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FXUS65 KBOI 101036 AFDBOI

Area Forecast Discussion National Weather Service Boise ID 336 AM MST Sun Dec 10 2023

SHORT TERM
Today through Tuesday night...Moist westerly flow will bring a mix of precipitation to SE Oregon and SW Idaho through Monday. The focus of rain/snow/mix will be from Baker county into the w-central mountains this morning, expanding southward tonight as an upper tracks in from the Oregon coast.
The mild flow off the Pacific will push in a wedge of warm air aloft, lifting snow levels up to 5500-7000 feet across SE Oregon and to 4500-5500 feet across SW Idaho. The changeover to rain at lower elevations will complicate the precip type as colder sheltered valleys in SE Oregon could see freezing rain or even sleet, depending on depth/strength of surface cold pool. Expect most valleys/basins to change over to rain by late morning.
Areas that could hold onto mixed precipitation past this morning would be across northern Malheur county north of Hwy 20 and far southern Baker County. Valley rain and mtn snow continues into Monday as a second upper wave moves into the Pac NW. A more northwesterly flow will narrow snow levels to between 4500-5500 feet across the region. Valleys will total an additional 0.10-0.25 inches in liquid with 0.3-0.7 inches of liquid equivalent in the mountains through Monday. Showers taper off north to south Monday night into Tuesday as the final trough exits. Without pause, an upper ridge looks to build over the region on Tuesday. The accompanying light winds and stable air mass won't allow for much drying which could result in areas of fog and stratus development Tuesday into Tuesday night.

LONG TERM
Wednesday through Sunday...An upper level trough over the southwest U.S. will have little effect on our area as an upper level ridge builds north from California over southeast Oregon and southwest Idaho on Wednesday. As the trough exits to the east on Thursday, models agree on expanding the ridge over the Intermountain Region, keeping our area under dry westerly flow under the northern periphery of the ridge. Light surface winds and stable air will favor development of an inversion in the lower valleys, at least during nighttime hours, but there is still some uncertainty. Ensembles hold onto the ridge through the weekend, but confidence decreases as deterministic models differ on timing a weak disturbance across our area on Saturday or Sunday. The chance of measurable precipitation in the valleys is still less than 10%. Temperatures will be a few degrees above normal.

AVIATION
Mainly VFR this morning south of a KBNO-KBOI-KPIH line. Further north, expect periods of IFR/MVFR conditions with mountains obscured in low clouds and precipitation. Snow levels 2k-3k feet. Slight chance of freezing rain at KBKE and KBNO.
Precipitation spreading south to the Nevada border this afternoon and continuing through Monday morning, with snow levels rising to 5k-7k feet MSL. Surface winds: S-SE 5-15 kts.
Winds aloft at 10k feet MSL: W 25-45 kts.

BOI WATCHES/WARNINGS/ADVISORIES
ID...Winter Weather Advisory until 5 PM MST this afternoon IDZ011.
Winter Weather Advisory until 11 AM MST this morning IDZ033.
OR...Winter Weather Advisory until 10 AM PST this morning ORZ062.

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