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FXUS66 KOTX 072338 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 338 PM PST Thu Dec 7 2023

SYNOPSIS
Temperatures will be near seasonal norms into the weekend. Snow showers return tonight and Friday for some areas then 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

Tonight through Friday night: The potential for precipitation will persist for the next 12-18 hours, before the area briefly dries out. Tonight into early Friday an upper trough will continue to migrate across the region, with embedded vorticity maxima.
This will work with some instability within that broader trough and the westerly flow to bring continued shower chances. The highest potential will remain around the mountains; more hit-and- miss transient showers are possible elsewhere. The higher focus in the mountains will mean some modest snow accumulations around places like Stevens and Lookout Pass. Winter weather advisories remain in place through 10 AM Friday near the Cascade crest and around the central Panhandle above 3000 feet, where more than 5 inches of snow is expected with some areas seeing a storm total of near a foot. The higher areas of the Camas Prairie, largely above 4000 feet, could see around 3 inches of snow. Lowland snow amounts are expected to be lighter, with less than a half inch over the northern valleys and eastern third of WA. The best potential for accumulating snow in the lowlands comes overnight into early Friday. Given the vorticity lobes pivoting through and the elevated instability, there is a chance some locally heavier snowfall amounts could be seen in the lowlands but this is expected to be isolated. Either way, with temperatures dropping to near the freezing or below in many areas there could be some slick spots in the lowlands during the morning commute. Drier air comes in behind the trough going into later Friday afternoon into Friday night, pushing the primary precipitation chances to the ID Panhandle mountains where it wanes and some clearing coming into the rest of the area through the day. Winds will remain breezy this evening, then decline. Lastly look for some patchy fog over the northern valleys tonight and again toward Friday night into Saturday morning over the northeast WA and ID valleys and near bodies of water. /Solveig

...WINTER IS SET TO RETURN TO MUCH OF THE INLAND NW THIS WEEKEND...

Saturday and Sunday...Winter is set to make its return across much of the Inland Northwest during this period. Yet another atmospheric river will have its sights set on the region beginning early Saturday night in the Cascades and spilling rapidly eastward overnight. Strong warm air advection and isentropic ascent on the 280k surface will result in prolonged lifting through the dendritic layer initially resulting in widespread snow. However just like the last system we will need to contend with a transition from snow to rain from southwest to northeast across the region. This is always a low confidence endeavor somewhere in the Inland NW. While we have good confidence it will readily change to rain over the Palouse, Southern Columbia Basin, LC Valley. Likewise we have high confidence it will remain as snow in the Methow Valley, the valleys of north- central and northeast WA and far northern ID. The area in between, ie Spokane, CdA, Coulee Dam is much trickier. Yes it should transition at some point, but when that happens is critical to determining the forecast snow amounts. The NBM wants to change the snow to rain in Spokane by 6-8am which based on the deterministic model data from the NAM, GFS, BUFR soundings, and other hi-res models is way too quick.

* QPF from a precipitation standpoint this system will be nowhere as wet as the last one which pummeled the region. However the amounts will still be appreciable. The NBM QPF fields are most likely quite reasonable for the event and show anywhere from 0.40-0.80” over the ID Panhandle and 1-1.5” of liquid near the Cascade Crest. For the Spokane area and Palouse totals range from 0.30-0.50 inches with less than a quarter inch over the Okanogan Valley south to Moses Lake. So while these amounts look good, there will be much more difficulty with the snow amounts.

* Snow...this is where the forecast gets quite tricky. Again for locations near the Canadian Border or in the immediate lee of the Cascades we have high confidence that much of the precipitation will fall as snow. But farther south the transition times from snow to rain are tricky. Our most likely scenario shows 3-4” snow amounts for Spokane, Davenport, Pullman, and the Camas Prairie. 4-8” amounts seem more plausible for CdA, Sandpoint, Deer Park, and Metaline Falls, and 16-24” at Stevens Pass. But this is not a high confidence forecast for Spokane, Pullman, CdA, Wenatchee due to how quickly the low- level wedge of above freezing air moves in. It’s conceivable that these locations could see more than forecast if the switch- over to rain is slower than expected. So how much uncertainty is there? It’s considerable. According to the NBM we could see anywhere from 0 to 4 inches of snow in Spokane. While the 0 inches is not plausible, I suspect the 4” range in amounts could be reasonable if the entire half inch of precipitation falls as snow. Leavenworth will be another tricky location. The NBM shows up to 2” of snow but it’s surface temperatures are far too high and based on nearby BUFR data it’s conceivable that amounts could range from 7-10”. Based on these forecasts its likely we will need to issue winter weather highlights during future forecast suites. The good news is most of this snow will fall overnight Saturday into Sunday morning and it won’t involve a workday commute so traffic should be relatively light.
Nonetheless if you have travel plans during this period, be prepared for winter driving conditions over the passes, ID Panhandle and over much of eastern WA from Highway 2 northward.

* Freezing Rain...Speaking of low confidence forecast, freezing rain is the toughest call. SREF guidance continues to show small probabilities of freezing rain near Wenatchee, the lower Columbia Basin and Waterville Plateau from late Saturday night into Sunday morning, but the period will be relatively brief and the amounts will be quite light. Nonetheless should we decide to issue winter highlights later, this will be added to the mix of wintry weather. fx

Monday through Thursday: We'll start off next week with a shortwave disturbance moving through, which will bring one last round of precip before we shift into a drier pattern. Snow will fall through Monday morning over the Cascades and throughout the eastern third of WA and the ID panhandle before gradually transitioning to a rain/snow mix and clearing from west to east into Monday afternoon and evening.

By Tuesday morning, precip will have moved out of the region with the exception of a few flurries possible over the Cascade Crest.
We'll come under the influence of a ridge, which will keep us dry for at least a couple days. Temperatures will remain near seasonal normals with highs in the mid to upper 30s and lows in the 20s.
/Fewkes

AVIATION
00Z TAFS: An upper trough migrating over the region and embedded shortwaves will bring periodic precipitation to a large portion of eastern WA and ID tonight into early tomorrow, while central WA will see limited chances. Precipitation will be mainly in the form of snow or a rain/snow mix. Some light snow accumulations possible for GEG/SFF/COE. Expect MVFR/IFR ceilings through the night into Friday morning. Wind will be breezy with gusts near 15-25 kts.

FORECAST CONFIDENCE AND ALTERNATE SCENARIOS: Recent rains and snow melt have left an extremely moist boundary layer in place. With cooler temperatures now in place, any lull in the winds combined with breaks in the clouds could lead to quick development in fog or low stratus. The other uncertainty comes with snow shower activity tonight-tomorrow morning east of a line from Spokane to Pullman. If the snow intensity is stronger than expected, this could overcome the warmer surface temperatures and lead to a quick inch of snow at the terminals.

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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 30 37 22 36 30 36 / 60 20 0 10 90 80 Coeur d'Alene 29 35 24 36 29 37 / 60 50 0 10 90 90 Pullman 29 36 23 35 30 36 / 60 20 0 10 90 90 Lewiston 33 43 28 41 35 43 / 50 10 0 0 80 80 Colville 23 37 21 34 26 36 / 50 20 0 40 100 70 Sandpoint 29 36 24 34 28 36 / 70 70 10 20 90 90 Kellogg 31 34 26 35 31 37 / 80 80 10 10 90 100 Moses Lake 27 42 24 37 30 38 / 10 0 0 30 80 60 Wenatchee 31 40 26 34 31 38 / 10 0 0 50 90 60 Omak 28 39 24 34 30 37 / 20 0 0 40 90 50

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

WA...Winter Weather Advisory until 10 AM PST Friday for Western Chelan County.

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