Area Forecast Discussion

KEY MESSAGES

- Mostly dry conditions through Wednesday.

- Rain chances develop later Wednesday into Thursday, with mountain snow.

- Additional systems arrive late week into the weekend will bring light rain and mountain snow chances.

SYNOPSIS

Temperatures will be seasonal and it will be mostly dry through Wednesday in the Inland Northwest. Mountain snow showers and rain showers return later half of the week as a couple systems move through. Unsettled conditions continue into the weekend and early next week.

DISCUSSION

Today to Thursday: the Inland NW will be in a brief col between low pressure to the south and a system off the coast today, before the offshore system moves in tonight into Thursday morning. Through this morning middle to higher clouds will continue to thin and this will enable some low clouds and patchy fog in the sheltered valleys and possibly the Columbia Basin to Palouse and L-C Valley; confidence is slower in these latter areas because higher clouds could be slower to erode. Later in the afternoon into tonight the higher clouds start to increase from the west as the system approaches. Rain and mountain snow chances will start into the Cascades by late afternoon to early evening. Rain chances expand to central Washington from late evening to overnight, impinging on the eastern third of WA late. By early Thursday morning rain chances peak over the east third of WA and ID, before the best chances shift into Idaho in the late morning and afternoon dissipate into Thursday evening.

This will be mainly mountain snow and valley rain; though some rain/snow mix is possible in the northern valleys, especially closer to the Canadian border. Precipitation amounts will be light, with rain amounts generally less than 0.05 inches and snow amounts generally less than 0.5 inches and those will be in the higher mountains. Some gusty winds are possible around south- central to east-central WA and lower ID Thursday afternoon in the wake of passing front, with gusts of 15-20 mph. Highs will largely in the upper 40s to lower 50s today and the upper 40s to upper 50s Thursday, with milder readings toward the deeper basin and cooler readings to the northeast CWA under the clouds and precipitation chances. Lows will largely be in the 30s, with some upper 20s in the sheltered northern valleys.

Friday to Tuesday: The Inland NW transitions to a more progressive westerly flow, with several embedded system tracking over the area every other day or so. However westerly flow and transient nature of these systems suggest limited precipitation potential. The next precipitation chances expand into the Cascades early Friday and expand toward northeast WA/north ID late Friday afternoon through evening. Snow levels start near 3.5-5kft, lowest in the Cascades and northern mountains, before rising to around 5-8kft by late afternoon. So some mountain snow accumulations are expected around the Cascades, around 0.5 to 1 inch locally higher in the higher peaks. Better chances will be early in the day. Clouds will be more abundant than precipitation elsewhere then.

The next more robust system coming on a 110-120Kt jet stream arrives Saturday night and Sunday. Precipitation chance rises over the Cascades first Saturday night, then expand east through the day Sunday. Probabilities still remain best in the Cascades and eastern mountains and are held to around 15-25 percent over the Upper Columbia Basin, Spokane/C'dA area, Palouse and lower ID. Still another system is forecast to come in later Monday into Tuesday. All of these have some degree of uncertainty in timing given the more progressive/quicker westerly flow as models can tend to have trouble with timing in such a flow pattern.

Snow levels Saturday night start near 6.5-8.5kft, drop to around 5-7kft by late Sunday morning, then 3-6kft by late Sunday night into Monday morning, before moderating back to 6-7kft by Tuesday afternoon. So this will mean occasional mountain snow, especially the Cascades. These are mainly above pass level, except Washington Pass. Saturday night into Sunday models show 2-4 inches in the mountains; higher peaks could see more. NBM guidance shows about a 30% chance of measurable snow at Stevens Pass and about 80% at Washington Pass; but for 1 inch probabilities are 10-15% at Stevens and 55% at Washington Pass. Monday night and Tuesday may see mountain snow around 0.5-2 inches. The NBM probabilities of measurable snow by this time are around 5% at Stevens and 45% at Washington Pass

The other notable feature heading toward Sunday is winds. The strong jet crossing the area, tightening gradient and increased mixing shows wind gusts of 20-30 mph Sunday afternoon over the Waterville Plateau to Upper Columbia Basin east toward the WA/ID border, as well as the Okanogan valley to Okanogan Highlands. Strongest winds are forecast for the northeast Waterville Plateau to Upper Columbia Basin. Highs will be in the upper 40s to mid-50s much of the period, with slight peak to the mid-50s to lower 60s Saturday. Lows largely in the 30s are forecast, with some 20s in the sheltered valleys; Saturday night/Sunday AM will be the exception with mainly 30s to lower 40s. /Solveig

AVIATION

06Z TAFS: Mid and high level clouds will continue to move into the region through tonight before thinning Wednesday morning. As the high clouds begin to thin with a moist boundary layer areas of fog and stratus are expected to again develop over the northern valleys. Models also show a chance for fog and stratus developing across the Columbia Basin into Spokane/Coeur d'Alene area, the Palouse, and LC Valley. But with probabilities at 50 percent or less confidence in coverage more than patchy is low so only went with a FEW-SCT low cloud deck for KMWH/KGEG/KSFF/KCOE/KPUW/KLWS.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: High confidence in VFR conditions for all TAF sites through this evening. Low confidence in ceiling and visibility for all TAF sites except KEAT after 11z Wednesday as areas of stratus and patchy fog develop.

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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 35 51 36 49 36 52 / 10 0 20 80 0 0 Coeur d'Alene 36 52 36 47 35 51 / 10 0 10 90 0 0 Pullman 34 51 35 49 35 51 / 10 0 10 80 0 0 Lewiston 38 53 40 55 38 54 / 0 0 10 70 0 0 Colville 28 53 32 49 31 50 / 0 0 20 60 0 10 Sandpoint 34 50 35 45 34 48 / 10 0 0 90 10 10 Kellogg 36 53 35 44 34 51 / 0 0 0 90 10 0 Moses Lake 30 53 35 57 34 54 / 10 0 30 10 0 0 Wenatchee 36 52 39 55 39 51 / 0 0 30 0 0 10 Omak 33 53 38 53 37 51 / 0 0 30 0 0 20

OTX WATCHES / WARNINGS / ADVISORIES

WA...None. ID...None.