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FXUS66 KOTX 090531 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 931 PM 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...

Rest of tonight: Tonight will be the calm before the storm. Quiet, dry conditions will prevail until snow begins to move in early tomorrow morning. Fog may develop over the course of the night in the Cascade valleys and northern valleys where breaks in the clouds allow for efficient radiational cooling. Overnight low temperatures will drop into the mid 20s for the southern half of WA and the central ID panhandle, and will be colder yet in the teens and low 20s for the northern half of WA and the ID panhandle north of Coeur d'Alene. /Fewkes

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

Saturday and Sunday...Winter is set to make its return across much of the Inland Northwest during this period. Another another atmospheric river will have its sights set on the region beginning in the Cascades late Saturday morning in the Cascades and spilling rapidly eastward overnight. Strong warm air advection and isentropic ascent on the 280k surface will result in prolonged strong 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 usually a low confidence endeavor somewhere in the Inland NW and this event will be no exception. We have fair 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 the northern third of the Idaho Panhandle. The area in between, including Spokane, CdA, Coulee Dam is much trickier. While it is likely to transition at some point it will likely do so after the threat of the heaviest precipitation moves through. The models are quite consistent on forecasting the heaviest times of snow from late afternoon through the evening in the Cascades and during the evening into early Sunday morning over the eastern third of WA into north ID. Just like yesterday the NBM guidance is probably too warm and far too eager to change the snow to rain. Yesterday the thought was to change the snow to rain around 7am Sunday and the latest run now has the transition at least 6 hours earlier. If we continue to use other deterministic models as well as BUFR soundings that is far too early. So we will continue to lean the forecast in that direction.

* QPF from a precipitation standpoint this system will be nowhere as wet as the last event which pummeled the region. However the amounts will still be appreciable. The NBM QPF fields are relatively reasonable with around 1.5 inches at the Cascade Crest and 0.40 to 0.80” over the Idaho Panhandle. For the Spokane area we are still looking at amounts from 0.30 to 0.50 inches. Over the past 24 hours the main change has been a subtle shift of the heaviest precipitation to the south. Now the heaviest band f precipitation will likely occur from the Blue Mountain to Palouse to Lookout Pass and locations southward.
Meanwhile over the Columbia Basin amounts will generally be a quarter inch or less.

* Snow...Again this is where the forecast gets quite tricky. For northern 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 with the heaviest amounts from late Sat afternoon into the evening near the Cascades and overnight Saturday into early Sunday over north Idaho. But farther south the transition times from snow to rain remain tricky. The most likely scenario for Spokane snow is still around 3 inches however various ensemble members show a range of snow from a half inch to nearly 5 inches for the entire event.
Farther north, snow amounts will generally range from 3 to 5 inches however a large variation in amounts is also possible.
For Sandpoint the most likely snow amount is around 5.5 inches however an even bigger deviation in possibilities exists here with ensemble members ranging from 1 to 8 inches! The consistency over the Mountains is quite a bit better. For Stevens Pass the forecast possibilities suggest a most likely amount ranging from 12 to 16 inches with the top end forecast nearing 20 inches. Lookout Pass is another easier location as it will most likely remain as snow for the entire event. Snow amounts should range from 8-12 inches with an upper end around 15 inches through Sunday. For this reason we have issued winter storm warnings or a winter storm watch with winter weather advisories for snow elsewhere. Although most of the Inland NW will be impacted by this snowy weather, the heaviest amounts are expected to fall overnight, when the traffic is generally light. If this were to fall during a weekday commute things would be much more impactful.

* Freezing Rain...This still remains a low confidence element.
SREF guidance continues to show small probabilities of freezing rain near Wenatchee, the lower Columbia Basin, Waterville Plateau, and southern end of the Okanogan Valley from Saturday evening through early Sunday morning, but the period will be relatively brief and the amounts will be quite light.
Additionally much of it will occur after the threat of heavy snow has ended. In cases like that freezing rain falling on snow is usually less impactful than it falling on bare roads. The main impact to travel will still be the threat of snow.
Nonetheless we included it in our winter weather highlights. fx

Monday through Friday: Ensembles will build a ridge over the Pacific Northwest. It will lead to a cooling, drying trend to the Inland Northwest. The period starts with remnants from the weekend system will continue to bring light shower activity to the region, mainly in the mountains. The main concern for the period will be fog creating icy, slick conditions for the morning commutes. Local air stagnation issues could bring some degraded air quality. A weak shortwave is expected to move through the region late Wednesday into Thursday. About 40% ensemble members bring precip into the region while 60% keep the shortwave dry. Confidence is low on the precip from the shortwave. The shortwave will bring a cold air push with it. Highs will start in the upper 30s before dropping to the low 30s by Friday. Overnight lows will start in the low 30s before dropping into the low 20s. /JDC

AVIATION
06Z TAFS: Cold conditionally unstable airmass remains over the aviation area and has allowed for quick moving hit and miss snow showers producing quick MVFR conditions at times but not at TAF sites tonigh. VFR conditions should prevail with some interruptions from late night and early morning low clouds and patchy fog in spots between 9Z-18Z Saturday. IFR producing snow is expected to arrive at KEAT near 21Z Saturday and near 2Z Sunday at KGEG. /Pelatti

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

Confidence has lowered some in the amounts of snow expected but not so much the arrival time of snow.

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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 24 35 30 37 33 38 / 0 20 90 70 40 20 Coeur d'Alene 24 35 30 36 33 39 / 0 10 90 80 60 30 Pullman 25 35 31 38 34 38 / 0 10 100 90 70 50 Lewiston 28 40 37 44 39 45 / 0 10 90 80 60 40 Colville 21 32 28 35 26 38 / 0 30 90 50 30 10 Sandpoint 22 32 29 35 32 35 / 0 10 90 80 60 40 Kellogg 24 33 30 36 36 40 / 10 10 100 100 90 60 Moses Lake 25 37 31 39 31 40 / 0 50 90 30 20 10 Wenatchee 27 33 32 38 33 40 / 0 80 90 30 20 10 Omak 27 34 28 37 32 40 / 0 60 90 20 20 10

OTX WATCHES/WARNINGS/ADVISORIES
ID...Winter Weather Advisory from 1 PM Saturday to 1 PM PST Sunday for Northern Panhandle.

Winter Weather Advisory from 4 PM Saturday to 10 AM PST Sunday for Coeur d'Alene Area-Idaho Palouse-Lewis and Southern Nez Perce Counties.

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

WA...Winter Weather Advisory from 1 PM Saturday to 1 PM PST Sunday for Northeast Mountains.

Winter Weather Advisory from 4 PM Saturday to 10 AM PST Sunday for Spokane Area-Washington Palouse-Western Okanogan County.

Winter Weather Advisory from 10 AM Saturday to 10 AM PST Sunday for Okanogan Highlands-Okanogan Valley-Upper Columbia Basin- Waterville Plateau-Wenatchee Area.

Winter Storm Warning from 10 AM Saturday to 10 AM PST Sunday for Central Chelan County-Western Chelan County.

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