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FXUS66 KOTX 182139 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 239 PM PDT Sat May 18 2024

SYNOPSIS
Cooler and unsettled weather will continue through the weekend.
Frost in the northern mountain valleys will be more patchy in nature with more cloud cover hanging around through the night. A brief break between weather systems Monday into Tuesday. Then a wet low pressure system is expected Wednesday into Thursday. The series of cold upper level low pressure systems through the week will keep temperatures below normal.

DISCUSSION

Tonight through Monday night: Cold pool aloft nested in a general trof pattern overhead consisting of 500mb temperatures hovering near -25 degrees C will allow for a conditionally unstable airmass to linger over Eastern Washington and North Idaho through at least Monday until some positively tilted shortwave ridging drops down from the northwest and allows the upper levels to warm up and bring some stability. Short term high resolution models suggest a vort maxes and other such small scale disturbances will utilize this instability to allow for continued showers and isolated weak low topped short lived pulse thunderstorms as the tropopause brought about by the cold trof overhead remains low. In addition the forcing at play, brought about by this vort max and any other additional disturbances, may allow for any convection to occur during the overnight hours. Thus pops don't drop off after sundown but more or less follow where the vortmax is expected to track which is northwest to southeast through Eastern Washington and North Idaho. Additional, though less resolved in the high res models, disturbances are expected to make similar travels through the cold trof overhead thus additional pops for spotty showers and thunderstorms remain in place Sunday and Sunday evening. Some clearing is suggested Sunday night into Monday morning, enough to merit mention of areas of frost. A repeat of minor mention of spotty convection lingers over portions of the Idaho Panhandle Monday afternoon as the earlier mentioned incoming upper level positively tilted ridge moves in and the resulting incoming stability should suppress convection over Eastern Washington.
Clouds then increase from west/northwest Monday night, mainly west of the upper level ridges positively tilted ridge axis but for the most part Monday night looks to remain mostly dry. Delayed onset of the incoming clouds may allow for another repeat of spotty frost late Monday night. As to be expected with a cold trof aloft forecast temperatures will tend to remain near or slightly on the cool side of what would be considered normal for this time of year. /Pelatti

Tuesday through Thursday: Confidence is increasing for a deep closed upper level low pressure system to drop down out of BC into the Northwest Tuesday night into Wednesday. The center of the low will pass across northwest Washington into north-central Oregon Tuesday night into Wednesday morning. A deformation band will track across the region and become hung up across the Idaho Panhandle, northeast Washington, and over into the northern Cascades. This area is expected to receive good soaking precipitation with a 70% chance for at least a quarter of inch of precipitation, a 50% for at least a half an inch, and a 20% chance of up to 0.75 inches of precipitation. The official forecast calls for between 0.50 to 0.75 inches of precipitation across the Idaho Panhandle into the Northeast Mountains and Okanogan Highlands. Extreme eastern Washington from the Spokane Area to the Palouse will more likely see rainfall amounts in the range of 0.25 to 0.50 inches.
Temperatures will be much cooler under the low with temperatures on Wednesday likely to struggle to warm into the upper 50s.

Thursday night through Saturday: There is more uncertainty with the ensembles on the overall pattern later in the extended portion of the forecast period, but the leading cluster is indicating another deep upper level low to drop down over the region around Memorial Day weekend. This second upper level low doesn't look quite as wet.
It will continue to keep the region unsettled though and cooler than normal. Anyone with camping plans for the holiday weekend should prepare for cool and showery conditions. /SVH

AVIATION
00Z TAFS: Conditionally unstable airmass with a disturbance dropping down northwest to southeast across the aviation area will allow for some mention of showers and possible weak short lived pulse thunderstorms. Still for the most part VFR conditions are expected. /Pelatti

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: Of course the disturbance dropping down could very well take a different trajectory through and miss some of the TAF sites that have mention of potential precip in them. In addition some stratus could form tonight into Monday morning due to any resulting rainfall today and tonight. /Pelatti

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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 40 61 39 68 44 66 / 40 40 20 10 0 10 Coeur d'Alene 39 57 39 65 42 66 / 40 50 20 20 0 10 Pullman 37 57 38 63 41 64 / 30 20 10 10 0 10 Lewiston 44 64 45 70 46 72 / 20 10 10 20 0 10 Colville 33 63 34 70 38 67 / 70 80 40 20 0 20 Sandpoint 37 55 38 63 41 64 / 60 80 30 40 10 20 Kellogg 40 52 40 60 44 62 / 50 60 20 40 10 20 Moses Lake 40 69 42 75 44 71 / 10 10 10 0 0 0 Wenatchee 43 67 44 72 49 68 / 10 10 0 0 0 10 Omak 39 69 41 75 45 71 / 30 30 10 0 0 10

OTX WATCHES/WARNINGS/ADVISORIES
ID...None.
WA...None.

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