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FXUS66 KOTX 280605 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 1005 PM PST Mon Nov 27 2023

SYNOPSIS
Persistent high pressure through the middle of the week will keep the region precipitation-free and stagnant with little variation in temperatures each day. Freezing fog and low clouds will be a daily concern as well. A pattern change at the end of the week will bring a round of snow for portions of the Inland Northwest.

DISCUSSION

As of 10PM, the mid cloud deck (3000-4000ft) across north central Washington has no dissipated as most of the models have suggested.
NAM forecast soundings hold onto a thin layer of moisture in that layer in the vicinity of a strong subsidence inversion. It is not surprising that most models want to disperse these clouds too fast. Our forecast for tonight into Tuesday has been updated for more persistent clouds for Wenatchee, Omak, Republic and Grand Coulee. Overnight temps have been warmed a few degrees with the anticipated cloud cover.

PREVIOUS DISCUSSION

An Air Stagnation Advisory has been issued through 11 AM Thursday for much of Central and Northeast Washington into the Idaho Panhandle. Deteriorating air quality conditions thanks to poor mixing and very light winds anticipated. Locally gusty winds on the Palouse and down through the LC Valley is expected to persist at times into mid-week and thus no advisory issued for this area.

Through Wednesday night: High pressure will continue across the Inland Northwest. This makes for generally light winds and a precipitation-free forecast, but also with stagnant conditions leading to nuisance freezing fog and periods of low clouds. The low stratus deck has mainly dissipated across northern ID and much of far eastern WA, but is proving more stubborn across the portions of the basin and far N WA and points westward into the eastern slopes of the Cascades.
Patchy fog development expected overnight in mountain-adjacent valleys. Daytime temps to run near to slightly below normal through Wednesday as well. /KD

Thursday through Saturday: The ridge begins to flatten and shift East. The Pacific Northwest will impacted by a moisture plume being pushed into the region by a Gulf of Alaska Low and a high pressure off the California Coast. Ensembles are bringing a series of shortwaves through the region starting Friday and lasting over the weekend. The Inland Northwest can expect several rounds of precip through the weekend. With temperatures hover near freezing, it is expected to be a widespread snow event for the region. The amounts across the Basin are near an inch. The Cascades mountain passes could see 8 to 14 inches. The Idaho Panhandle could get 1 to 4 inches. Some ensembles are indicating breezy winds for the Eastern Basin with gusts into the 30s possible late Sunday. It will be something to monitor. Highs for the period will be in the 30s. Lows will be in the upper teens and 20s. /JDC

AVIATION
06Z TAFs: Mainly clear skies should be the prevailing condition across the Idaho Panhandle and east central Washington. Dry surface dewpoints and light easterly (downslope flow) is a good combination for keeping skies mainly clear for Coeur d'Alene, Spokane and Pullman. A 3000-4000ft deck has persisted longer than most model guidance has suggested for Omak, Colville, Chelan, Grand Coulee and Wenatchee. A slow erosion of these clouds will be possible as subsidence increases over the next 24 hours, but clearing has been significantly slower than HRRR and other ensemble based guidance suggests.

FORECAST CONFIDENCE AND ALTERNATE SCENARIOS: Low confidence in fog development for Spokane given the low dewpoints and relatively dry ground. As of 06z, river fog was developing on the Snake River in SE Washington and expanding toward Lewiston. At times, river fog doesn't climb to the elevation of the airport, but fog has been added to the Lewiston TAF. Confidence is high that fog will spread into Moses Lake, but the timing of arrival could be as early as 08z or as late as 15z depending on subtle shifts between east or northeast wind directions. /GKoch

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 20 36 19 31 19 33 / 0 0 0 0 0 0 Coeur d'Alene 20 38 21 34 20 34 / 0 0 0 0 0 0 Pullman 22 39 21 36 20 35 / 0 0 0 0 0 10 Lewiston 25 42 26 37 25 39 / 0 0 0 0 0 0 Colville 24 36 15 34 13 34 / 0 0 0 0 0 10 Sandpoint 20 36 20 34 19 32 / 0 0 0 0 0 10 Kellogg 27 42 27 38 25 37 / 0 0 0 0 0 10 Moses Lake 17 34 18 31 17 34 / 0 0 0 0 0 0 Wenatchee 28 38 27 37 25 36 / 0 0 0 0 0 0 Omak 27 39 22 36 22 35 / 0 0 0 0 0 0

OTX WATCHES/WARNINGS/ADVISORIES
ID...Air Stagnation Advisory until 11 AM PST Thursday for Central Panhandle Mountains-Coeur d'Alene Area-Northern Panhandle.

WA...Air Stagnation Advisory until 11 AM PST Thursday for Central Chelan County-Moses Lake Area-Northeast Mountains-Okanogan Highlands-Okanogan Valley-Spokane Area-Upper Columbia Basin- Waterville Plateau-Wenatchee Area-Western Chelan County- Western Okanogan County.

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