West Clarkston-Highland, WA Weather

Key Messages

- Active weather pattern with chances for precipitation increasing on Monday and chances continuing throughout the work week.

- Snow levels lowering to between 3,000 and 4,500 feet with snow in the mountains by Tuesday night and continuing through Thursday. Winter travel over mountain passes in the Cascades to Sherman Pass in north-central WA will be possible.

Synopsis

The weather pattern turns more active this week with a pair of low pressure systems impacting the region. The first weather system moves across on Monday and is expected to bring light precipitation, especially across southeast WA and into the ID Panhandle. A second, colder weather system for Tuesday night through Thursday night will drop snow levels to as low as 3000 feet over the Cascades and north- central WA and down to around 4500 feet in the ID Panhandle. Valley rain and mountain snow is expected with the potential for travel impacts over mountain passes.

Discussion

Monday through Tuesday: A vertically stacked low off the Washington coast will weaken and slide southeast today (Monday). As it does so increased lift ahead of the low combined with mid level instability will result in a band of rain showers passing through the region with snow levels around 5000 to 6000 feet. As the low exits Monday Night residual moisture and upslope flow into the Cascades, Northeast Washington, and the Idaho Panhandle will keep showers persisting. With the weakening nature of the low and quick movement through the area precipitation amounts are expected to be light, mainly between .05 to .15 inches. Exception is around the Blue Mountains into the Central Panhandle Mountains where stronger lift as well as surface based instability with CAPE of 200-400 J/KG has the potential to produce locally heavier downpours with rain amounts of a quarter inch. A few thunderstorms can not be ruled out but confidence remains too low to include in the forecast.

Wednesday through Thursday: An upper ridge amplifies along 150W in the Eastern Pacific forcing a trough to settle into the region, with a stronger closed low dropping south along the WA coast on Wednesday and remaining in place Thursday. There is good ensemble agreement leading to high confidence of a cooler and showery pattern as snow levels fall Wednesday to 3000-4500 feet. This will allow for snow in the mountains including the passes, especially in the Cascades. Increasing instability with 500mb temperatures dropping to near -30C combined with upslope flow into the Cascade crest will be favorable for several inches of snow at Stevens Pass and Washington Pass. Below are the NBM chances for various snow thresholds being reached for the mountain passes from 5 PM Tuesday to 5 PM Thursday:

Mountain Pass 2+ inches 4+ inches 6+ inches 8+ inches Stevens Pass 100% 75% 40% 5% Washington Pass 100% 60% 10% 2% Sherman Pass 60% 10% 1% 0% Lookout Pass 50% 10% 0% 0%

Snow is likely to cause winter driving conditions at times in the mountains and drivers should be prepared for winter travel. With the influence of the low and showers high temperatures will drop into the mid 40s to low 50s, with overnight lows mainly in the 30s.

Friday through Sunday: The low remains off the coast Friday and Saturday but gradually weakens resulting in a drying trend. Although impulses rotating into the area will keep a chance for rain and mountain snow showers persisting. Snow levels overnight fall as low as 2000 feet in the northern valleys such that some wet snow could mix in at the lower elevations as well but little to no accumulations are expected at this time. On Sunday the low digs further south leading to drier conditions across the Inland NW but with continued cool temperatures. JW

Aviation

06Z TAFS: A band of precipitation associated with a low pressure system will remain nearly stationary through 15z over the Cascades, with TAF sites remaining dry. Dry northeast winds in the boundary layer will continue to promote VFR conditions tonight. On Monday as the low pressure system moves east bands of showers will expand across the region with areas of MVFR conditions developing in the afternoon over Northeast Washington into the North Idaho as the boundary layer moistens and winds switch to an upslope southwest direction.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: High confidence for VFR conditions at all TAF sites through 18z Monday. As rain showers increase across Eastern Washington and North Idaho HREF probabilities are for a 20-40 percent chance of MVFR conditions at KGEG/KSFF/KCOE/KPUW between 20z-01z then increasing to 60-80 percent chance between 02z-06z. JW

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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