textproduct: Boise

DISCUSSION

Precipitation has tapered off this evening, leaving behind a moist atmosphere across east Oregon and southwest Idaho. Mid to low clouds continue to engulf much of eastern Oregon, the terrain of central/west-central Idaho, and near the ID-NV border, with satellite imagery already showing some expansion from nighttime radiational cooling. Elsewhere, clearing did occur across much of the western Snake Plain around sunset, which will now pave the way for areas of fog to form where winds remain light. The exception will be around Huntington, Ontario, and Weiser where northwest winds at or above 10 mph will persist into the early morning hours. Current hi-res models are favoring the development of fog (60-90% chance) from the Boise metro to the Magic Valley overnight, as well as in portions of eastern Oregon where cloud decks are expected to lower to the surface. Fog should lift and improve throughout the day Friday due to daytime heating, but light winds with moist, inverted conditions could limit the amount of stratus erosion across the valleys and basins.

AVIATION

Low to mid level VFR/MVFR stratus will continue to decrease overnight. This will allow areas of IFR/LIFR low stratus and fog to develop across valleys of SE Oregon and SW Idaho. Fog most likely in sheltered valleys where winds are light. Conditions improving Friday afternoon, but may redevelop Friday night. Surface winds: light and variable, except locally up to 10 kt from the N-NW overnight. Winds aloft at 10kft MSL: variable less than 10 kt.

KBOI...Mid level stratus has cleared and light winds are expected overnight. This should allow LIFR fog or low stratus to develop (60% chance through the night) and persist into Friday morning. Surface winds light and variable.

Weekend Outlook...Areas of valley fog/stratus each morning with improvement each afternoon. On Sunday evening a cold front will move into SE Oregon. This will bring light precipitation mainly to northern areas and mountain obscuration late Sunday night into Monday. Snow levels 5500-7000 ft MSL. Surface winds: variable under 10 kt, except SW-NW 5-15 kt late Sunday night into Monday as the cold front moves through.

AIR STAGNATION

High pressure will support stable conditions through Sunday night, with daytime mixing heights peaking near or slightly above 2000 feet AGL. Light winds and strengthening near-surface inversions will result in relatively poor ventilation in the valleys. A cold front will arrive late Sunday into Monday and lead to improved mixing and ventilation.

PREV DISCUSSION

SHORT TERM...Tonight through Saturday night...At 1 PM MST radar still showed light rain echoes from Baker County through Malheur County into Owyhee County but slowly decreasing. Expect most of it to end by sunset. A few more weak echoes in the Boise Mountains and Upper Weiser River Valley should also end by sunset. Today's rain has added moisture to the lowest atmospheric levels which should lead to more fog than previously forecast. Difficult to determine where fog may become dense so no advisory will be issued with this package. Expect little day-to-day wx change through the short term, with a weak upper high creating an inversion in the valleys below, with the usual fog and low clouds. Except for a slight chance (15-20%) of rain near the ID/NV border tonight there should be no pcpn through Saturday night. Light winds through the period.

LONG TERM...Sunday through Thursday...High pressure and weak flow aloft will keep Sunday mostly dry with temps remaining above normal. However, that evening and early Monday a broad Alaskan low will swing mostly north of the area. High terrain in SW Idaho and SE Oregon will see a 20-40% chance of precipitation. The strong northwesterly flow behind the low will lower snow levels to 3.5 to 5 kft MSL, lower to the north. This means most mountain towns could see snow with minimal accumulation Monday morning. Temps also drop behind the low on Tuesday, down to normal for this time of year. Weak high pressure will again build into the area, but flow will remain strong and moist enough for another shot at precipitation Wednesday. Precipitation is likely to be very orographic as only a few model members show a shortwave providing dynamics. Precipitation will last through much of Thanksgiving day, but that night another brief dry spell will set in before what is shaping up to be a much colder weekend.

BOI WATCHES/WARNINGS/ADVISORIES

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