textproduct: Boise

DISCUSSION

Persistence through Friday morning, with cold air, low clouds, and patchy fog continuing below about 5000 feet MSL, and mostly clear skies (or cirrus clouds) with mild daytime temperatures above 5000 feet. Meanwhile, NBM keeps forecasting decreasing clouds and warmer daytime highs in the valleys. We don't expect that to happen until possibly Friday when a dry cold front and upper level short wave trough come in from the north. No changes to current forecast.

AVIATION

MVFR-IFR low clouds and fog will persist in valleys below 5-6kft MSL. Surface winds variable 10kt or less. Winds at 10kft: W-NW 10-20 kt.

KBOI...MVFR-IFR stratus through the day. Surface winds variable or SE less than 10kt.

AIR STAGNATION

Stagnant conditions remain in the valleys of southeast Oregon and southwest Idaho. Mixing heights will remain below 2000 feet AGL. Cooling aloft will weaken the inversion Friday, and light to moderate northwest winds behind a dry surface cold front will increase mixing, so we are hopeful that the current Air Stagnation Advisory will end on schedule at 5 AM MST Friday. Even a few hours of vertical mixing should clean the air before a new stagnation event begins about Sunday.

PREV DISCUSSION

SHORT TERM...Today through Friday night...Low clouds and fog remain in the valleys generally below 5000 feet this morning and will persist over the next several days. The clouds may produce flurries at times. In the mountains, mostly clear skies expected with mild temperatures during the day and cold temperatures at night. Temperatures will remain well below guidance in the Snake Basin and have used persistence in the forecast as the NBM and most of the high resolution guidance cannot resolve the Snake Basin cold pool. Clouds should persist through Friday below 5000 feet.

LONG TERM...Saturday through Wednesday...Dry conditions continue and persistence was used as the ensemble and NBM guidance remains useless in the Snake Basin. Cooler air aloft will begin to move into the area on Saturday which may help to clean out the low clouds and fog, and actually warm up temperatures in the valleys, although forecast confidence is low. A ridge builds into the area again by Tuesday, with warmer air aloft and a temperature inversion strengthening again. With persistence used, temperatures in the valleys remain below normal with above normal temperatures over the mountains.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Air Stagnation Advisory until 5 AM MST Friday IDZ011>014-016-028-033. OR...Air Stagnation Advisory until 5 AM MST /4 AM PST/ Friday ORZ061>064.