textproduct: Boise

DISCUSSION

High pressure will build into the West today, bringing generally clearing skies to the region, except for stratus trapped in the lower valleys. Lingering instability and moisture aloft will support scattered cumulus clouds and light upslope snow showers over the West Central Mountains. These showers are expected to taper off by early afternoon with minimal additional snow accumulations of less than an inch. No updates needed at this time.

AVIATION

VFR-MVFR in a stratus deck over much of the region, expected to clear this evening. Mountains obscured in the stratus. Sporadic light snow/drizzle is possible, but unlikely to cause impacts beyond the ceilings. After the stratus clears this evening, mountain valley fog and lowland stratus becomes possible again tonight and tomorrow morning. Surface winds: variable less than 10 kt, except the Western Magic Valley maintaining W-SW 15-25 kt. Winds aloft at 10kft MSL: N 5-15 kt.

KBOI...VFR with lower clouds clearing this afternoon. Surface winds: E-SE 3-7 kt.

Weekend Outlook...Mostly VFR with clear skies on Saturday and increasing clouds on Sunday. Localized LIFR-IFR conditions in patchy morning fog in sheltered valleys. Generally variable surface winds less than 10 kt.

AIR STAGNATION

The building high pressure ridge will promote the development of steep low level inversions across valley locations starting late Saturday. Forecast soundings indicate a trend toward light transport winds and mixing heights dropping below 1500 feet AGL. These conditions will limit vertical mixing and could lead to the accumulation of pollutants in the Snake River Valley and other sheltered basins. While ventilation rates may remain marginal on Saturday, the potential for stagnation increases Sunday and Monday as the ridge axis centers overhead. An Air Stagnation Advisory will likely be needed.

PREV DISCUSSION

SHORT TERM...Today through Sunday night...A ridge of high pressure will build into the West today bringing generally clearing skies with scattered cumulus clouds over the central mountains and some low level clouds along the Boise Foothills. Increasing mid level clouds as of 2 AM MST will generally keep the risk of fog low across the area this morning, with sheltered mountain valleys seeing any fog that forms mix out by mid morning. Better valley fog chances will occur under clear skies on Saturday and Sunday morning.

Some lingering instability and moisture aloft will allow for light upslope snow showers in the West Central Mountains today, tapering off by the early afternoon with minimal accumulations of less than an inch. Winds will generally weaken across the region, with breezy westerly winds in the Magic Valley and on ridgetops near the Nevada border through this evening. A surface inversion will begin to build in this weekend over regional valleys, keeping low elevations cool and bringing air stagnation concerns due to limited mixing by Sunday.

LONG TERM...Monday through Friday...Dry and stagnant conditions will likely prevail through the work week thanks to persistent high pressure over the region. A plume of moisture extending from the Central Pacific along the ridge will push moisture north of our region on Monday. However, this moisture will allow widespread clouds to briefly move in over the area with a 20% chance of precipitation from stray showers generally north of a line from Baker City to McCall. High pressure will then close off over the California Coast, bringing dry conditions and allowing for a surface inversion to continue to develop through at least Thursday thanks to light winds and limited daytime mixing. Cold pools will develop in sheltered valleys, bringing a chance of valley fog and keeping temperatures below normal in most lower elevation valleys while mid slopes and ridgetops will see above normal temperatures.

A shift from the persistent stagnant conditions could develop late next week, thanks to a broad trough over the Central Plains creating a tight pressure gradient over the state by Friday. This might be enough to push colder air and breezy winds into the region, clearing out any stagnant conditions that formed. However, forecast uncertainty remains high among cluster solutions on the depth of this trough and the resulting impact on the broad high over West.

BOI WATCHES/WARNINGS/ADVISORIES

ID...None. OR...None.