Clarkston Heights-Vineland, WA Weather

Key Messages

- Cool and showery weather pattern continues through the week with several rounds of valley rain and mountain snow showers expected.

- Snow levels fall to 3000 to 4000 feet by Wednesday morning. Expect winter travel conditions over the Cascade passes Tuesday night into Wednesday.

Synopsis

Another weather system will move into the region Tuesday night through Thursday. This will drop snow levels to between 2500 and 3000 feet over the Cascades and North-Central WA. For the ID Panhandle, snow levels will drop to around 4500 feet. Valley rain and mountain snow is expected, bringing winter travel conditions at times over mountain passes.

Discussion

Tuesday through Thursday: The Inland NW will be under the influence of an upper trough resulting in a cool and showery weather pattern. A deep closed low dropping out of southern British Columbia will bring cooler temperatures to the region, along with increasing showers as the mid level become more unstable as 500mb temperatures plunge Tuesday Night and Wednesday to -30C over the northern half of Washington into the North Idaho Panhandle. This combined with lower level upslope flow from west-southwest winds at 850mb into the Cascade crest, and mountains of Northeast Washington and the North ID Panhandle will produce several inches of snow. Bands of convergence into the Cascades also has the potential to produce brief periods of heavier snow with HREF showing around a 30-40 percent chance of 1 inch per hour snow rates Tuesday Night into Wednesday morning. A Winter Weather Advisory remains in place for elevations above 3000 feet in the Cascades from 5 PM Tuesday to 5 PM Wednesday with the highest amounts expected over Stevens and Washington Passes. Wednesday Night into Thursday as the closed low drops south to off the WA coast the flow turns southwest. And some subtle mid level drying will result in a decrease in the intensity and coverage of showers, but enough moisture lingers over the northern mountains and Cascades for additional light rain and mountain snow amounts. Below are the NBM chances for various snow thresholds being reached for the mountain passes from 5 PM Tuesday to 5 PM Thursday, which have increased compared to yesterday's probabilities:

Mountain Pass 2+ inches 4+ inches 6+ inches 8+ inches Stevens Pass 100% 95% 80% 50% Washington Pass 100% 70% 35% 10% Sherman Pass 90% 40% 1% 0% Lookout Pass 90% 50% 0% 0%

Friday through Monday: The influence of the upper low continues Friday and Saturday as another wave associated with the low passes through for more cool and showery conditions with mountain snow showers, and a rain/snow mix in the valleys. On Sunday the low starts to push out and is replaced by an upper ridge on Monday bringing dry weather and slightly warmer temperatures.

Aviation

06Z TAFS: Isolated to scattered shower activity will linger into Tuesday mainly over the ID Panhandle and eastern third of Washington, increasing Tuesday evening as the next weather system moves in. Rain has moistened the boundary layer considerably, with high resolution model data suggesting an increase in IFR and MVFR stratus over Northeast Washington and the Idaho Panhandle tonight including KGEG/KSFF/KCOE, and eventually KPUW later tonight (around 12z). Drier air in the boundary layer in Central WA has a better potential for continued VFR conditions including KEAT/KMWH. Stratus will linger into Tuesday morning before increasing southwest winds in the boundary layer help lift and break up the stratus.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: There is high confidence in increasing IFR and MVFR stratus over Northeast Washington into the Idaho Panhandle tonight into Tuesday morning including KGEG/KSFF/KCOE. Confidence in restrictions is lower for KPUW/KLWS as the boundary layer is moist but boundary layer winds not as favorable for low ceilings. Shallow ground fog could also form in the LC Valley with any breaks in the cloud cover tonight. 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