Valley, WA Weather

Key messages

- Light snow in the east slopes of the Cascades tonight into Friday morning above 3000 feet. Less than half of an inch of new snow expected.

- A chance (20-50%) of rain primarily over southeast WA and the southern ID Panhandle Saturday evening and night. Light rain accumulations up to a tenth to one quarter inch expected.

Synopsis

Low pressure will remain off the coast through Friday then eject inland over the weekend. Valley rain and mountain snow is forecast at times through the week with periods of light snow impacting the Cascade passes. The incoming system will bring increasing chances for light rain Saturday evening and night across Idaho and eastern third of Washington.

Discussion

Little in the way of impactful weather is expected for the rest of this week into next week. An upper level low pressure system will become cut off from the Polar Jet and sit off of the WA coast through Saturday. The low then deepens and digs as it becomes entrained in an active Sub-Tropical Jet moving across California into the Great Basin. The digging motion will divert much of the energy and dynamics associated with the low south of the Inland Northwest as the trough ejects across the Northwest. Ridging of high pressure then looks to establish itself over the the region by early next week. Cluster analysis of the model ensembles suggests high confidence for a strong ridge to dominate the weather pattern for at least Monday and Tuesday. The general consensus indicates upper level energy undercutting the ridge on Wednesday with active weather impacting California. It won't be until the next shortwave disturbance around Wednesday night into Thursday that the ridge displaces to the east, and this looks to be when we will look to see precipitation chances increasing again over the Inland Northwest. This far out in the forecast carries low confidence and even fringe scenarios indicate little more than minor impacts in the far extended.

Rest of Thursday through Sunday: Low pressure spinning off of the WA coast will promote easterly flow at low levels. The east slopes of the Cascades will see weak orographic lift tonight and will be supported with a weak warm front that pushes in over the Cascades by Friday morning. Both of these lifting mechanisms will be weak and moisture will be limited resulting in a 50-70% chance for light precipitation. Snow levels will start out around 2500-3000 feet and rise to around 4000-4500 feet by the afternoon. At most, looking at snow accumulations of up to an 1 inch for travel over Stevens Pass and Washington Pass Friday morning into Friday afternoon.

The longwave trough of low pressure ejecting across Saturday into Sunday night will favor precipitation over southeast WA and the southern ID Panhandle. Snow levels will have climbed above mountain pass level by then, and probabilities for measurable rainfall is only 20-40% for Spokane to Lookout Pass and down to Pullman. Probabilities increase to around 50-60% on the Camas Prairie. Chances for rain will be increasing through the evening for Trick-or- Treaters, but overall doesn't look to be a wet Halloween evening. Total rainfall will generally be around an tenth of an inch or less and will bring little to no impacts.

Temperatures will be near normal through the rest of the week with highs in the upper 40s to low 50s and lows generally in the upper 20s to low 30s.

Sunday night through Thursday: Next week looks even more benign. Subsidence under the rideg of high pressure to begin the week will suppress potential for precipitation. The ridge does look "dirty" with high clouds passing over the top and across the Northwest region. The boundary layer will remain moist under the ridge and that typically means fog and stratus cover is expected. The prospects of high clouds makes forecasting fog and stratus more challenging as that will act to inhibit radiational cooling a bit. Regardless, this ridge doesn't look to be a big concern for producing stagnant conditions that would necessitate an Air Stagnation Advisory. If strong inversions do develop, then that would most likely result in a lot of stratus forming and we tend to mix a bit better each day under stratus cover. The ridge also may not be in place long enough to meet the time requirements of 72 hours of stagnant air for an advisory as well. Model ensembles are in general agreement for the ridge to be displaced around Wednesday night into Thursday. Potential for precipitation is low though as the ridge displaces as the prognosis looks more favorable for energy to dig south of the region. Temperatures will see little change from this weekend through next week. /SVH

Aviation

18Z TAFS: Low stratus and patchy fog will result in IFR conditions around KEPH and KMWH, as well as KGEG/KSFF/KCOE and KSZT to K65S in North Idaho through the rest of this morning. Low clouds are expected to lift transitioning to VFR conditions between 20-22Z this afternoon. In addition to the low stratus clouds, a band of very light rain will develop along a narrow boundary south of KGEG to KCOE and north of KPUW. This boundary will lift north throughout the day with brief light rain possible (20-40% chance) at KGEG/KSFF/KCOE early this afternoon. Precipitation is not expected to be heavy enough to lower cigs- vis. Easterly flow in the boundary layer will pool moisture in the boundary at KEAT with MVFR ceilings expected tonight after 02Z. Much of the rest of Eastern WA and the ID Panhandle will see a drying trend in the boundary layer and thickening mid level clouds with cigs above 10 kft agl. and VFR condtions expected prevailing.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: High confidence that stratus clouds will lift by 20Z with VFR conditions prevailing at all TAF sites. There is low to moderate confidence that low stratus with MVFR ceilings will develop at KEAT tonight, and a 20% chance that ceilings lower below 2 kft agl. /SVH

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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 36 51 33 51 35 50 / 0 0 0 0 30 20 Coeur d'Alene 34 52 33 53 35 50 / 10 0 20 0 30 20 Pullman 36 49 32 51 33 44 / 0 0 20 0 30 40 Lewiston 39 54 37 52 40 50 / 0 0 20 0 40 50 Colville 34 50 27 52 24 52 / 0 10 10 0 20 10 Sandpoint 34 49 30 52 31 50 / 0 0 10 0 10 10 Kellogg 33 53 33 55 36 49 / 0 0 30 0 20 30 Moses Lake 36 52 28 53 31 55 / 0 10 0 0 20 0 Wenatchee 37 49 34 49 37 54 / 20 40 0 10 20 0 Omak 36 47 31 48 32 51 / 10 10 10 0 20 10

Watches, warnings and advisories

WA: None. ID: None.