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FXUS66 KOTX 270940 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 240 AM PDT Sat Apr 27 2024

SYNOPSIS
The weekend and early next week will feature showery weather over the mountains with seasonal like temperatures. Sunday and Monday have the potential to be windy with the passage of a frontal system. This system will also bring snow to the Cascade mountain passes Sunday night and Monday morning. Temperatures begin to warm mid next week onward, but showers remain in the forecast as well.

DISCUSSION

Today through Monday: The next few days will feature breezy to locally windy conditions as several shortwaves swing across the PacNW. The first shortwave is evident on water vapor near 130W this morning and will arrive today. This wave is encountering split flow which will split its energy into Oregon and along the Canadian Border. A second shortwave arrives Sunday but will only offer a glancing blow with majority of its energy once again deflected along the Canadian border as midlevel flow in buckling to the southwest in response to the third most compact shortwave diving toward the region. This third, most aggressive system, will arrive Sunday night and Monday. Each of the waves will force pressure falls on the lee side of the Canadian Rockies while cooler, maritime piles up west of the Cascades creating surface high pressure. This will create a tight pressure gradient across the forecast area delivering our multi-day winds. On average, the pressure gradient from say Portland, OR to Kalispell, MT will average around 7-8mb for Saturday and Sunday with winds generally blowing out of the south and southwest. In general, wind speeds will be as follows: Saturday - South/Southwest 10-15 gusts to 25-30 mph; Sunday - South/southwest 15-20 mph G 30-35 mph; Monday - Southwest to West/Northwest 15-25 mph G 30-40 mph. There will be a few areas with locally stronger speeds including the Waterville Plateau, exposed ridgelines of the Cascades, and our wind prone areas in the foothills of the Blue Mountains.

We cannot rule out the threat for patchy blowing dust but this comes with very confidence and is very hard to predict, especially this time of year. Thinking Monday will carry the main risk in the Western Columbia Basin around Moses Lake and Othello due to areas most active with planting.

Precipitation will come in the form of showers with a few weak thunderstorms. Showers on Saturday and Sunday will mainly focus over the Cascade Crest and into the rising terrain of Northeastern WA and North Idaho. Showers will expand at times into the northern Basin for Saturday and Sunday. On Monday, most areas will be vulnerable to precipitation given the stronger forcing. Not overly excited about the lightning threat but tough to call it none with 100-300 J/kg of CAPE each afternoon. Greatest risk on Saturday will be over the northern mountains. T-storms have been removed on Sunday then including for much of the Idaho Panhandle and eastern third of WA for Monday.

Precipitation type will mainly be rain in the lowlands and mix of rain/snow in the mountains through Sunday. Sunday night and Monday, snow levels come crashing down closer to 3000 feet or slightly lower. Would not rule out some wet snow mixing down to 2500 feet at times. The main impacts will be snow returning to the mountain passes Sunday afternoon through Monday. My main concern for any impacts will be after sunset Sunday into Monday morning when 2-3 inches will be possible for Stevens Pass. This comes with a 30% chance of occurring. Washington Pass is expected to receive 1-2 inches with similar amounts for Lookout Pass. Motorists planning overnight trips across the passes should be cautious of winter travel conditions.

Temperatures will be a few degrees cooler today vs Friday with little changes into Sunday. Readings will be close to 30-year averages or in the upper 50s to mid 60s. Temperatures will drop a handful of degrees on Monday as the air mass cools roughly 4 Celsius topping out in the 50s for all locations. If clouds clear and winds decrease Monday night, there will be a good chance for freezing temperatures Tuesday morning. /sb

Tuesday through Friday...The extended forecast remains uncertain with a division in ensemble predictions beginning Wednesday night.
For Tuesday through daytime Wednesday, the atmosphere remains unsettled with afternoon showers and the potential for isolated lightning strikes over mountainous terrain and far Eastern WA.
Temperatures Tuesday night will drop into the mid 30s, leaving the area with the potential for frost. Those with sensitive plants may want to keep an eye on Tuesday morning lows near freezing within the mountain valleys and across the upper Basin. Showers will return Wednesday afternoon, but the coverage will remain over the mountainous terrain. Winds will increase each afternoon, with gusts of 15 to 20 mph, before decreasing each evening.

Beginning Wednesday night, ensembles begin to differ. Some ensembles continue with a troughing pattern and the arrival of a shortwave ridge, while others slide a ridge into the region. If the ridge remains off the coast, temperatures will be cooler than the current forecast with the region remaining in the trough. But confidence is increasing that ridge will shift onshore, warming temperatures more than the current forecast by the end of the week. Afternoon will also contain enough instability that could allow for spring showers to pop up. Details remain uncertain on specifics this far out, so stay tuned. /KM

AVIATION
00Z TAFS: VFR conditions tonight for all TAF locations with a 10-20% chance of MVFR cigs redeveloping for GEG/SFF/COE/PUW towards 15Z, though confidence isn't high and therefore have it as a few/sct deck instead of bkn. Will see patchy to areas of fog develop across the northeastern WA/north ID valleys aft 06Z. There is a 15-30% chance of thunderstorms Saturday afternoon, though highest confidence is along the Canadian border. Do not have mention in the GEG/SFF/COE TAFs.

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: Low to Moderate confidence of showers for COE Saturday afternoon.

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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 59 40 58 40 53 33 / 40 30 10 10 60 10 Coeur d'Alene 57 41 55 38 50 32 / 40 40 40 20 80 30 Pullman 56 39 56 38 48 33 / 20 10 20 30 80 30 Lewiston 63 46 64 44 56 38 / 20 20 10 20 60 30 Colville 57 34 58 38 52 30 / 70 60 30 30 70 20 Sandpoint 56 41 52 39 48 32 / 70 70 70 50 90 40 Kellogg 54 43 52 36 45 32 / 70 40 70 50 90 50 Moses Lake 64 39 64 42 58 36 / 20 10 0 0 20 0 Wenatchee 60 42 61 39 55 37 / 10 10 0 10 10 0 Omak 62 39 63 38 58 33 / 40 10 0 10 30 10

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

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