textproduct: Boise

LONG TERM...Sunday through Thursday

A broad ridge of high pressure will build over the Western US on Sunday, bringing air stagnation concerns and dry conditions through the middle of next week. Cold pools and patchy fog with low stratus clouds will likely develop in area valleys due to limited daytime mixing, with above normal temperatures midslope and on ridgetops. Model agreement remains consistent on the development of the ridge and the dry pattern for early next week, with some discrepancies in the path of a trough by the end of the long term period (Thursday). Some members are digging the trough into the region, bringing cold air and the potential for snow on valley floors, while others keep the coldest air to our north and west, keeping us in warm southwest flow which would keep snow levels higher around 5000-6000 feet MSL. At this point, agreement exists on the pattern shift back to wet weather, but the track of the coldest air remains uncertain. The Climate Prediction Center 6-10 day outlook favors above normal temperatures continuing (60% chance) with above normal precipitation.

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.AVIATION..Mostly VFR. Scattered rain/snow showers today through tomorrow bringing periods of MVFR/IFR and localized LIFR conditions. Snow levels: 5k-7kft MSL dropping to 3k-6kft MSL overnight. Low confidence in lightning (20% chance) and gusty outflow winds up to 40 kt, primarily over high terrain in Boise Mtns. Mtns obscured. Areas of LLWS tomorrow morning, primarily over Snake River Valley. Surface winds: SE-SW 15-25 kt with 25-45 kt gusts. Strongest gusts up to 55 kt will be near the NV border. Winds aloft at 10kft MSL: SW 35-60 kt.

KBOI...Generally VFR. Brief MVFR/IFR conditions in convective showers developing late this afternoon/evening. Low confidence in lightning and gusty outflow winds up to 40 kt (20% chance) with stronger showers this evening. Surface winds: SE-E 5-15 kt. Gusts to 20-30 kt, tapering off after sunset.

Weekend Outlook...Widespread snow showers in high terrain Fri thru Sat afternoon, with IFR-LIFR conditions in snow. Snow levels 3-4 kft MSL drop to valley floors Sat morning. Scattered showers in lower elevations during this time. Precip clears by Sat night with patchy fog/stratus that morning as an inversion builds. Surface winds: SE- SE 15-20 kt with gusts to 40 kt on ridges Fri, becoming W 15-25 kt Saturday, and finally light and variable Sunday.

AIR STAGNATION

High pressure is forecast to build over the region beginning Saturday night and will strengthen through the early part of next week. This pattern will favor the development of a strong and persistent surface based temperature inversion from Sunday through at least next Tuesday. With light winds and low mixing heights (1000-2000 feet) expected under the ridge, there is increasing concern for deteriorating air quality and poor ventilation in the sheltered valleys of southeast Oregon and southwest Idaho. While it may be too early for a formal advisory, conditions will be monitored closely as the stagnant air mass becomes established.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Winter Weather Advisory from 5 AM Friday to 5 AM MST Saturday IDZ011-013. OR...None.