962
FXUS21 KWNC 042005
PMDTHR
US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EST December 04 2025

SYNOPSIS: Mid-level low pressure across the northeastern Pacific and eastern
Alaska downstream from a narrow but strong mid-level high pressure ridge
centered just west of the Alaska Mainland are expected to entrench very cold
air and dangerous wind chills across central and southern Mainland Alaska,
along with episodes of high winds across southern Alaska from the Aleutians
into Southeast Alaska. This mid-level low pressure should enhance moist Pacific
flow across the northwestern Contiguous U.S. (CONUS) late week-1, with
increased chances of heavy precipitation, heavy high-elevation snow, and high
winds lingering into week-2 across this area. Farther east, the pressure
gradient between surface low pressure moving away from the Northeast and cold
high pressure in its wake increases the odds for high winds over parts of the
Great Lakes, Northeast, central Appalachians, and mid-Atlantic. This set-up
also enhances the likelihood of heavy lake-effect snows on the lee side of the
Great Lakes and in the central Appalachians.

HAZARDS

Moderate risk of much below normal temperatures for most of southeastern
Mainland Alaska and the Panhandle, Fri-Mon, Dec 12-15.

Slight risk of much below normal temperatures for much of central, eastern, and
southern AK, Fri-Thu, Dec 12-18.

Moderate risk of high winds across southern Alaska from the eastern Aleutians
through the central Panhandle, Fri-Thu, Dec 12-18.

Slight risk of high winds, southern Alaska from the central Aleutians through
Southeast Alaska, Fri-Thu, Dec 12-18.

Slight risk of high winds, northern and central Rockies and adjacent High
Plains, northern Great Basin, and the Northwest, Fri, Dec 12.

Slight risk of heavy precipitation across the Pacific Northwest, northern
Intermountain West, and northern Rockies, Fri, Dec 12.

Slight risk of heavy snow across the Washington Cascades, Fri, Dec 12.

Slight risk of heavy snow across part of the northern Rockies and adjacent High
Plains, Fri, Dec 12.

Slight risk of heavy snow across the Upper Peninsula of Michigan, the lee side
of Lake Michigan, and adjacent areas, Fri-Sun, Dec 12-14.

Slight risk of heavy snow across the central Appalachians and the lee sides of
Lakes Erie and Ontario, Fri-Sun, Dec 12-14.

Slight risk of high winds for the Great Lakes, Northeast, and northern portions
of the mid-Atlantic, Fri-Sun, Dec 12-14.

Flooding possible across part of the Pacific Northwest.

DETAILED SUMMARY

FOR SUNDAY DECEMBER 07 - THURSDAY DECEMBER 11:
https://www.wpc.ncep.noaa.gov/threats/threats.php

FOR FRIDAY DECEMBER 12 - THURSDAY DECEMBER 18: The 0z ECENS, GEFS, and Canadian
ensemble means all depict anomalous 500-hPa troughing moving into the Atlantic
through early week-2. This should strengthen surface low pressure heading away
from the Northeast. Meanwhile, cold high pressure is expected to push into the
Great Lakes and eastern CONUS behind the storm system. The pressure gradient
between these two systems may result in periods of high winds across the Great
Lakes, Northeast, and northern mid-Atlantic, where a slight risk of high winds
is posted for the first few days of week-2. These winds will advect cold air
southward from northern Canada into the region, potentially resulting in low
wind chills and scattered heavy lake-effect snow squalls. A slight risk of
heavy snow is posted for the first few days of week-2 in areas climatologically
vulnerable to such conditions, on the lee side of each Great Lake. The heavy
snow may reach as far south as the central Appalachians, where upsloping flow
may often enhance snowfall amounts. As the low pressure moves further away from
the CONUS, the surface high pressure is expected to slide southeastward and
slowly weaken. This should relax the pressure gradient, ending the high wind
and heavy snow threats by the middle of the period.



The 0z ECWMF, GEFS, and Canadian ensembles depict a narrow but highly-amplified
mid-level ridge centered west of Alaska, with a fairly amplified downstream
mid-level trough established over southeastern Alaska and the adjacent
northeastern Pacific. This set-up favors persistent surface high pressure
across much of the state that keeps exceptionally cold air entrenched over much
of Alaska outside the northern tier (where extremely cold air is more common)
and western tier (closer to the mid-level ridge). The Probabilistic Extremes
Tools (PETs) from all three ensemble suites have shifted the highest chances
for much below normal temperatures (below the 15th percentile) farther
southeast today. The GEFS PET shows a 60 to 80 percent chance for temperatures
below the 15th percentile from southeastern Mainland Alaska through the
Panhandle on day 8 (Dec 12). The ECENS PET is less robust again today, keeping
odds for such temperatures in the 40 to 55 percent range over approximately the
same area. The Canadian PET approximately splits the difference. The GEFS and
ECENS means keep temperatures slightly above hazards thresholds, but the
Canadian ensemble mean is colder, and the dynamical ECMWF, GFS, and Canadian
models all show temperatures dropping below -40 deg F periodically in parts of
east-central and southeastern Alaska, with isolated areas falling below -55 deg
F at times. Wind chills are expected to be even lower, and a parallel run of
the Canadian model shows wind chills briefly dropping to between -55 deg F and
-65 deg F in a few spots. There is some uncertainty regarding how conditions
evolve over the course of week-2. The ECENS and GEFS means allow temperatures
to slowly moderate over the course of the period, especially in southwestern
parts of the state closer to the mid-level ridge. In contrast, the colder
Canadian ensemble mean changes little over the course of the week, with slight
changes limited to some moderation in western parts of the Mainland and in the
coldest valleys. Given the consistency in the guidance, a moderate risk of much
below normal temperatures is posted from the southeastern Mainland through the
Panhandle, with a slight risk extending into much of the eastern, central, and
southern parts of the state. Model trends have been to keep the colder air in
place longer, so the moderate risk extends into the middle of week-2, and the
slight risk is in place through all of the week-2 period. Meanwhile, south of
the strong surface high pressure covering most of the state, low surface
pressures are favored near the mid-level trough in the North Pacific near the
southern part of the state. The higher pressure over the Mainland and lower
pressure to the south set up a tight pressure gradient along the southern tier
of the state, increasing the risk of high winds there. The models are not
consistent in their depictions of individual surface storm systems over or near
the Gulf of Alaska. The pressure gradient and thus winds will wax and wane at
any given location as these surface storm systems evolve, but the pattern
favoring high winds across the southern tier of the state should stay in place
throughout week-2. As a result, a moderate risk of high winds is posted across
southern Alaska near the coastline from from the eastern Aleutians to central
portions of the Panhandle for the entirety of week-2. Models show episodes of
strong winds most consistently affecting this region. A slight risk of high
winds surrounds this region, stretching farther west into the Aleutians and
farther southeast through the entire Alaska Panhandle. The slight risk also
extends farther away from the coastline into some of the higher elevations of
the southern interior. The combination of very cold weather and high winds
along the southern tier of the state increases the threat for extreme amounts
of freezing spray at times near larger bodies of water.



The mid-level low pressure near the North Pacific will be at least temporarily
easing its grip on the northwestern CONUS early week-2. There is some disparity
among the dynamical models, a few of which have precipitation out of the
Pacific Northwest later week-1, although this minority solution is an outlier
and not preferred at this time. Overall the PETs and ensemble means are similar
to yesterday on day 8 (Dec 12), showing conditions in the process of improving.
Still, enhanced chances for heavy precipitation, heavy high-elevation snowfall,
and high winds appear to linger into week-2 on most of the guidance, so hazards
for slight risks of heavy precipitation and high winds are in place for the
first day of week-2. Some adjustments were made along the southern tier of the
heavy precipitation risk to better align with today`s PETs, but the slight risk
of high winds covers the same areas as yesterday. Slight risks for heavy snow
remain in place across the higher elevations of the Washington Cascades and the
northern Rockies, but these areas do not extend as far south as yesterday,
consistent with changes in the GEFS Snow Water Equivalent (SWE) PET, the ECENS
and CNENS means, and a slightly warmer temperature profile. Later in week-2,
multiple models and PETs indicate the potential for stormy weather to redevelop
over part of the western CONUS, but the models are in fair agreement at best
regarding the evolution, timing, and intensity of conditions. In general, the
PETs and ensemble means favor heavier precipitation to shift farther south
across northern and central California around the middle of the week, then pull
back farther to the north toward the end of the period, but not much else can
be specified at this time. Due to expected heavy rainfall late week-1 into the
first day of week-2, the potential for flooding continues across western
Washington and adjacent Oregon.

FORECASTER: Rich Tinker

$$