Area Forecast Discussion

KEY MESSAGES

- Increasing westerly winds and widespread rain showers will be seen through Wednesday evening. Thunderstorms are possible through early Wednesday morning.

- Widespread moderate rain Wednesday will bring potential for small stream and urban flooding in Southeast WA.

- Strong wind gusts continue through Wednesday and into early Thursday morning.

SYNOPSIS

Unsettled weather returns Tuesday evening into Wednesday with potential for moderate rain, thunderstorms, and gusty winds. Main risks with thunderstorms will be gusty outflow winds, small hail, and infrequent lightning. Moderate rainfall on Wednesday could result in small stream flooding over the Palouse. Gusty winds will continue through Thursday. Conditions trend drier heading into the weekend.

DISCUSSION

Today through Thursday: The upper level low has moved inland, bringing active weather to the Pacific Northwest that will continue through Thursday night. The incoming trough paired with warm air left in the wake of the trough has resulted in thunderstorm chances across the area. While the highest chance for thunderstorms has passed, there remains a 10-20% chance for thunderstorms overnight and into later this morning. Main impacts with these thunderstorms will be gusty outflow winds up to 40+ mph and small hail. Afternoon thunderstorm chances continue on Thursday, though will be confined to far eastern Washington and the Idaho Panhandle as the low pressure system moves northeastward.

Alongside thunderstorms, this system will also bring widespread rain showers. In particular, a deformation band setting up looks to bring moderate rainfall to southeastern Washington and the Idaho Panhandle. 24-hr rainfall totals through late tonight show this area receiving 0.5-0.75 inches of precipitation, with localized higher totals near 1 inch in areas such as the Palouse and in the mountains. The Palouse receiving this rainfall in particular will be a concern, especially since soils are already saturated and the area has already seen flooding issues earlier in the year. As a result, a Flood Watch has been issued for the Palouse area through tonight. Main impacts will be rises on rivers and streams, and possible minor small stream and urban flooding in poor drainage areas. Other specific areas that are expected to or already seeing flooding include Kootenai River at Bonners Ferry, and Stehekin River. Kootenai River is expected to reach Action Stage later today, while Stehekin River has experienced ongoing flooding issues.

With the frontal passage, winds will increase Wednesday afternoon, especially across the Cascades, Okanogan Valley and Highlands, Waterville Plateau, Moses Lake, Basin, and northern Blues areas, with gusts 35-40 mph expected and locally higher wind gusts near 45- 50 mph. Areas that will experience the strongest winds with gusts near 45-50 mph include the Moses Lake, Wenatchee, Waterville Plateau, and Lower Garfield and Asotin county areas. A Wind Advisory has been issued for these areas. Main impacts include choppy waters on any lakes, driving impacts to high profile vehicles, blown objects, and damage to tree branches. The advisories will end tonight and early tomorrow morning, but continued elevated winds will continue through Thursday night. Additionally, high temperatures will drop considerably, nearly 15-20 degrees, from yesterday to today. Low temperatures will not see quite as dramatic a drop, though it will still be noticeable at 10-15 degrees.

Friday through Sunday: As the low pressure moves into the northern central US and a trough strengthens off the coast of Canada, dry conditions will be seen through the weekend. The one exception will be showers in mountain areas due to orographic lift. Clusters are in disagreement about whether the low will retrograde westward as it interacts with another low off the California coast, so the main message right now is that there will overall be a warming and drying trend through the weekend. /AS

AVIATION

06Z TAFS: A system moving through will result in rain showers, lowered ceilings, and gusty winds through the night and into tomorrow. Radar shows rain showers about 2 hours out. Rain is projected to continue through the forecast period for all sites but MWH and EAT, where PoPs decrease at 18Z. Chances (10-20%) for thunderstorms throughout the forecast area continue through around 15Z, but there isn't enough confidence to include them at any specific terminal. A slight decrease in winds combined with a moist boundary layer will result in MVFR/IFR conditions for GEG, SFF, COE, and PUW starting around 12-14Z. Timing on when ceilings will lift is uncertain, but HREF has MVFR conditions returning right near the end of the TAF period around 02-04Z. There is less confidence in ceilings dropping at LWS, MWH, and EAT due to stronger winds persisting through the night. Should any thunderstorms form, main impacts to terminals will be gusty outflow winds and small hail.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: Low to moderate confidence in the timing of rain starting at each TAF site. High confidence in rain at each TAF site through at least 18Z. Low confidence in timing for ceilings lifting at GEG, SFF, COE, and PUW. Most impactful alternate scenario would be a thunderstorm directly impacting a terminal, resulting in amendments. /AS

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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 55 39 56 35 54 33 / 100 20 40 20 20 10 Coeur d'Alene 51 37 52 33 52 31 / 100 50 60 30 20 10 Pullman 46 37 50 33 51 32 / 100 60 70 40 30 10 Lewiston 52 42 55 39 56 36 / 100 70 60 30 20 10 Colville 66 38 60 33 57 31 / 80 10 30 10 20 10 Sandpoint 51 37 50 33 50 31 / 100 60 80 50 30 10 Kellogg 49 37 48 33 49 30 / 100 80 90 50 50 10 Moses Lake 67 43 65 40 62 38 / 90 20 20 10 10 0 Wenatchee 63 45 63 42 61 43 / 70 10 10 0 10 0 Omak 71 42 64 38 61 36 / 40 20 0 0 0 0

OTX WATCHES / WARNINGS / ADVISORIES

WA...Flood Watch from 5 AM PDT early this morning through this evening for Washington Palouse. Wind Advisory from 8 AM this morning to 5 AM PDT Thursday for Lower Garfield and Asotin Counties. Wind Advisory until 11 PM PDT this evening for Moses Lake Area- Waterville Plateau-Wenatchee Area. ID...Flood Watch from 5 AM PDT early this morning through this evening for Central Panhandle Mountains-Idaho Palouse.