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FXUS66 KOTX 081730 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 930 AM PST Fri Dec 8 2023

SYNOPSIS
Temperatures will be near seasonal norms into the weekend. Widespread snow arrives this weekend with potential for moderate snow amounts. Next week looks seasonal and drier than normal as a ridge of high pressure settles over the region.

DISCUSSION

...WIDESPREAD SNOW SATURDAY AFTERNOON THROUGH SUNDAY. HEAVY SNOW OVER THE CASCADE PASSES...

Today and tonight: Much different atmosphere has taken residence over the Inland NW compared to 24 hours ago. Noticeably cooler, drier, and the evening balloon data showing a more unstable environment.
We are monitoring a vort max dropping into Northern Idaho this morning bringing snow showers. This feature has been rather transient and valley temperatures have ranged from 32-35F thus impacts have been minimal so far for elevations below 3000 feet.
Above 3000 feet, snow is impacting roadways with several cams showing slushy accumulations to snow covered roads. Snow showers are also falling along the Cascade Crest and at Stevens Pass. An upper- level trough is over the region and will begin to slide east today. As this occurs, midlevel flow will start to buckle from southwest to northwest. I expect snow showers to continue in the Central Panhandle Mountains, Camas Prairie, Blue Mountains, and Cascade Crest with a general drying trend for the remainder Central WA and Eastern WA. Cannot rule out a sneaky vort max coming down from the northwest stirring up a cluster of showers yet with surface dewpoints drying out in the 20s, thinking cloud bases will be on the rise for Central and Eastern WA suggesting any additional shower activity will be extremely light or virga. /sb

Saturday and Sunday: A complex scenario remains on the table for the atmospheric river coming this weekend. The evolution of the expected warm air advection in the Columbia Basin and the Palouse makes for a challenging forecast for the Inland Northwest. There still remains large differences between the NBM and the global ensembles for the Spokane area. Given the recent warm bias of the NBM and BUFR soundings showing mostly snow for Spokane, I still think the NBM changes to rain Sunday morning far too fast. NBM warm bias continues to plague areas such as Leavenworth and the Methow Valley as well. With that in mind, the most likely scenario is a sharp cutoff of snow totals southwest of Spokane and southeast of Wenatchee.

Using the global ensembles as guidance for probabilities, there is a 50-80% chance of >= 3 inches of snow for Spokane to Cda. Cda to Sandpoint has a 30% chance of >= 6 inches of snow. Ensemble guidance has a 0% chance of >= 3 inches of snow along and south of Interstate 90 from Moses Lake to Ritzville and west of highway 195 which seems reasonable given soundings show mostly rain in the Basin. The heaviest snowfall will be late Saturday night for the region. Stevens Pass is forecasted 18-24 inches and has a 70% chance of an inch an hour Saturday evening.

SREF guidance continues to show freezing rain potential in Central WA where surface temps dip below freezing while warm air advection occurs aloft. Ice accumulations look the greatest for the Waterville Plateau. There is a 30% chance of a tenth of ice along highway 2 between Waterville and Douglas. With all this in mind, highlights will be considered later this morning once the current highlights end. /db

Monday through Saturday: High pressure begins to blossom along the coast Monday placing the Inland NW under northwest flow aloft. A shortwave dropping down the eastern flank of the ridge will usher another round of rain/snow showers. Without adequate cooling in the wake of Sunday's AR event, snow levels will remain elevated with snow mainly impacting the mountains and rain or rain/snow mix in the lowlands; overall a low impact event. There is good agreement amongst the ensembles for the ridge to come inland promoting drier weather for Tuesday, Wednesday, and likely Thursday. We see larger spread in the models by late in the week with about 60% of the models bringing precipitation back into the picture while 40% dry and holding on to the ridge. In general, not seeing any significant storm systems through Friday. Ridge patterns this time of year tend to promote an abundance of low clouds and fog, especially following our recent wet weather.
Temperatures will be "capable" of residing on the warm side of normal given midlevel warming but models tend to struggle with these stagnant weather patterns and mixing is often poor resulting in temps closer to normal or mid to upper 30s. /sb



AVIATION
18Z TAFS: Scattered snow showers over the Idaho Panhandle will decrease throughout the day from north to south. Drier air advection in concert with steady winds are delivering VFR skies for much of Central and Eastern WA. MVFR ceilings will remain possible across the Central Panhandle Mountains Friday afternoon, otherwise VFR skies are expected. The next aviation concern will be a bank of low clouds developing between Pullman and Spokane 5-9Z Saturday. Low level winds shifting to the southeast will likely carry this bank of clouds into Spokane with HRRR showing a 50% for LIFR conditions. This is supported by the latest MOS output.

FORECAST CONFIDENCE AND ALTERNATE SCENARIOS: There is moderate confidence for the low clouds redeveloping north of Pullman Friday night and impacting Spokane. Past events have brought vis as low as 1/2 mile so this will need to be monitored closely.

Heads up...widespread snow returns to the region Saturday afternoon through Sunday.

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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 38 24 36 30 36 32 / 10 0 10 100 80 60 Coeur d'Alene 36 24 36 29 37 33 / 40 0 0 100 80 70 Pullman 36 24 35 30 36 34 / 10 0 0 90 90 80 Lewiston 44 28 41 35 43 39 / 10 0 0 90 80 70 Colville 38 19 34 26 36 27 / 10 0 40 100 60 50 Sandpoint 36 22 34 28 36 32 / 70 10 20 100 90 70 Kellogg 34 24 35 31 37 34 / 80 10 0 100 100 90 Moses Lake 43 25 37 30 38 31 / 0 0 30 80 50 40 Wenatchee 41 27 34 31 38 34 / 0 0 50 90 60 40 Omak 41 27 34 30 37 32 / 0 0 50 90 40 40

OTX WATCHES/WARNINGS/ADVISORIES
ID...Winter Weather Advisory until 10 AM PST this morning for Central Panhandle Mountains.

WA...Winter Weather Advisory until 1 PM PST this afternoon for Western Chelan County.

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