textproduct: Boise

DISCUSSION

The upper ridge over the west has flattened this evening as the storm track moves through WA/BC Canada. This has brought a layer of high clouds across E Oregon/SW Idaho today, and has slightly weakened the broader-valley surface inversions this afternoon from the increased pressure gradient. This supported breezy easterly surface winds earlier this afternoon particularly across northeast Oregon and portions of the western Snake Plain. Meanwhile, cold drainage winds from southeast ID will strengthen overnight through the Magic Valley and keep easterly winds elevated for this area through morning. The upper ridge will gradually rebuild Sunday and into this week, which should reinforce stable conditions and promote air stagnation.

AVIATION

VFR, except patchy fog/low stratus in sheltered mountain-valleys overnight. Mtn obscuration in fog/stratus. Surface winds: variable or easterly up to 10 kt, except 10-20 kt around KJER/KGNG through Sun AM. Winds aloft at 10kft MSL: SW to W 5-20 kt.

KBOI...VFR. Surface winds: SE 5-10 kt, becoming light and variable Sunday afternoon.

AIR STAGNATION

High pressure will remain centered over the region through the end of the week. This pattern will maintain valley temperature inversions and very weak transport winds. Forecast mixing heights are expected to remain below 1500 feet AGL for an extended period. These conditions will severely limit ventilation, leading to the accumulation of pollutants in valley locations. The Air Stagnation Advisory is currently in effect until 5 am MST Friday for all valley zones in southeast Oregon and southwest Idaho. Long range models suggest these conditions may persist beyond Friday.

PREV DISCUSSION

SHORT TERM...Tonight through Monday night...A high amplitude upper level ridge remains the dominant weather feature over the Intermountain West. This pattern will maintain dry and stable conditions across southeast Oregon and southwest Idaho through the short term period. High pressure is firmly situated along the West Coast and will continue to build into the region, further stabilizing the atmosphere. Higher than anticipated southeasterly wind has helped temperatures across much of the Treasure Valley reach the low 40s this afternoon. With high clouds clearing tonight though temperatures will drop below freezing again. Mostly clear skies and light winds aloft, should allow cold air to continue to pool in the lower valleys. This setup is highly conducive to persistent surface inversions and the formation of patchy fog and low stratus, particularly in the Long Valley and other sheltered mountain basins. While mountain locations will enjoy abundant sunshine and temperatures above seasonal normals, some valley locations will remain cooler due to limited daytime mixing. No significant precipitation is expected through early next week.

LONG TERM...Tuesday through Saturday...More of the same through the long term, with high pressure and stagnant conditions dominating through the extended. Come Thursday, guidance is hinting at a shortwave swinging northeast of our area. While no precip is expected (better forcing remain northeast of our area), a more westerly track could work to increase transport winds to help mix valley inversions and bring cooler temperatures into our area. However, out of the Grand Ensemble, only 21% of members favor a more westerly track (mostly out of the GFS Suite). The EC-AI, EC, and GFS- AI keep it east which would do little to help mix out our valleys. Beyond Friday, the blocking pattern sets back in the high pressure setting up over the Pacific Northwest, maintaining the dry and stagnant conditions.

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.