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FXUS66 KOTX 192303 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 403 PM PDT Sun May 19 2024

SYNOPSIS
Showers and isolated thunderstorms will quickly subside this evening. Cold temperatures will lend to frost formation for portions of the region overnight, especially across the northern mountain valleys. There will be a brief break between weather systems Monday into Tuesday. Then a wet low pressure system is expected Late Tuesday night and Wednesday, lingering Thursday. The series of cold upper level low pressure systems through the week will keep temperatures below normal.

DISCUSSION

Tonight through Monday night: A low pressure trough over the Inland Northwest with colder temperatures aloft will keep the atmosphere conditionally unstable. Shower development this afternoon will dissipate quickly with the loss of surface heating. Only a handful of cloud to ground lightning strikes have been observed from the Northeast Mountains to the Northern Panhandle. I don't expect much more coverage of thunderstorms for the rest of this afternoon with weak surface based CAPE and little to no wind shear. Clouds are expected to clear out across much of eastern Washington through the evening with some lingering over the Idaho Panhandle. Dew points sitting in the lower 30s will lead to frost formation across the northern mountain valleys as clear skies and weak winds promote strong radiational cooling. A frost advisory has been issued for the Okanogan Highlands and Northeast Mountains in effect late tonight through early Monday morning. This includes the communities of Republic, Northport, Metaline Falls, Colville, Chewelah, and Deer Park. Tonight is expected to be the last cold night this week with widespread frost potential. It's also possible that patchy frost will be observed over the northern portions of the Upper Columbia Basin, Palouse, and Northern Panhandle. Elected not to include these areas in the frost advisory as it will be much more patchy in nature.

Chances for showers and isolated thunderstorms will mainly be confined to the Idaho Panhandle into extreme eastern Washington for Monday. These areas will see a -24 Celsius cold pool aloft at 500 mbs. Shortwave ridging will be building across central Washington with temperatures warming aloft and having a stabilizing effect on the atmosphere. Temperatures will see a bump up with highs warming into the mid 60s to mid 70s across much of the Inland Northwest on Monday. /SVH

Tuesday through Sunday: As a low pressure slides into the region, rain showers will increase throughout the day on Tuesday. Rain will start over the Cascades, spreading eastward by the afternoon. Rain will become widespread overnight and into Wednesday morning. Some areas, like the lower lee of the Cascades and the deeper Columbia Basin will receive less rain. Rain totals will be upwards of 0.5 inches to over an inch in the mountains, with a quarter to half an inch in the lower elevations. The Columbia Basin will miss out on much of the rain with less than 0.05 inches expected. Snow will be limited to the highest terrain, with less than an inch forecasted at Washington Pass, and Stevens Pass will remain as all rain, with the possibility of a few snowflakes mixed in. This rain, along with additional snowmelt from the mountains will lead to rivers rising, but should remain below action stage. During the day on Wednesday, an isolated lightning strike is possible but don't expect to see any. Tuesdays temperatures will be near normal, dropping about 10 degrees cooler for highs on Wednesday as the system passes through.
In areas that receive limited rain could see breezy winds with gusts up to 25 mph.

For Thursday and beyond, scattered showers will continue, especially in northeast WA and ID Panhandle. Another low arrives late Friday into Saturday, with additional rain to the area. Temperatures will conintue to stay slightly below normal for this time of year. /KM

AVIATION
00Z TAFS: Scattered showers with isolated embedded thunderstorms will wane this evening with loss of surface heating. Main airports that may be impacts by convection this evening will be KCQV-KDEW- KSZT-KGEG-KSFF-KCOE. Hazards may include infrequent lightning, brief heavy downpours, pea sized hail, and gusty erratic winds up to 25-30 kts with stronger thunderstorms. Showers with isolated thunderstorms will mainly be confined to extreme eastern Washington and the Idaho Panhandle for Monday afternoon. Patchy fog or low stratus will be possible Monday morning in mountain valleys of northeast Washington and the Idaho Panhandle.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: Confidence is low with a 15-20% chance that thunderstorms will develop at KGEG- KSFF-KCOE this evening through 03Z. There's a 15% chance for low stratus producing MVFR conditions along the Spokane-Coeur d'Alene corridor Monday morning. This will be dependent on additional rain that this evening that moistens the boundary layer. /SVH

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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 37 68 41 62 45 53 / 30 20 10 20 70 90 Coeur d'Alene 37 65 41 61 45 51 / 50 30 20 20 80 90 Pullman 37 62 42 60 44 52 / 10 20 10 10 80 90 Lewiston 44 70 46 69 50 60 / 10 20 20 10 80 80 Colville 35 69 42 63 40 53 / 40 20 10 30 80 90 Sandpoint 36 62 39 61 45 51 / 60 50 20 20 80 100 Kellogg 38 60 42 59 47 49 / 50 40 20 20 80 100 Moses Lake 41 75 44 66 45 63 / 10 0 0 10 20 50 Wenatchee 44 72 49 63 47 64 / 10 0 0 20 20 40 Omak 39 74 44 66 45 63 / 30 10 10 20 50 70

OTX WATCHES/WARNINGS/ADVISORIES
ID...None.
WA...Frost Advisory from 2 AM to 8 AM PDT Monday for Northeast Mountains-Okanogan Highlands.

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