949 FXUS65 KBOI 120908 AFDBOIArea Forecast Discussion National Weather Service Boise ID 308 AM MDT Fri Sep 12 2025
.SHORT TERM...Today through Sunday night...The upper-level low has elongated over the northern Rockies this morning, extending from southeast Oregon into north-central Montana. This feature is maintaining isolated showers and thunderstorms, mainly across Baker County and the west-central Idaho mountains this morning. The low will remain over the region today with sufficient moisture to support additional showers and thunderstorms this afternoon. Convection will initiate over higher terrain, with a few storms drifting into the lower valleys by evening. Storms may produce heavy rain and small hail. Precipitation chances will diminish tonight with the loss of daytime heating.
An upper-level ridge will briefly build over the area Saturday, bringing a break from precipitation and allowing temperatures to rebound to near normal.
Latest guidance trends faster and stronger with the next low pressure system expected to move through on Sunday. While some timing differences remain, conditions appear favorable for thunderstorm development (3050 percent chance) as the trough passes during peak heating. Precipitable water values approaching the 85th percentile, along with modest shear and instability, will support a risk for heavy rain, small hail, and gusty outflow winds.
.LONG TERM...Monday through Friday...Shower chances diminish Monday as an upper level trough moves east of the area. Temperatures are expected to be 5-10 degrees below normal Monday and Tuesday behind the front. By Wednesday, upper level ridging should bring warmer temperatures within a few degrees of normal. However, forecast confidence decreases by Thursday and Friday with models split between another trough moving into the Pacific Northwest, resulting in slightly cooler temperatures, or the upper level ridge hanging on and bringing continued warmer and dry conditions.
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.AVIATION...Areas of fog are expected to form in the mountain valley locations by early Friday morning with reduced visibility at KBKE, KBNO and KMYL through 16Z. Scattered showers and thunderstorms, with brief MVFR and IFR conditions will develop in the afternoon across the Baker Valley and in the Boise and West Central Mountains of Idaho late Saturday afternoon with heavy rain, small hail, and gusts to 35 kt. Surface winds: mainly SW-NW 5-15 kt. Winds aloft at 10kft MSL: SE-SW 5-15 kt.
KBOI...VFR. Scattered mid level clouds. Surface winds: NW 5-10 kt.
Weekend Outlook.. Drier conditions will return Saturday with VFR conditions expected. Expect early morning fog in mountain valleys. There is about a 30 percent chance of showers and thunderstorms on Sunday afternoon. Surface winds mainly SW-NW 5-15 kt.
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.BOI WATCHES/WARNINGS/ADVISORIES... ID...None. OR...None.
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SHORT TERM...JDS LONG TERM....JB AVIATION.....JB
NWS BOI Office Area Forecast Discussion