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Area Forecast Discussion
Issued by NWS Spokane, WA
Issued by NWS Spokane, WA
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751 FXUS66 KOTX 162332 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 432 PM PDT Fri Aug 16 2024 .SYNOPSIS... Thunderstorms will continue into the evening across the northern mountains with concerns for flash flooding over burn scars near the Cascades. More showers and thunderstorms are expected Saturday evening and night for central Washington with the possibility of strong outflow winds, especially over the southern half of eastern Washington. Temperatures will be quite warm on Saturday with dry and breezy northeast winds across the Columbia Basin, West Plains, and southeast Washington. Unsettled weather will continue near the Cascades on Sunday with drier weather expected for Monday and Tuesday. Wetter and unsettled weather returns by midweek. && .DISCUSSION... Tonight...Interesting forecast through this evening as slow moving shortwave trough pivots slowly northward into extreme SE BC. The presence of this feature combined with orographic enhancement from southerly winds and MUCAPE values of 500-1000 j/kg should keep thunderstorms going through the evening. Although these storms wont be severe, due to limited shear (less than 10 kts) the main threat will continue to be locally heavy rains and the possibility of flash flooding. This is a direct result of precipitable water values in excess of 150% of normal for this time of year and storm motions of less than 10 mph. So far the heaviest rainfall per radar estimations has occurred over western Okanogan County. We will continue with our flash flood watch through 11 pm after which the threat of thunderstorms will taper off rapidly. While tonight`s weather will prove interesting the weather for Saturday and Saturday night will be potentially even more dynamic. The culprit will be a strong upper level low currently poised off the WA/OR coast between 130-135w. All the models dig this low southward overnight and into Saturday morning before pivoting it north-northeast during the late evening and overnight hours. Initially this amplifies a strong shortwave ridge over the Inland NW which will bring generally sunny skies and increasing amounts of east-northeast winds. High temperatures will surge into the mid 80s and 90s for all valley locations and could even touch triple digits in the LC Valley, Snake River Valley and the lower Columbia Basin. According to the NBM probabilities theres a 60% chance of exceeding 100F at the Lewiston Airport and a higher chance in downtown Lewiston with a 10-20% chance over SW Whitman and extreme southern Grant Counties. In addition to the heat, much of the Columbia Basin will see NE winds of 10 to 15 mph with gust of 20-25 mph possible during the hours of peak heating. There will also be relative humidity values dropping into the upper teens to mid 20s This will lead to enhanced fire weather conditions but will likely be too marginal to issue a fire weather watch. Saturday evening/night should be when most of the thunderstorm action develops as the low ejects NE and brings enhanced upper level difluence combined with strong synoptic scale forcing and MUCAPEs from 500-1000 j/kg. This will also be accompanied by 0-6km shear values of 40-50 kts which is more than enough to bring the possibility of severe weather. However the main threat will lie from the Cascades westward and thus we dont foresee terribly strong thunderstorms over our portion of eastern WA and north Idaho. Much more concerning will be the potential of strong outflow winds developing near the Columbia Gorge late in the evening and overspreading the Columbia Basin, Palouse, Blues, and perhaps into the Spokane area overnight. Some of the finer resolution CAM models are forecasting outflow winds out of the southwest of 40-50 mph and perhaps locally stronger close to the WA/OR border. We shall see. Whether or not these winds actually materialize is questionable however the DCAPE values in excess of 1500 j/kg certainly supports this notion. If the winds do occur they could cause areas of blowing dust, an abrupt wind shift, and the possibility of local tree or property damage. Most of the potential impacts in the Inland NW will occur after 9pm, which is fairly unusual for a severe/strong weather event. Nonetheless we should not disregard what some of the CAMs models are depicting. As far as other threats, much of the area near the Cascades will see another round of precipitation and locally heavy rains. However unlike todays threat of heavy rain, tomorrows will feature much faster moving storms and thus a smaller possibility of flooding rain. Consequently we wont issue a flash flood watch at this time. fx Sunday through Friday: The majority of the extended forecast will see low pressure to our west along the coast and a ridge just to the east of the Inland Northwest. Chances for showers and strong thunderstorms will linger into Sunday, but look to be confined to the Cascades at that point. Forecast then turns drier for Monday through Tuesday. Chances for storms return Wednesday and Wednesday night as a shortwave impacts the region. Enough lingering moisture (and uncertainty in the finer details of the weather pattern) means we are keeping slight chances for mountain showers in the forecast for Thursday and Friday. Daytime temps near daily normals through much of the extended, though Monday looks the warmest with temps around 5 degrees above average. Best chances for gusty winds will be near any thunderstorms that develop through the period, though locally breezy winds are anticipated Sunday across central Washington. /KD && .AVIATION... 00Z TAFS: Rain showers will continue across portions of northern WA, extreme eastern WA and the ID Panhandle, with isolated embedded thunderstorms possible through the evening. Thunderstorms are not expected to impact TAF sites, but could go over smaller fields like Winthrop, Omak, Republic and Colville. Clouds will clear south to north through the late afternoon and evening hours. Saturday will be relatively quiet through the afternoon. An incoming system will begin to impact the MWH area with gusty winds near the end of the TAF period. FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: High confidence for VFR conditions for the TAF period. /JDC ----------------------- 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 90 59 86 58 89 / 0 0 10 0 0 0 Coeur d`Alene 53 88 58 84 57 88 / 0 0 10 0 0 10 Pullman 52 90 53 83 53 84 / 0 0 10 0 0 0 Lewiston 62 99 65 92 64 92 / 0 0 10 0 0 0 Colville 45 87 48 85 47 90 / 20 0 20 0 0 0 Sandpoint 50 84 56 83 53 87 / 10 0 10 0 0 10 Kellogg 56 86 59 83 61 88 / 10 0 10 0 10 10 Moses Lake 55 94 59 86 57 89 / 0 0 30 0 0 0 Wenatchee 63 91 66 85 64 88 / 0 0 40 10 0 0 Omak 58 93 63 91 60 93 / 20 0 30 10 0 0 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...Flash Flood Watch until 11 PM PDT this evening for Central Chelan County-Okanogan Highlands-Okanogan Valley-Upper Columbia Basin-Wenatchee Area-Western Chelan County-Western Okanogan County. && $$