textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Current: High pressure is reinforcing valley inversions, air stagnation. Fog/low stratus persists especially north of I-90.

- Next hazard: Messy system arrives Friday. Warmer air overrunning valley cold pools creates a risk for freezing rain.

- Outlook: Cool, showery weather continues through the weekend and into early next week.

A strong upper level ridge remains firmly anchored overhead this afternoon. This pattern reinforces a classic wintertime subsidence and valley inversion setup. While mountains and mid-slopes are basking in the sunshine and relatively warm temperatures, valleys have remained decoupled and cold.

Satellite imagery this afternoon confirms stubborn valley fog and low stratus north of I-90. Elsewhere, pockets of morning fog have lifted but we anticipate a "reload" of fog and low clouds again overnight tonight to ring in the new year.

With light winds and limited vertical mixing, air stagnation is becoming increasingly noticeable. Pollutants from industry, automotives, wood stoves, and eventually holiday fireworks will only continue to worsen air quality and hazy skies near the most populated areas.

The ridge begins to break down late Thursday night, opening the door for another Pacific system with relatively "mild" winter time temperatures. The primary challenge with this forecast will be the thermal profiles. Warm air advects into the region with valley cold pools and cold ground/road surfaces firmly in place.

Snow levels are progged to surge quickly above 4,500 ft Thursday afternoon and into the evening south of I-90. Precipitation will likely begin as snow or sleet but rapidly transition to freezing rain as the "warm nose" deepens and extends northward.

North of I-90 will be a particularly tricky forecast as snow levels may not rise as fast leading to a prolonged period of snow or sleet into mid-morning Friday before the transition to freezing rain.

Overall, this is not a major snow producer. The main hazard is ice. Even at pass levels, expect a slushy trace to couple inches before the changeover to freezing or plain rain.

The flow becomes a little more progressive through the weekend as further prolonged ridging is not anticipated.

AVIATION

High pressure continues to enforce strong valley inversions and light winds. KGPI remains the primary site impacted by persistent IFR/MVFR stratus and fog that may dissipate temporarily later this afternoon/early evening before reloading into the morning of New Year's Day. Other airfields will remain in good flying weather conditions through sunset but likely deteriorate back to fog and low ceilings overnight as well. KBTM and KSMN will generally prevail with VFR conditions although localized haze may reduce slant-range visibility at times.

MSO WATCHES/WARNINGS/ADVISORIES

MT...ID...Air Stagnation Advisory until noon PST Thursday for Lower Hells Canyon/Salmon River Region...Northern Clearwater Mountains...Orofino/Grangeville Region...Southern Clearwater Mountains.