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FXUS66 KOTX 271133 AFDOTX

Area Forecast Discussion National Weather Service Spokane WA 433 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 air piles up west of the Cascades creating surface high pressure. This will setup a tight pressure gradient across the forecast area delivering the 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. On Monday, the pressure gradient increases toward 12 mb. 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 becoming 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
12Z TAFS: One shortwave is traversing the region this morning producing bkn-ovc skies across the northern mountains and Idaho Panhandle and spotty light showers. A second shortwave arrives 17- 04Z and combined with afternoon heating will result in expanding ceilings between 5-8k ft AGL region-wide along with widely scattered light showers. A few cells in the afternoon and early evening will be capable of a slightly deeper depth producing heavier downpours and lcl ceilings as low as 3000 ft AGL. This also comes with a 10-20% chance for lightning strikes with the focus north of a line from Coeur D Alene to Omak. Another area that will need to be monitored for lightning strikes is south of a line from Lewiston to Mullan. Winds will increase this afternoon with speeds of 10-15 mph across the Columbia Basin and Okanogan Valley. Gusts will be in the vicinity of 25 mph. Showers decrease after 03-04z for most sites but a completely dry forecast is not expected given the unsettled nature of the upper-level trough over the region. /sb

FORECAST CONFIDENCE AND/OR ALTERNATE SCENARIOS: There is a small threat for scattered to broken MVFR stratus developing this morning 15-18z between Pullman and Spokane. This comes with low confidence to impact the terminals but HRRR carries a 50% prob of MVFR cigs for areas in between. Confidence is low for lightning strikes. SREF has a 5-10% chance for general thunder; HRRR keeps the best chance along the Canadian Border but produces enough afternoon CAPE to keep from removing in the general forecast. Given the low probabilities, did not mention in the TAF forecasts. As mentioned above, will need to keep an eye south of Lewiston for cells developing around the Camas Prairie capable of enough growth for lightning strikes. /sb

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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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