textproduct: Boise

DISCUSSION

High pressure ridge continues to build over the region this morning and will remain the dominant feature through next week. Satellite imagery shows high clouds streaming over the top of the ridge this morning, but this will have little effect on afternoon temperatures. Satellite also shows stratus and fog in sheltered valleys this morning which is expected to largely dissipate by noon. This pattern will reinforce valley inversions, keeping lower valley temperatures near seasonal normals while higher elevations see a warming trend. Winds will remain generally light, though some breezy southeast winds may persist briefly in the open terrain before diminishing this evening.

AVIATION

Mostly VFR. Areas of fog/low stratus in sheltered valleys resulting in some MVFR/LIFR and mountain obscuration. Fog/low stratus dissipating through the morning. Surface winds: Variable up to 10 kt, except E-SE 10-20 kt with gusts up to 28 kt near KJER and portions of the Snake Basin this morning. Winds aloft at 10kft MSL: W-SW 5-25 kt, highest north.

KBOI...VFR. Surface winds: SE 4-8 kt this morning, becoming light and variable this afternoon.

AIR STAGNATION

A strong ridge of high pressure will remain centered over the Pacific Northwest and Intermountain West through the upcoming week. This setup will maintain strong valley temperature inversions and very weak transport winds across the region. Mixing heights are forecast to remain suppressed, generally below 1500 feet AGL, for an extended period. These conditions will severely limit ventilation and lead to a buildup of pollutants in valley locations. The Air Stagnation Advisory remains in effect for all valley zones in southeast Oregon and southwest Idaho until 5 AM MST Friday.

PREV DISCUSSION

SHORT TERM...Today through Tuesday night...A ridge of high pressure will build overhead through the short term, pushing the storm track well to our north. The ridge will cause valley inversions to strengthen, keeping lower valley temperatures near normal. Meanwhile, higher elevations will gradually warm, with temperatures 5-15 degrees above normal by Monday and Tuesday. Mostly to partly clear skies and light winds will allow patchy valley fog to develop each night, mainly in mountain valleys, lower basins across southeast Oregon, and the Snake Basin. No precipitation is expected under this pattern.

LONG TERM...Wednesday through Sunday...High pressure will continue to build over the region through the work week, with air stagnation concerns from a persistent surface inversion expected through at least Thursday. Temperatures will remain warm during the daytime and likely above freezing for higher elevations (5000-7000 feet MSL) above the inversion. Cold pools with below normal temperatures will linger in lower elevation valleys, with the potential for low clouds and fog to develop each morning, primarily in sheltered mountain valleys.

Relief from the stagnant conditions could come as early as Friday with a dry cold frontal passage to our east potentially bringing breezy winds and cooler air into the valley. The big question mark in this forecast outcome is how far west the cold front/upper level trough will track, with the bulk of the ECMWF ensembles and EC AI solution keeping the frontal passage further east. This would keep the stagnant conditions in place over the area through the weekend. Regardless, precipitation and a significant change in weather is not anticipated in the long term period.

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.