are posted over
California through Dec 26 and 27, respectively.



Later in the period, much of the mean mid-level troughing is favored to
gradually deamplify, resulting in lesser precipitation amounts over the
southern half of the West Coast towards the middle of the period.  However
there continues to be ensemble support for the mean troughing to restrengthen
near British Columbia, leading to a renewal of enhanced onshore flow that
becomes more focused over the Pacific Northwest by the middle of week-2.
Again, the GEFS is weaker with this potential, though there is good agreement
between the ECMWF and Canadian raw tools depicting the return of well
above-normal precipitation amounts by day 11 (Dec 26). Therefore, separate
moderate risk areas of heavy precipitation and high elevation heavy snow are
posted over the Pacific Northwest and are valid for Dec 26-29, and Dec 25-29,
respectively.  To capture the evolution of the AR activity throughout the
period, broader slight risk areas of heavy precipitation and high elevation
heavy snow are posted over the West Coast for the entirety of week-2, with the
heavy precipitation risk now extending into the Desert Southwest based on
increased support of 3-day amounts exceeding the 85th percentile and half an
inch in the PETs.  It is worth noting that should the strongest AR potential be
realized over the more heavily populated areas of coastal California, this is
likely to cause major disruptions to travel over the holiday season. In
addition, a flooding possible hazard remains issued and now covers the entire
West Coast in the updated outlook. Flooding would be likely especially in areas
where antecedent conditions are abnormally wet. Continued precipitation
accumulations on saturated soils also brings the additional risk of landslides
and debris flows in steep terrain or burn scars.



The incoming moisture associated with this enhanced jet is favored to
overspread much of the Interior West, leading to an elevated risk of heavy snow
for many parts of the Rockies, Great Basin and Northern Intermountain. This
continues to be supported by both the precipitation and Snow Water Equivalent
PETs, as well as the raw snow tools from the GEFS and ECMWF.  A slight risk of
heavy snow remains posted for a broad area of the Interior West, along with a
moderate risk embedded within this area over portions of the Central and
Northern Rockies where signals are highest for amounts exceeding a foot, both
valid for all of week-2. Relative to yesterday, the corresponding moderate risk
area is expanded northward to the Canadian border due to the aforementioned
potential for renewed onshore over the Pacific Northwest later in week-2.  In
addition, a separate slight risk area of heavy snow is posted over the lower
Four Corners (Dec 24-26) associated with the deep troughing favored initially
in the period, as locally heavy snowfall is possible over the higher elevations
of Arizona and New Mexico.



Accompanying the risks of heavy precipitation and heavy snow, the elevated
potential for AR activity also supports periods of high winds reaching hazard
criteria throughout the period.  At the base and ahead of the mean troughing, a
moderate risk remains posted for much of the western CONUS and is expanded
southward to cover more of the southwestern CONUS where PETs show elevated
chances for winds exceeding the 85th percentile. Farther east, individual
shortwaves will periodically be ejected eastward from the North Pacific trough,
inducing one or more episodes of cyclogenesis over the Plains. Surface high
pressure should be fairly persistent downstream from the axis of the mid-level
ridge covering the CONUS. Between these systems, a tight pressure gradient may
periodically be established across the central CONUS, somewhat increasing the
odds for episodes of high winds across the central CONUS. With the pattern
changing little over the course of week-2, a slight risk of high winds is
supported for the entire period.



Over Alaska, much of the anomalous 500-hPa ridging over the Bering Strait late
in week-1 is favored to retrograde and allow for more troughing and a southerly
flow component to develop over the Mainland and Southeast heading into week-2.
As a result, a moderation of the coldest temperatures are expected, as both
GEFS and ECMWF PETs indicate lower chances for minimum temperatures falling
below the 15th percentile relative to yesterday. However, this warming may be
short-lived as the ridging looks to return to the western Mainland and promote
more northerly, Arctic flow into the state. PETs are fairly muted with the
return of extreme cold, though there is a more of a colder signal emerging
across the southwestern Mainland by this lead.  Taken all together, a moderate
risk of much below normal temperatures remained posted, but is now valid for
Dec 23, with a broader slight risk area remaining posted and expanded westward
to the Norton Sound for all of week-2.  A moderate risk of much below normal
temperatures may be added for future dates should there be better support in
the PETs with the evolving pattern. High winds also remain possible in gaps and
inlets, and close to the lowest surface pressures, but the precise locations of
the highest winds will vary from day to day. Probabilistic Extremes Tools
(PETs) show enhanced chances of wind speeds exceeding the 85th climatological
percentile. Therefore, a slight risk of high winds is posted for most of
southern and southeastern Alaska for all of week-2. The very cold weather and
high winds along the southern tier of the state increases the threat for heavy
freezing spray at times near larger bodies of water.

FORECASTER: Nick Novella

$$