Sunrise 6:14 AM   Sunset 9:12 PM Moonrise 5:37 PM   Moonset 3:42 AM

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FXUS65 KBOI 200926 AFDBOI

Area Forecast Discussion National Weather Service Boise ID 326 AM MDT Mon May 20 2024

SHORT TERM
Today through Wednesday night...An active weather pattern will persist in the short term, bringing early spring conditions. An upper-level trough today will be followed by a much stronger closed low-pressure system on Wednesday. Overall, temperatures will remain well below normal.

This morning, the upper-level trough will gradually shift southeastward across eastern Washington and Oregon, reaching south-central Idaho by the afternoon. This will result in the highest instability and moisture in central Idaho zones (West Central and Boise Mountains), offering the best chance for convective showers and thunderstorms. There is a possibility of convective initiation extending further west over the higher terrain of Baker County and northern Malheur and Harney counties, although confidence in this is low. Some showers and thunderstorms may also reach the Upper Treasure and Western Magic Valleys later this afternoon. Despite weak overall convective parameters, with CAPEs around 200 J/KG, conditions are favorable for gusty outflow winds from mountain convection.
The most favorable outflow winds are expected across the Western Magic Valley and Eastern Upper Treasure Valley, with gusts up to 50 MPH potentially causing localized blowing dust.

A transient ridge is forecast for Tuesday as the region remains between systems. Temperatures are expected to rise approximately 6 degrees above today’s highs but will still be about 4 degrees below normal.

A much stronger low-pressure system is anticipated to reach the Pacific Northwest late Tuesday evening, pushing a cold front through the forecast area on Wednesday morning. Widespread precipitation will precede the front on Wednesday morning, followed by gusty westerly winds in the afternoon as the upper- level low center reaches northern zones. Winds have increased significantly from previous model runs, approaching wind advisory levels across Southeast Oregon. Additionally, a strong upper-level jet will promote orographic lift across the Central Idaho mountains, leading to prolonged precipitation into Wednesday evening. The mountains could receive up to 0.75 inches of rain, while the Western Snake River Plain and areas south and west could see between 0.10 and 0.20 inches. Snow levels will start around 7500 feet on Tuesday, gradually lowering to around 5500 feet by Wednesday evening. Current guidance suggests a total of 2-4 inches of snow above 6500 feet from Tuesday night through Wednesday night, with about a 45% chance of McCall receiving 0.1 inches of snow or more (mainly Wednesday night).

LONG TERM
Thursday through Monday...As a closed low tracks eastward, cool northerly flow and wrap-around moisture will continue over the forecast area into Thursday. The best chance for lingering precipitation will be over northeast Oregon and southwest Idaho, where a 50-75% chance is forecast. The coldest air, and therefore the lowest snow levels of 4500-5000 ft MSL, are expected early Thursday morning...resulting in the potential for light snow to accumulate down to these elevations. Several inches of snow is likely over the high mountain terrain of east Oregon and southwest Idaho through Thursday, with greater than 50% chance of seeing at least 6 inches of snow above 7000 ft MSL for the west-central Idaho mountains. Precipitation should taper off throughout the day on Thursday as the low continues east, but breezy to windy conditions can be expected in south-central Idaho in the wake of the low.
Thereafter, slightly warmer temperatures are anticipated Friday with the return of brief ridging. The next low pressure system is expected to arrive Friday night through Saturday night, supporting a chance (15-35%) of showers to return to the area. Models still diverge on how far south this trough will progress, and therefore the temperatures, wind speeds, and shower extent is still a bit uncertain. Warmer, drier weather is on track to return Sunday/Monday as high pressure builds.

AVIATION
Mainly VFR. Scattered showers and isolated thunderstorms will develop this afternoon across SW Idaho and far E Oregon.
Mountains obscured in showers. Thunderstorms capable of small hail and outflow gusts to 30-40 kt. Snow levels 5500-6500 ft MSL. Surface winds: W-NW 5-15 kt, becoming 10-20 kt with gusts to 25-30 kt this afternoon. Winds aloft at 10k ft MSL: N-NW 10-20 kt.

KBOI...VFR. Showers developing after Mon/18Z over the Boise foothills, with a 20% chance of showers hitting the terminal.
However, nearby shower and thunderstorm activity will be capable of producing outflow wind gusts 30-40 kt that could travel to KBOI.
Best chance after Mon/19Z, ending Tue/03Z. W-NW winds becoming gusty by late morning.

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

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