Area Forecast Discussion
Issued by NWS Spokane, WA

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000 FXUS66 KOTX 061144 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 344 AM PST Wed Dec 6 2023 .SYNOPSIS...
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Rain will continue today however the air mass will begin cooling with snow slowly returning to the mountains from west to east. More seasonal temperatures return Thursday into the weekend. Snow showers return Thursday night and Friday for some areas then widespread snow arrives this weekend with another atmospheric river. Next week looks seasonal and drier than normal.
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&& .DISCUSSION... Today: Broad upper-level trough will move onshore today ushering a cold front through the region and bringing another day of steady rainfall. As of 2AM, the main axis of rain stretched from the Tri- Cities northeast through Deer Park and into Bonners Ferry in North Idaho. This main baroclinic band is expected to buckle northwest slightly this morning then make a slow track from west to east. Due to the duration of the rain aimed into NE WA and N ID, these areas carry the highest probabilities for another 0.50-1.00" of QPF. Most other locations will be in the range of 0.20-0.50" except locally lower amounts in the L-C Valley and portions of the North Cascades. Rainfall intensities will not be similar to those experienced on Tuesday as the moisture plume is losing its ties to the subtropics. Additionally, the air mass will begin cooling. Snow will be returning to the Cascade passes by mid to late afternoon and Idaho Panhandle mountains Wednesday evening-night. In the lower atmosphere, the cooling will be much more subtle such that p-type will remain all rain through tonight. In North Idaho, Northeastern WA, and around Chelan County, additional moderate rainfall will keep minor flooding concerns ongoing including flooded fields, creeks rising near bankfull, and potential for rock-slides. The Stehekin River which experienced swift rises to bankfull on Tuesday looks to have crested and will begin to slowly recede. We are starting off quite mild this morning with 40s and low 50s common across the eastern third of WA and North Idaho. Temperatures will only rise a handful of degrees but as the rain returns, these temperatures will begin cooling. In Central WA, the cold air has been stubborn to mix out with temperatures still in the mid to upper 30s. Today`s frontal passage will bring another chance for some mixing but not completely sold temperatures will reach full potential and kept forecast highs on the lower side of guidance. Thursday and Friday: The trough will be over the Northwest on Thursday and Friday with cooler air slowly returning to the Inland Northwest and snow levels eventually falling back to valley floors. 850mb temperatures cool back near -1 to -2C on Thursday then -4C on Friday. This will equate to a rain-snow mix for the lowlands on Thursday the increasing potential for snow Thursday night into Friday. Precipitation will be less organized and more chaotic with the trough overhead. Focus will be on a pair of smaller perturbations rounding the base of the trough and bringing bands of showers. Details with these features will change over the next 24-48 hours but given that we can already identify these features off the coast, there is moderate confidence that these waves will bring bands of showers to the INW over the next few days. The main impacts on Thursday will be on and off snow on the mountain passes and potential for slick travel conditions. By Thursday night and Friday, may be dealing with slick travel conditions down to valley floors across the Camas Prairie, Palouse, Idaho Panhandle, and into the Spokane-Cd`A area with a lower threat for snow into Central WA away from the Cascade Crest. At this time, there is a 10-30% chance for at least 0.1" of new snow for Bonners Ferry, Sandpoint, Cd`A, and Spokane. Probabilities increase toward 50-70% for locations like Kellogg, Deary, Pullman, Moscow, and Winchester. When the dial is turned up to 1", probabilities for the lowlands decrease below 10% except St Maries, Deary, and Clarkia which remain near 50% or higher suggesting the Central Panhandle Mountains and Idaho Palouse are most likely for winter travel conditions Friday morning. /sb Saturday and Sunday: * High confidence in a weak/moderate strength atmospheric river in the PNW this weekend. * Moderate confidence for snow for the valley locations (30-70% chance of 2" of snow in Eastern WA/North ID) except a rain snow mix in the Eastern Columbia Basin/Palouse Saturday through Sunday. There is high confidence for snow in the mountain passes. * Probability of 8, 12, 18” of snow Saturday-Sunday: Stevens Pass: 90%, 70%, 25% Lookout Pass: 50%, 15%, 5% Monday through Wednesday: * Seasonal temperatures with below average precipitation expected early next week with a ridge building in the Northwest US. /db && .AVIATION...
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12Z TAFs: A plume of Pacific moisture will remain directed into the Pacific Northwest through Wednesday evening. It will make slow progress to the east Wednesday afternoon as a cold front crosses the Cascades between 21-00z. Until then, the Spokane, Coeur d`Alene, Pullman and Lewiston airports will be in the warm sector of this system with ceilings generally at or above 4000 feet. As the cold front spreads toward the Idaho/Washington border between 00-03z, precipitation will be accompanied by a lowering cloud deck as well as a swath of rain and lower visibilities. More persistent rain in central and eastern/northeast Washington Wednesday morning and afternoon will promote ceilings in the 500 to 2500ft range with the lowest cloud deck banked against the Cascades including Wenatchee. In the absence of rain, periods of LIFR fog will be possible but these lower visibilities have been inconsistent over the last 6 hours. Fog is also impacting other northern mountain valleys including Deer Park, Colville, Republic, and Winthrop. As the cold front presses through, winds will ramp up across the eastern Columbia Basin and Palouse with sustained winds of 15-20 mph possible for a few hours. The wind direction is favorable for low stratus but should keep the visibilities elevated given these speeds. This includes KGEG, KPUW, KCOE. FORECAST CONFIDENCE AND ALTERNATE SCENARIOS: Confidence is moderate at KGEG/KSFF/KCOE that ceilings will remain above 3000 feet until the cold front arrival Wednesday afternoon/evening. Main uncertainty for these sites will be duration of rain throughout the day. Thinking any breaks will be short-lived with persistent showers forming south of the main rain axis. Confidence is moderate/high for IFR/MVFR conditions along the East Slopes of the Cascades and northern mountain valleys given what has been occurring and lack of strong winds to mix out these lower cigs through 00z. After 00z, there could be enough mixing to scrub out the low clouds but thinking this will redevelop after 06z. Confidence is moderate/high for VFR conditions until the cold front arrives late in the afternoon/early evening at KPUW/KLWS. /sb 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:
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&& .PRELIMINARY POINT TEMPS/POPS... Spokane 49 34 40 29 35 25 / 80 100 70 50 20 0 Coeur d`Alene 49 35 40 30 35 24 / 80 100 80 60 40 0 Pullman 51 36 42 30 36 25 / 30 100 70 70 40 0 Lewiston 57 41 49 35 44 29 / 20 90 50 60 30 0 Colville 43 29 38 25 38 19 / 100 100 60 40 20 0 Sandpoint 45 35 39 30 36 23 / 100 100 90 70 70 10 Kellogg 47 35 38 32 34 27 / 60 100 90 80 80 20 Moses Lake 49 33 45 29 41 25 / 90 80 20 10 0 0 Wenatchee 42 33 42 31 40 27 / 90 60 30 20 0 0 Omak 43 31 40 29 39 25 / 90 70 20 20 0 0 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...None. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.