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FXUS65 KBOI 020353 AFDBOI

Area Forecast Discussion National Weather Service Boise ID 853 PM MST Fri Dec 1 2023

DISCUSSION
Cool northwest flow continues tonight. Some breaks in the clouds have allowed temperatures to cool back below freezing across much of the area with patchy dense fog forming over the fresh snow. Strong radiational cooling will continue for a few more hours. The next shortwave off the WA coast will move inland with increasing clouds and precipitation becoming widespread on Saturday morning. Precipitation will generally start as snow in the Treasure Valley but changeover to rain during the afternoon, with snow accumulations generally under an inch. A frontal boundary will stall out from about Baker City OR though Stanley ID on Saturday afternoon with cold air to the north. Some of the (30 percent) ensembles bring that front south across the Treasure Valley on Saturday night into Sunday morning which would change the precipitation back over to snow in the Boise Metro, with moderate to heavy snow for a few hours.
The rest of the models (70 percent) keep the front northeast of BOI which would keep the precipitation as rain in the Treasure Valley. Nonetheless, expect heavy mountain precipitation with significant snows in the central ID mountains above 6000ft.
Snow levels slowly rise on Sunday and Monday to 7000-9000ft, with much warmer conditions on tap. The forecast is on track with no updates planned.

AVIATION
MVFR/IFR and local LIFR due to widespread snow/low ceilings. More snow in a N-S band over SW Idaho is moving eastward, reaching KTWF/KJER by 01/21z. Mountains obscured. Fog/mist may set in between snow showers. Areas of LLWS in excess of 20 kt. Surface winds: W-SW 5-15 kt with gusts 30-35 kt in higher elevations. Winds aloft at 10k feet MSL: W-NW 30-50 kt.

Sunday Outlook...MVFR to LIFR conditions through Sunday due to widespread precipitation, with low elevation rain and snow over higher elevations. Mountains obscured. Patchy fog is possible between snow showers. Snow levels rise to 5500-6500 ft Sunday.
Surface Winds: generally SW-SE 10-20 kt, except SW 25-35 kt with gusts 35-50 kt for E Oregon and S of Snake Plain. Winds aloft at 10k feet MSL: W to NW 30-60 kt.

PREV DISCUSSION
SHORT TERM
Tonight through Sunday night
Active weather pattern to continue through the Sunday as a series of warm moist systems moves through the region. At 2 PM, light-moderate snow, associated with a shortwave, will continue east into south- central Idaho through late this afternoon. This will be the last of the widespread precipitation today before the next system arrives Saturday morning. Expect up to an inch of snow on cold surfaces this afternoon though most roads will remain wet. The next system will bring a warm front through the area late Saturday morning for another round of precipitation. Snow levels will start out around the valley floors with surface temperatures near freezing but will quickly rise through the morning changing the snow to rain below 4000 feet. Up to an inch of wet snow will be possible in the valley Saturday morning though most areas see less than 0.5". The upper level dynamics increases Saturday afternoon as the trough moves across western Canada which will continue precipitation across the area as well as increase the surface winds especially along the Nevada border. Have expanded the wind advisory into Southeast Oregon Saturday for wind gusts up to 55 MPH. Despite the stronger winds, blowing snow will be less likely given warming temperatures and wet snow. A third system pushes another warm front through on Sunday which will increase the snow levels from 4000-4500 feet Sunday morning to 6000-7000 feet Sunday afternoon. Expect periods of heavy snowfall in the West Central and Boise Mountains from Saturday morning through Sunday Morning for 12" to 24" with locally higher amounts. However, snow will change to rain below 6000 feet Sunday afternoon which will limit snowfall in the mountain valleys.

LONG TERM
Monday through Friday
Widespread precipitation will taper off Monday as a ridge builds over us. Light precip may continue in northern parts of our area, as moisture continues to flow to our north. By Tuesday, the pattern will be dominated by a deep low in the Gulf of Alaska. The ridge and southwest flow will help warm temperatures and raise snow levels to 8000-9000 ft MSL. A period of short lived stagnant conditions may be possible in the upper Snake Valley on Tuesday under the ridge. As the deep low slowly moves towards the PacNW coast, most of our area will stay dry until Wednesday afternoon, when the ridge exits eastward. After which precipitation chances increase to 40-60%, almost all of which will be rain.
Precipitation chances remain the same through the rest of the period, but snow levels begin to drop Friday as the Pacific low moves inland. By Friday afternoon, snow levels will drop to 3000- 4000 ft MSL. Temperatures during the ridge pattern will be 10-15 degrees above normal, dropping to 5 degrees above normal on Friday.

BOI WATCHES/WARNINGS/ADVISORIES
ID...Winter Storm Warning until 5 PM MST Sunday IDZ011-013-028.
Wind Advisory from 11 AM to 11 PM MST Saturday IDZ015-016-029-030.
Winter Weather Advisory until 11 AM MST Saturday IDZ033.
OR...Wind Advisory from 11 AM MST /10 AM PST/ to 11 PM MST /10 PM PST/ Saturday ORZ061-063.

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