Area Forecast Discussion
Issued by NWS Spokane, WA
Issued by NWS Spokane, WA
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379 FXUS66 KOTX 280959 AFDOTX Area Forecast Discussion National Weather Service Spokane WA 259 AM PDT Tue May 28 2024 .SYNOPSIS... A return of showers and thunderstorms today with a cold front passage. A few storms could be strong with gusty outflow winds in southeastern Washington and the Idaho Panhandle. Frosty conditions is expected Wednesday night into Thursday morning. Cool and showery conditions will continue through the end of the week with drier and temperatures warming back up to near normal for the start of the weekend. && .DISCUSSION... Today and Tonight: A cold front will push into the Inland Northwest today. The front is just off the coast and will push east of the Cascades this afternoon. There is weak instability at mid levels east of a line from Moses Lake to Omak. Radar is picking up on isolated convection across much of this area including into the Idaho Panhandle. Axis of best mid level instability with most unstable CAPEs of 200-300 J/kg is located from Ritzville to Coeur d`Alene. This will remain as an area for the best chances (up to 20%) for isolated thunderstorms into the early morning hours. CAPEs will then be retard to around 100 J/kg after 8AM or so this morning as mixing occurs in the boundary layer and the drier sub-cloud air entrain into the moisture at mid levels. There will still be the potential for isolated cloud to ground lightning strikes across much of the eastern half of the forecast area into the afternoon. Much of this convection will remain elevated. Best potential for surface based convection will be over the far southeast portion of the forecast area: essentially over the Northeast Blue Mountains, Camas Prairie and southern to central Idaho Panhandle. Higher clouds will be thickening up through the afternoon further northwest, which will decrease the potential for surface based convection to initiate and remain capped. This area where we may see surface based convection and points southeastward will be the area to watch for stronger thunderstorm development. CAPE values of 500-800 J/kg and shear values of around 40 kts up to 6 km will support the potential for stronger thunderstorms to develop. Main hazard will be for damaging wind gusts from convection. Could also see penny to nickle sized hail, but severe hail is less likely due to higher LCLs of 8 kft and higher and a drier sub- cloud layer. This direr air will result in evaporative cooling that will strengthen cold pools and increase the potential for damaging wind gusts. Instability may be limited though and may not be enough for strong to sever thunderstorms to develop this far northwest. Better chances will be south and east of the Lewiston- Clarkston Valley. Today will be a breezy day through the Cascades and out over the exposed areas of the Columbia Basin. Expect westerly winds to increase this afternoon with gusts in the range of 25-35 mph absent of convective showers and/or thunderstorms. Wednesday and Wednesday night: The upper level trough of lower pressure will move in over the Inland Northwest with a cold pool of minus 28 degrees Celsius over the northern half of forecast area. Showers with embedded thunderstorms are expected to develop under this cold pool with diurnal heating. Shear will be absent where convection is most likely tomorrow. Much less likely for thunderstorms to become strong and will be of the garden variety pulse type storms. There will be a lot of dry air that pushes in behind the cold front passage today. The drying of the boundary layer will make it more difficult for convection to develop in the Columbia Basin. The northern mountains of eastern Washington and the Idaho Panhandle will see the best potential for showers up to 40- 70%. Wednesday will see a drop in temperatures from the 70s today to the upper 50s and low 60s by tomorrow. Skies look to clear out Wednesday night with winds weakening. This will result in strong radiational cooling with dew points falling into the upper 20s. Temperatures will be chilly by Thursday morning with widespread frost conditions possible, especially in the sheltered mountain valleys. We will have to consider frost/freeze messaging for this period. Anyone with sensitive vegetation should consider covering their plants to protect them from frost/freeze damage. /SVH Thursday through Monday: Generally dry conditions are forecast for Thursday and Friday in the wake of Wednesday`s trough passage. And while Thursday will be a bit cooler than normal, temperatures look to rebound back to seasonal norms for the rest of the extended forecast. Moderate uncertainty does remain for the weekend into early next week, but we are anticipating increasing chances for showers, especially by Monday. Occasionally breezy winds are also expected each day, though any gusts are currently looking to remain below 30 mph. /KD && .AVIATION... 06Z TAFS: A weather system will spread an increase chance for showers and thunderstorms mainly east of a line from Moses Lake to Republic into Tuesday. Elevated instability will help contribute to showers developing between 9-18z mainly from the Tri Cities area up through the Columbia Basin and NE Washington. There is a 20% chance of thunderstorms at KGEG/KSFF with this round of convection, and a 10% chance at KMWH/KPUW/KLWS. Stronger storms are possible Tuesday afternoon and evening, but confidence is low on whether or not any of the TAF sites will be impacted. The most likely areas are KGEG/KSFF/KCOE/KLWS with a 30% chance of thunderstorms. The best chances will be over far NE WA/N Idaho with a 40-50% chance at Colville, Sandpoint, and Bonners Ferry. Locations south and east of Lewiston will also carry a 40-50% chance of thunderstorms. A few strong storms are possible, but none of the CAM`s models are showing strong convective gusts impacting any of the TAF sites. FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: There is high confidence for VFR conditions to continue across the region. The one exception is under heavier showers or thunderstorms where there is a moderate chance of visibility being reduced to MVFR. JW ----------------------- 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 73 42 59 36 65 42 / 30 10 20 10 0 0 Coeur d`Alene 73 43 57 37 63 41 / 40 30 40 30 0 0 Pullman 71 42 56 36 61 41 / 20 40 10 20 0 0 Lewiston 79 50 66 44 71 48 / 30 50 10 10 0 0 Colville 71 37 58 34 65 35 / 50 40 60 20 0 0 Sandpoint 71 43 57 34 61 39 / 50 60 70 40 10 0 Kellogg 74 45 54 41 59 42 / 30 60 40 40 0 0 Moses Lake 77 42 66 39 72 42 / 20 0 10 0 0 0 Wenatchee 72 45 64 42 69 45 / 10 0 10 0 0 0 Omak 76 41 64 37 70 42 / 20 10 10 0 0 0 && .OTX WATCHES/WARNINGS/ADVISORIES... ID...None. WA...None. && $$