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FXUS66 KOTX 042217 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 317 PM PDT Sat May 4 2024

SYNOPSIS
A slow moving low pressure system will bring widespread rain to the Inland Northwest Sunday and Sunday night. As the low pulls out of the region on Monday and Tuesday, the weather will become cool, showery, and breezy. Warmer and drier weather will arrive late next week. Next weekend has the potential to deliver our warmest temperatures of the spring so far.

DISCUSSION

Tonight and Sunday: As of 2PM, an elongated upper low making landfall along the coast of Oregon and northern California was spreading rain inland from southwest BC to as far south as central California. Water vapor satellite imagery shows two distinct circulations with one off the coast of central OR dropping south and the other pivoting inland over northern California. As these circulations move inland overnight, the morning model runs generate an band of wrap around precipitation that will spread northward overnight into our region. Around sunrise, ensemble consensus has the rain band extending from about Bend Oregon to Tri Cities to Spokane. By afternoon, significant precipitation is forecast to pivot west toward Moses Lake, Wilbur, and Republic before migrating eastward Sunday night. If the band evolves this way, the majority of central/eastern Washington and north Idaho will receive much needed rainfall. The National Blend of Models (NBM) gives north Idaho and the eastern third of Washington a 60 to 80 percent chance of a quarter inch by Monday morning. And the NBM generates a 50 to 60 percent chance of a half inch or more from Ritzville to Tri Cities by the evening. Precipitation deficits were common region-wide during the month of April, so rain of this magnitude will be important for our dryland farmers in the Columbia Basin, Palouse, and West Plains. Unfortunately, a chilly rain will not be pleasant for outdoor activities on Sunday.
For Spokane, our latest guidance generates between a tenth and a quarter inch of rain across the metro in the morning with temperatures in the mid to upper 40s.

Monday and Tuesday: By Monday, our low will be emerging east of the Rockies. Surface pressure falls over Montana will contribute to a tight westerly pressure gradient across the Inland Northwest.
West or southwest winds of 20 to 25 mph with gusts to 35 mph will be common from the Waterville Plateau across the Columbia Basin to the Palouse and West Plains. Mid-level instability will be reinforced by an incoming Pacific shortwave. With 500mb temperatures as cold as -27C in the afternoon, rain and mountain snow showers will develop with the most concentrated shower activity over the mountainous terrain of north Idaho, northeast Washington and along the Cascade crest. But even widely scattered showers in central Washington before surface dewpoints in the 30s are replaced by dewpoints in the 20s Monday night and Tuesday. A few lightning strikes will be possible over the mountainous terrain around Republic, Colville and Northport in the afternoon as the NAM and GFS generate up to 100 to 300 J/Kg of surface based CAPE.

Cooler and drier air will spill east of the Cascades Monday night into Tuesday. Snow levels will fall as low as 2500 to 3000 ft Tuesday and Tuesday night. Precipitation amounts by Tuesday and Tuesday night will be light and mainly orographic. What little snow falls will be in the mountains of north Idaho and along the Cascade crest. Tuesday afternoon has the potential to be a graupel or small hail day in north Idaho with highs in the low to mid 50s, steep lapse rates, and low freezing levels. And it will be windy Tuesday with more west winds between 15 and 20 mph with gusts of 30 mph or more across similar areas as Monday. The deep low over Montana won't move much until Wednesday.

Wednesday through Saturday: To quote the Beatles' song, "Here Comes the Sun". The medium range models are in good agreement that the Pacific Northwest will experience a warming and drying trend over the second half of the week. The formation of a Rex Block over the western U.S. will likely deliver our warmest temperatures of the season by Friday into the weekend. The National Blend advertises widespread 70s by Friday and our first 80s of the season for central WA and the L-C Valley. /GKoch

AVIATION
18Z TAFS: Mid to high level clouds will continue to increase across the Inland NW and gradually lower this afternoon and evening as bands of precipitation move up from the south. Light rain is expected across the Cascades by the morning hours on Saturday with rain at KEAT by 16Z with local MVFR ceilings. South and easterly winds will pick up with the front approaching.
Ceilings will continue to lower overnight. Rain bands will increase across KLWS, KPUW, KCOE to KGEG after 05Z.
There is a 90% probability of MVFR ceilings for KGEG-KCOE-KPUW after 10Z.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: High confidence for VFR conditions through Saturday night. Chances for MVFR ceilings expand early Sunday morning to KLWS, KPUW, and KGEG. There is low probability of visibility dropping below 3SM at Taf sites.
Ensembles have brief periods of 2SM at KPUW. JDC

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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 43 50 40 57 36 59 / 50 100 80 40 10 10 Coeur d'Alene 43 50 39 53 36 55 / 40 100 90 70 20 20 Pullman 40 47 38 52 34 53 / 80 70 90 60 30 20 Lewiston 48 56 45 60 41 59 / 80 70 90 60 30 20 Colville 43 52 39 60 34 61 / 20 90 90 60 30 20 Sandpoint 44 50 40 52 36 54 / 30 100 90 80 40 40 Kellogg 46 47 40 48 37 49 / 50 90 100 90 50 50 Moses Lake 47 58 41 64 38 64 / 60 90 30 10 0 0 Wenatchee 50 59 44 61 41 59 / 40 60 20 20 0 0 Omak 49 62 41 65 38 66 / 20 60 30 20 10 0

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

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