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FXUS66 KOTX 112240 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 240 PM PST Mon Dec 11 2023

SYNOPSIS
Stratus and fog will be the norm as high pressure settles over the region. Anticipate patchy light wintry precipitation Tuesday and again late the week, although accumulations look to be minimal. Meanwhile temperatures should fluctuate either side of the freezing mark through the week.

DISCUSSION

Tonight through Tuesday night: In the wake of the departing upper level shortwave, a ridge of high pressure flops back over the Inland NW with light northwest flow aloft. This pattern persists into Tuesday as the upper ridge axis shifts through the region.
Although the mid and upper levels will be dry, the boundary has an abundance of moisture and will be allowed to expand across much of the lowlands. The pesky fog and and stratus over Spokane will spread its tendrils across the Palouse, Coeur d’Alene area, and Columbia Basin overnight into Tuesday morning. Could see a return of drizzle across parts of the Palouse to the Columbia Basin overnight. Temperatures will be slow to fall overnight across the lower elevations given the stratus deck, although could see lows similar if not a degree or two cooler than this morning, still teetering around the freezing mark.

By Tuesday, the low level winds increase out of the east as surface high pressure increases in western MT, and this will help push the stratus into the east slopes of the northern Cascades. Upslope flow may be enough to generate light rain or snow, or pockets of drizzle. These impacts look to minor with light accumulations that will struggle to measure. Stratus and fog will gradually lift across extreme eastern WA into north Idaho and could see some thinning of the cloud deck by Tuesday night as the low level flow veers to the southeast. Right now, it appears that lows Tuesday night will trend cooler with temperatures dipping into the mid 20s under less cloud cover and locally slick conditions. /rfox.

Wednesday through Sunday: The upper level pattern introduces a ridge positively tilting over the PacNW midweek but not strengthening nor remaining in one place for any length of time. A deep broad trough will dig south out of the Gulf of Alaska through late week and allow for several shortwave features to slide through the PacNW. The first wave Wednesday evening into Thursday brings a rather weak plume of moisture up and over the Cascades (precipitable water anomalies briefly climb to 150% of normal). While the boundary layer temperatures (850mb) have a nudge of warm air pushing north into the Columbia Basin, most of the Cascade Valleys and northern mountains and valleys will remain cold. The upper levels (700-500mb) do see some warmer air push east with the shortwave, but overall the precipitation is just light enough to be of little impact. This said, the precipitation type for these lower elevations and The Basin have the potential to see some freezing rain (only the NBM has small 15-20% percentages) though confidence is just not very good at this time. Snow amounts for the Thursday shortwave are around 1" for the Cascade Crest and the far northern northeast mountains in WA.
Even consulting the high end potential (75-90th percentile) for snow, the snow amounts are 1-3" in the Cascade mountains (not the valleys) and 1-3" in the northern and northeast WA mountains, with similar values in the northern ID Panhandle mountains.

Friday into the weekend the deep trough will cut off a Low in the Eastern Pacific while the mean trough moves through British Columbia and Alberta, leaving the Inland Northwest in a backdoor ridge and rather quiet, cloudy pattern. Temperatures will see little day to day change through the week though the potential is high for busting the forecast if the stratus is thicker or clears up. Persistence temperatures in the 30s and low 40s with overnight temperatures in the 20s and 30s has the highest confidence. /Dewey



AVIATION
18Z TAFS: Many TAF sites experienced improvements to mainly MVFR conditions during the morning hours as the shortwave slowly tracks through the region. The exception being the Spokane area where areas of LIFR conditions will linger longer. Spotty light rian/snow will drift across the Palouse and southern ID Panhandle through the day. Drier air to the west should keep the valleys of north central WA and even northeast WA fog free thorugh the day.
Overnight, high pressure rebuilds and stratus field should expand giving way to renewed degradation of ceilings and visibilities by Tuesday morning.

FORECAST CONFIDENCE AND ALTERNATE SCENARIOS: There is high confidence that stratus will persist with restrictions across the Columbia Basin and reduced levels in the Spokane area. There is moderate to high confidence TAF sites will stay in IFR/MVFR conditions through the period and some VFR conditions could develop near EAT late this afternoon and early evening. There is low confidence that VFR conditions will persist overnight. /rfox

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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 33 37 26 35 28 38 / 10 0 0 0 10 30 Coeur d'Alene 32 38 24 36 27 38 / 10 0 0 0 10 30 Pullman 34 40 27 38 30 39 / 10 0 0 0 0 10 Lewiston 38 45 31 43 32 44 / 10 0 0 0 0 0 Colville 26 36 22 35 24 37 / 0 0 0 0 20 50 Sandpoint 30 37 25 38 26 37 / 0 0 0 0 10 40 Kellogg 33 39 24 40 29 40 / 10 0 0 0 0 20 Moses Lake 33 41 30 39 30 41 / 0 0 0 0 10 10 Wenatchee 32 38 34 37 32 38 / 0 10 10 0 10 10 Omak 30 38 30 38 30 36 / 0 10 10 0 20 30

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

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