Winter Weather in Washington, December 5 to 7, 2024

Area Forecast Discussion
National Weather Service Spokane WA
909 PM PST Wed Dec 4 2024

.SYNOPSIS...
A dry and stable weather pattern will persist with temperatures
seasonably cool in the mid 20s to mid 30s. Chances are increasing
that the ridge of high pressure will weaken by the weekend with a
return of light wintry precipitation.

&&

.DISCUSSION...
Tonight through Friday: The ridge will continue which will promote
stratus and fog for all the valleys and lowlands. A near persistent
forecast is expected for the next 24 hours, where the temperatures
only vary a few degrees and the cloud heights may move up or down
a couple hundred feet at best. We have had some periods of very
light snow or freezing drizzle fall out of the stratus, and that
will continue at times through Friday. Late Thursday a weak wave
that is shearing apart will move over the ridge. This will be
enough to perturb the stratus to bring additional very light
freezing drizzle or flurries. Friday the ridge begins to flatten.
We will see a 20% chance of light wintry precipitation in the
Cascades during the afternoon. /Nisbet

Friday night through Sunday: A shortwave trough will come barreling
into the Northwest flattening the upper-level ridge. Our string
of "dry" days will come to a close as moist frontal band passes
through. Precipitation will reach the Cascades first starting
around midnight or shortly thereafter and spread east to west
Saturday morning and be exiting the east around sunset. QPF
amounts will vary from less than a tenth in the lee of the
Cascades and and western Basin to 0.10-0.35" in the rising terrain
of Eastern WA increasing closer to 0.50" into North Idaho. The
Cascade Crest will intercept 1 to 2 inches of QPF while the higher
peaks in N Idaho and far NE WA are closer to the 0.50-1.00". So
the million dollar question is...What precipitation type does this
fall as?

The current ridge has created a strong subsidence inversion with
temperatures between 4000-8000 feet well above freezing. Below
the inversion, temperatures are in the upper 20s to lower 30s. The
air mass aloft is dry and will cool as it moistens however this
will be competing with increasing southwest flow at 850 mb and
warm air advection. All things considered, it will be a messy
precipitation event with not only thermal profiles dictating
precip type but also precip intensity. Given the current data,
best chance for a wintry mix of rain, sleet, and freezing rain
will be over the lowlands of Central WA into the northern valleys
(30-60%) chance for at least a brief period of freezing rain or
sleet. Would not rule out the remainder of the Columbia Basin and
into the West Plains though this comes with more of a 10-30%
chance for at least a brief period. Probabilities lower closer to
10% for locations like Pullman, Lewiston, and Winchester.

There is moderate confidence for this warm layer to initially melt
precipitation aloft. The uncertainty is what will the surface
temperatures be and how deep will the cold air be? If shallow,
freezing rain could be more common vs sleet. Southerly winds
developing over SE WA and the lower ID Panhandle are responsible
for the lower aforementioned probabilities for freezing rain and
potential to be above freezing.

In the mountains, I also anticipate a wintry mix initially or
potentially rain. As the air mass moistens/wet-bulbs, snow levels
will lower. Values will be lowest along Highway 20 toward the
Canadian border which will dip near 3000 feet. For areas
southward, snow levels will lower between 3500-5500 feet with
time. Precip type at Stevens Pass will start as rain or freezing
rain and transition back to snow around midday. Motorists should
be prepared for winter driving conditions over Stevens Pass
Saturday afternoon into Saturday night. A similar message for
Lookout Pass with light snow amounts.

The main precipitation band will depart to the east Saturday evening
with a cold, unstable trough settling into the region. 500mb
temperatures will crash from -15C to -34C. Snow levels will lower
back near valley floors and the stagnant inversion will be a story
of the past. Valleys and mountains can expect temperatures more
common of December. Additional precipitation chances will come in
the form of clusters of convective showers drifting northwest to
southeast. There is low confidence for placement and timing of
this activity. Mountain passes will carry the highest probability
for an inch or more but in this unstable environment, would not
rule local accumulations in other areas.

Monday through Thursday: There will be a subtle battle ongoing between
another ridge nosing northward and shortwaves dropping down from
the northwest. Majority of the ensembles are leaning toward light
snow showers in the Idaho Panhandle Monday then dry region-wide
Tuesday. Confidence falters by midweek as ensemble members show
nearly a 50/50 split of the ridge flattening and more
precipitation arriving OR holding strong and remaining dry.
Ensemble means seem to be trending drier (less QPF) but still
enough to have the National Blend of models maintain appreciable
PoPs for the Dec 12-13th period. Stay tuned. /sb

&&

.AVIATION...
06Z TAFS: LIFR/IFR stratus continues to cover most of the region,
and is not expected to budge through at least Friday morning. The
lowest visibilities will occur where the stratus intersects the
terrain. Lower elevation TAF sites will have VFR vis, while
slightly higher elevation sites including GEG and PUW will
fluctuate between LIFR and MVFR vis. The forecast was based
largely on persistence with the stratus sloshing around the lower
elevations and ceiling heights only varying a couple hundred feet
through the period. There is potential for very light freezing
drizzle to fall out of low stratus layers. Highest chances for
freezing drizzle will be across the Spokane area and Palouse.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS:
Confidence is high of stratus plaguing the region through the
period. Models have a pretty good handle on the stratus across the
Basin into the Palouse and Spokane area, but in the valleys to the
north and in the Cascades they are not doing a good job. There is
a 50% chance of vis going to 1 mile at GEG overnight and a 40%
chance at PUW.

-----------------------

Confidence descriptors:
Low - Less than a 30 percent chance
Moderate - 30 to 70 percent chance
High - Greater than a 70 percent chance

For additional probabilistic information for NWS Spokane airports,
please refer to the Aviation Dashboard on our webpage:
https:/www.weather.gov/otx/avndashboard

&&

.PRELIMINARY POINT TEMPS/POPS...
Spokane        28  33  29  34  30  40 /   0   0   0   0  10  80
Coeur d'Alene  30  34  31  35  30  41 /   0   0   0   0  10  80
Pullman        28  32  29  38  31  43 /   0   0   0   0  10  80
Lewiston       32  37  33  40  35  48 /   0   0   0   0   0  60
Colville       26  33  30  33  25  33 /   0   0   0   0  30  80
Sandpoint      29  33  31  34  29  35 /   0   0   0   0  30  90
Kellogg        28  36  29  39  35  42 /   0   0   0   0  10  90
Moses Lake     31  33  31  32  28  37 /   0   0   0   0  10  70
Wenatchee      29  33  30  31  31  37 /   0   0   0   0  30  70
Omak           31  36  32  32  30  34 /   0   0   0   0  40  60

&&

.OTX WATCHES/WARNINGS/ADVISORIES...
ID...None.
WA...None.
&&