textproduct: Boise

DISCUSSION

Mostly clear skies tonight should allow temperatures to reach or fall below freezing in most places. The upper-level ridge will continue to weaken overnight as the next system moves inland across the Pacific Northwest. A weak associated cold front will push east late Monday morning, bringing light, isolated showers. Precipitation amounts are expected to be minimal, with only a few hundredths of an inch of rainfall forecast, mainly below 4,500 feet across Baker County and into the Central Idaho Mountains. Pockets of cold air trapped in the lower valleys of eastern Oregon and far western Idaho late Monday morning will create a slight chance of freezing rain should precipitation reach the surface. Snow levels will quickly rise, however, to 7,000 to 9,000 feet by Monday afternoon. Because this frontal system does not appear strong enough to fully clear the valley inversion, the Air Stagnation Advisory will continue through Tuesday morning.

A much stronger, more moist system remains on track for late Tuesday into Wednesday, which will bring breezy conditions and widespread precipitation favoring the Central Idaho Mountains. Due to persistently high snow levels (above 7,000 feet for most of the event), areas below 7,000 feet are likely to see moderate rainfall, totaling 0.50 to 1.00 inch. Strong westerly flow aloft will create a rain-shadowing effect on the Treasure Valley, limiting total precipitation to around 0.10 inches. There is strong potential for Wind Headlines on Tuesday, especially south of the Snake River Plain, where guidance over the past 48 hours continues to show advisory-level winds (gusts of 45 to 57 MPH). The forecast remains on track, and no significant updates are needed at this time.

AVIATION

MVFR-LIFR conditions in patchy valley fog/stratus tonight into Monday morning, mainly between KONO-KEUL. Clouds increase as precipitation moves west-to-east in SE OR starting Mon morning and in SW ID Mon afternoon. Snow levels 4-6 kft MSL rise to 7-9 kft MSL Monday evening. 15-30% chance of freezing rain across valleys Mon 12-18Z. Surface winds: light/variable. Winds aloft at 10kft MSL: W-SW 15-25 kt.

KBOI...VFR. Clouds increase with a slight chance of rain beginning Monday afternoon. Surface winds becoming southeast tonight, but generally light and variable.

AIR STAGNATION

A system will move across the area on Monday, weakening the ridge that has been in place and bringing light precipitation to the area. However, this system is not expected to be strong enough to remove the inversion. As a result, mixing heights will remain less than 1,500 feet AGL on Monday, and winds will generally remain less than 10 mph resulting in poor ventilation. An active pattern will bring increased mixing and winds, as well as widespread precipitation, Tuesday and Wednesday which will end the stagnant conditions.

PREV DISCUSSION

SHORT TERM...Tonight through Tuesday night...A trend toward more active weather will develop over the next couple of days. An upper level ridge will remain in place tonight, with patchy fog redeveloping in the lower Treasure Valley. On Monday morning, a system will begin to move inland and bring light precipitation to eastern Oregon and west-central Idaho. It will weaken as it encounters the ridge, so amounts from this activity will be minimal (trace up to 0.05"). Cold air trapped in the lower valleys of eastern Oregon and far western Idaho will result in a slight chance of freezing rain. However, any amounts will be very light, limiting any impacts. Light snow will be possible down to around 4000 feet before snow levels rise to 6500-8000 feet north and 7500-9500 feet south by Monday evening. The system will continue to bring a chance of rain and high elevation snow into Tuesday with light amounts expected. Temperatures on Monday will cool several degrees from today, but remain 5-15 degrees above normal. However, on Tuesday, high temperatures especially at lower elevations will rise by 5-10 degrees as the inversion is finally eroded.

A deep trough will move onshore into British Columbia Tuesday night, sending a more significant plume of moisture into our region. Precipitation chances will rise to 40-60% in the lower elevations, and 60-90% in the mountains. Snow levels will remain high through Tuesday night at around 6500-8000 feet, but 2-4 inches of snow are expected by late Tuesday night on higher peaks. Meanwhile, a tight surface pressure gradient will develop over our area as a deep surface low tracks inland near the U.S./Canada border. This will combine with strong winds aloft (50-70kt at 700 mb) to produce windy conditions Tuesday night, strongest on ridgelines and across southeast Oregon and far southwest Idaho south of the Snake River where gusts 40-60 mph will develop. Wind headlines will likely be needed in some areas as this system approaches.

LONG TERM...Wednesday through Sunday...A potent, upper-level disturbance will move over the area on Wednesday. Significant Pacific moisture will accompany this trough, but so will a very strong mid-level jet. Precipitation will be under the effects of shadowing in valley areas causing lower amounts, however, a cold front moving through the area Wednesday morning and afternoon should help PoP chances areawide. Snow levels will begin around 6000-8000 ft MSL Wednesday morning and rapidly drop from north to south to 3000-5000 ft MSL by afternoon. Surface winds will respond accordingly to this strong mid-level jet and cold front Wednesday morning and afternoon, with widespread wind gusts of 30+ mph (with higher ridgetops getting 50+ mph) across eastern Oregon and the mountains of southwest Idaho. This would result in blowing/drifting snow and visibility issues for the higher terrain. Additionally, temperatures will become several degrees cooler on Wednesday/Wednesday night compared to Tuesday, particularly for the northern areas and also the mountains where precipitation is heaviest. Compared to the normal, however, it will still be above average temperatures.

Drier northwesterly flow will establish behind the trough Wednesday night into early Thursday, with a chance (10-25%) of lingering showers. This brief respite from precipitation will be short lived, as a significant plume of subtropical moisture (atmospheric river) impacts the Pacific NW on Thursday. This event will be longer duration than the previous, lasting from Thursday through at least early Saturday. Snow levels will oscillate quite a bit during this period: starting 3000-5000 ft MSL Thursday morning, approximately 4500-6500 feet on Thursday into Friday, then lowering to 2500-5000 feet (lowest along northern CWA border) by late Friday/early Saturday with another cold front as precip begins to decrease. Overall, mountains could see 1 to 2 inches of liquid equivalent throughout this period, along with hefty snowfall totals depending on the snow level. Notable rainfall over saturated, steep terrain will also introduce rockslide concerns. While flooding is currently not expected, rivers and streams at low/mid elevations will swell resulting from multiple days of above average rainfall. Meanwhile, temperatures will stay above normal for most areas Thursday and Friday, with the coldest air expected across west- central/central Idaho.

BOI WATCHES/WARNINGS/ADVISORIES

ID...Air Stagnation Advisory until 5 AM MST Tuesday IDZ011>014-016-028-033. OR...Air Stagnation Advisory until 5 AM MST /4 AM PST/ Tuesday ORZ061>064.