textproduct: Boise

LONG TERM...Monday through Friday

Dry and stagnant conditions will prevail through the week as high pressure remains persistent over the region. Models have continued to push the slight chance in northern areas further north on Monday, leaving only a sliver of 10% chance of snow/rain showers in Valley county. The high pressure will then close off the California coast and amplify as it moves into the Gulf of Alaska. This will set up a strong blocking pattern continuing inverted/stagnant conditions through Friday. 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. Models begin to show a weakening in the blocking pattern around next Friday the 16th. Interestingly, shortly after this hint of weakening, while deterministic and ensemble height charts come out of agreement, the teleconnections all show a sharp trend to negative values (Primarily EPO, PNAO, WPO. I'm mentioning teleconnections in a forecast discussion, can you tell the long term is dry and stagnant yet?). The negative TC values favor the breakdown of the blocking pattern, especially as we trend towards ENSO neutral.

AVIATION

VFR in a stratus deck over much of the region, expected to clear this evening. Mountains obscured in the stratus. 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 clouds clearing. 10% chance of low clouds tonight development, if any, will favor the river. 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

A strong upper level ridge will remain centered over the region through the weekend and into next week. This pattern will support the development of steep low level inversions and very weak transport winds. Forecast mixing heights are expected to remain well below 1500 feet AGL for extended periods. These conditions will severely limit ventilation, leading to the accumulation of pollutants in valley locations. An Air Stagnation Advisory has been issued for all valley zones in southeast Oregon and southwest Idaho and remains in effect until Thursday morning though there is low confidence (30% chance) atmospheric mixing will improve by that time.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Air Stagnation Advisory from 11 PM this evening to 5 AM MST Thursday IDZ011>014-016-028-033. OR...Air Stagnation Advisory from 11 PM MST /10 PM PST/ this evening to 5 AM MST /4 AM PST/ Thursday ORZ061>064.