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FXUS65 KBOI 030207 AFDBOI

Area Forecast Discussion National Weather Service Boise ID 807 PM MDT Thu May 2 2024

DISCUSSION
Isolated to scattered showers will persist for much of the area through late evening, then a short-wave ridge of high pressure is expected to move overhead for much of Friday. This will lead to a warming/drying trend for Friday, with temperatures rising 10-15 degrees from high temps today.
The next weather-maker, a deep low pressure system, is expected to usher in gusty SE winds for much of Saturday, along with increasing precipitation chances Saturday afternoon into mid day Sunday. Current guidance indicates a 60-70% chance of at least 0.5 inches of precipitation with the weekend system, and a 30-40% chance of an inch or more. Those with outdoor plans this weekend should follow the situation closely.



AVIATION
FR, except MVFR in scattered showers and/or low clouds mainly impacting SW Idaho. Areas of mountain obscuration. Showers decreasing overnight. Snow levels 4500-5500 ft MSL. Surface winds: variable or NW 5-15 kt, then S to SE 5-15 kt Fri afternoon.
Winds aloft at 10kft MSL: NW to NE 15-25 kt, veering to S after Fri/15Z.

KBOI...Isolated rain showers at/near terminal could bring brief MVFR ceilings. Best chance for rain after Fri/04Z, ending by Fri/09Z. VFR and clearing Friday. Light NW or variable winds, becoming SE 5-15 kt around Fri/21Z.

Weekend Outlook...Widespread precipitation moves into SE Oregon mid- morning Saturday, then into SW Idaho Saturday afternoon. A 20% chance of thunderstorms Saturday afternoon in SW Idaho. Scattered to numerous showers continue Sunday. Snow levels 6000-7000 ft MSL, lowering to 4000-5000 ft Sunday. Mountains obscured. Surface winds: SE 15-25 kt with gusts 25-35 kt Saturday afternoon, W 5-15 kt with gusts to 25 kt Sunday.



PREV DISCUSSION
SHORT TERM
Tonight through Saturday night
A dynamic weather pattern is unfolding over the next few days, characterized by a shortwave passage today followed by the arrival of a robust cold core low pressure system over the weekend. This setup indicates a high potential for significant rainfall during the upcoming weekend.

The shortwave is currently tracking east-southeast, expected to reach southeast Oregon later today and southwest Idaho tonight.
Convective activity, including showers and thunderstorms, has already commenced across southeast Oregon, with further development anticipated south of the Treasure Valley through this evening. While eastern portions of the Upper Treasure Valley and Western Magic Valley may experience some activity later today, the majority of the Treasure Valley should remain dry until late tonight. Showers and thunderstorms could bring brief heavy rain, along with graupel below 4000 feet. Higher elevations may see a transition to graupel or snow with minimal accumulation.

As the shortwave moves out of the region tonight, wrap-around showers from the east will persist across the West Central and Boise Mountains, extending into much of the Western Snake River Plain into early Friday. Friday itself is expected to be warm and dry as a weak ridge builds over the area, with temperatures reaching the mid to upper 60s in the lower valleys.

The subsequent weather system, characterized by a deep closed low, will sweep across the forecast area late Saturday. A substantial precipitation band will form along the cold front preceding the low, spreading from southeast Oregon early Saturday to southwest Idaho by late afternoon. Recent model runs suggest increased precipitation amounts, with a 60-80% probability of 0.25" or more of liquid across much of the area, and a 10-20% chance of exceeding 1.00”. Snow levels are expected to remain elevated above 6500 feet, limiting snowfall to the highest mountain peaks.

Winds will intensify on Saturday, initially from the east as the low draws in air, then shifting to the northwest behind the cold front Saturday night. Sustained winds of 20-30 mph, with gusts of 30-40 mph, are anticipated ahead of the front along the I-84 corridor from Twin Falls to Baker City, as well as over higher open terrain in southeast Oregon. Additionally, there is a potential for thunderstorms ahead of the cold front across south-central Idaho on Saturday afternoon, with forecasted surface temperatures in the low 70s and favorable atmospheric conditions supporting a few stronger thunderstorms.

LONG TERM
Sunday through Thursday
Unsettled weather from a deep low continues to dominate the earlier parts of the long- term forecast. Sunday's max temp drops 10-20 degrees below normal under the very cold airmass, though the min temps are 'only' 5 degrees below normal. A 60-80% chance of precipitation exists throughout the area late Sunday, dropping to 20-40% in lower valleys and 50-70% for higher elevations early Monday through later Tuesday. There is a brief peak in activity Monday evening as an embedded upper shortwave is carried through in the northwest flow aloft. Late Sunday through Tuesday, lower elevation liquid precip totals will be 0.05-0.20 inches. Higher elevation liquid totals of 0.3-0.5 inches will likely fall as 1-6 inches varying greatly with elevation as snow levels are forecast to be 4000-5000 ft MSL. There is a very small chance (<5%) that parts of the Snake Plain see an early morning flurry, but accumulation is not expected even if it does happen.

Conditions become more stable early Wednesday as the upper gradient weakens and most of the energy heads east. A few models are resolving a very positively-tilted weak trough moving through Wednesday evening, but some models show close to nothing, thus there exists only a 10-20% chance of precipitation. Thursday and potentially onwards models generally agree on a very mundane pattern as a closed high develops to our northwest and upper gradients weaken significantly.

BOI WATCHES/WARNINGS/ADVISORIES
ID...None.
OR...None.

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