textproduct: Boise

DISCUSSION

A ridge will continue to expand over the region over the next several days with warming temperatures aloft. Patchy fog is possible, especially in the lower basins and sheltered mountain valleys as the inversion strengthens. The ridge retrogrades offshore around Friday and allows cooler air to move down the Snake Plain from the east next weekend. No precipitation in sight and no updates to the forecast.

AVIATION

VFR, except patchy fog/low stratus overnight for sheltered mountain-valleys and lower basins. MVFR-LIFR in fog/low stratus, with mountain obscuration. Surface winds: variable up to 10 kt. Winds aloft at 10kft MSL: W to NW 10-25 kt.

KBOI...VFR. Mist/fog may develop in the vicinity between 12/10Z-15Z. Surface winds: light and variable.

PREV DISCUSSION

SHORT TERM...Tonight through Tuesday night...A strong upper level ridge will remain in place as the dominant feature over the Intermountain West through the period. This feature will keep the storm track well to our north and promote dry conditions across southeast Oregon and southwest Idaho. As the ridge strengthens, valley inversions will deepen and trap cold air near the valley floors while temperatures aloft warm significantly. This setup will result in a continuation of near normal temperatures in the lower valleys, particularly the Snake Plain and Baker Valley, while mountain locations and ridgetops experience unseasonably warm conditions. With light winds and clear skies expected overnight, radiational cooling will be efficient. This will lead to the development of patchy fog and low stratus in sheltered valleys and basins, including the Treasure Valley and Lower Snake Plain, by Monday morning. Any fog that forms may be slow to erode under the strengthening subsidence inversion. Winds will remain generally light and terrain driven, though a weak southeasterly drainage wind may persist in the Magic Valley.

LONG TERM...Wednesday through Sunday...The forecast period will continue to be dry under high pressure, with continued air stagnation concerns under an inversion. Valley cold pools will keep lower elevation temperatures leaning near to above normal (up to around 5 degrees above), while higher elevations will be 10-15 degrees above normal. Have adjusted higher elevation temps up to better capture the inversion affects. Daytime highs will be above freezing across much of our area, barring the highest peaks. Another implication of the valley cold pool, paired with light winds and generally clear skies, will be patchy fog each morning. The favored locations for fog development will be sheltered mountain valleys and lower basins.

Uncertainty still remains come late Thursday/Friday. As has been advertised, guidance has been in good agreement of a dry cold front/shortwave crossing to our east...with the uncertainty in the westward extent of it. The GFS has been the most ambitious in its western extend, which would bring increased winds and help mix some of the valley inversions, but appears to be trending east towards the Euro and EC-AI solutions. The majority of the Grand Ensemble members still favor that farther east track. Either way, the ridging is expected to persist throughout the long-term with no precipitation chances to be found. Beyond the long-term period, early next week, there are some hints that we may finally break out of this pattern. Stay tuned!

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.