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FXUS66 KOTX 300619 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 1019 PM PST Wed Nov 29 2023

SYNOPSIS
Stagnant conditions with periods of freezing fog and low clouds will continue through early Thursday. A significant pattern change at the end of the week will lead to several rounds of snow for most of the Inland Northwest with the potential for heavy snow near the Cascades. Much warmer weather will develop early next week with the wet weather continuing.

DISCUSSION

Areas of freezing drizzle has been added to the forecast for tomorrow morning (Thursday) across the Palouse, Spokane area, and West Plains. Ceilings at or below 500 feet have been common in these areas over the last 12 to 24 hours and the stratus depth has been gradually increasing. Spotter reports have been relayed to the NWS of spotty, light freezing drizzle this evening. Drizzle forecasting is tough/low confidence endeavor, but there are a couple of factors that support increasing chances by morning. A slow moving shortwave tracking across southern Washington is evident on satellite imagery. Not only will this shortwave have the potential to disturb our low cloud deck, but it will introduce a deeper cloud layer over the top of our resident low stratus. Of the deterministic models we have examined this evening the NAM has one of the better initializations of our stratus deck, and the NAM does generate some light precipitation Thursday morning in areas we would expect drizzle (spots with the lowest ceilings). /GKoch



PREVIOUS DISCUSSION

Periods of heavy mountain snow Thursday through Saturday

Tonight through Saturday: Air stagnation pattern will continue overnight and through Thursday morning. It will keep the stratus and fog in place. Ensembles are in good agreement of the incoming pattern change. The ridge flattens and shifts East as a trough begins to dig south along the coast. The change will push a decent moisture plume in the Pacific Northwest late Thursday into Saturday. It will bring widespread snow to the region with the heaviest snow amounts in the Cascades. Ensembles have steadily increased the expected amounts between forecast runs. The Cascades mountain passes could see 20 to 30 inches. The amounts across the Basin are 2 to 4 inches. Northern Valleys and Idaho Panhandle could get 3 to 6 inches. Lookout Pass is expected to receive 10 to 14 inches. Morning commutes across the region will slick and more hazardous than normal.

As the system exits on Saturday, warmer air will begin to infiltrate the region. It will slowly transition the snow across the Basin to rain. It will also bring breezy winds to Southeast WA. Winds are expected to be sustained in the teens low 20s with gusts into the upper 30s and low 40s possible. Depending on the timing, blowing snow could affect roadways in Southeast WA. Highs for the period will be in the upper 20s and 30s. Lows will be in the upper 10s and 20s. /JDC

Saturday night through Wednesday: An active weather pattern will continue well into next week with a potent atmospheric river expected to take aim directly at the Northwest for several days in a row, sending a stream of moisture in from the Pacific and bringing copious amounts of precipitation regionwide.

There is high confidence that we'll see an impressive amount of precipitation fall over the weekend and into next week, but there is uncertainty as to the form in which that precipitation will fall, especially in the Northern and Cascade Valleys. For the Basin eastward to Spokane and Coeur d'Alene, snowfall Saturday night looks to transition to a rain-snow mix by Sunday morning, and eventually to all rain by Sunday afternoon. For the Northern and Cascade Valleys, however, what form the precipitation takes will be highly dependent on how fast the cold air from this week mixes out. Some valley locations could see snow linger for quite a bit longer than the rest of the region if that cold air stays trapped.

Overnight lows will be several degrees below freezing Saturday night, but will hover right around freezing each night after that through at least mid next week. Daytime highs will see a similar warming trend starting out in the upper 30s for most of the region on Sunday and rising into the mid 40s by Tuesday.

A combination of snowmelt and lots of additional rainfall once temperatures rise above freezing will likely result in substantial river rises, particularly in southeast WA and north-central ID. No flooding is currently in the forecast, but that could change depending on how models trend over the next couple of days with regard to temperatures and snowmelt expected. /Fewkes

AVIATION
06Z TAFs: Our low cloud deck has become entrenched across central/eastern Washington and north Idaho. In general, the visibility and ceilings aren't moving around as much as they were 24 hours ago with the lowest ceilings at GEG and PUW...at or below 500 feet. Spotty reports of light freezing drizzle have come in this evening around Spokane and north Idaho. The arrival of a weak upper level disturbance early in the morning will deepen the cloud layer potentially increasing freezing drizzle coverage so it has been added to GEG and PUW where cigs are lowest.

FORECAST CONFIDENCE AND ALTERNATE SCENARIOS: Drizzle forecasting is almost always a low confidence endeavor. The high end will be a hundredth or two...enough for a thin glaze on aircraft. /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 23 28 24 33 28 37 / 0 20 60 80 80 80 Coeur d'Alene 24 30 24 33 28 37 / 0 20 60 80 80 90 Pullman 22 29 23 34 28 38 / 0 10 70 80 90 90 Lewiston 27 34 29 39 33 44 / 0 10 50 60 80 90 Colville 24 30 19 32 20 35 / 0 10 50 80 80 80 Sandpoint 24 30 24 30 25 35 / 0 20 70 90 90 90 Kellogg 22 31 28 33 31 36 / 0 20 80 90 90 100 Moses Lake 26 32 22 35 24 40 / 0 10 20 40 40 30 Wenatchee 29 31 25 34 27 40 / 0 10 20 60 60 50 Omak 28 33 24 34 25 39 / 0 10 20 40 50 50

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

Winter Storm Watch from Friday afternoon through Sunday afternoon for Central Panhandle Mountains.

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.

Winter Storm Watch from Thursday morning through Sunday morning for Western Chelan County-Western Okanogan County.

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