textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Moderate to heavy snow continues through this morning for the mountains of central Idaho and northwest Montana.

- Another shortwave Thursday will cause some light valley snows.

- A ridge builds back in quickly this weekend causing a return valley cold pools and stagnant conditions.

Zonal, or straight west-to-east flow is impinging on the mountains of central Idaho and western Montana this morning, and it has a lot of moisture with it. A subtle short wave embedded in that flow is crossing the region this morning, causing heavier snow yet again, which will last through mid morning. Strong southerly winds ahead of that shortwave have caused temperatures to warm in the valleys more than expected, and so much of the night we've had temperatures in the mid 30s from I-90 south. Once that shortwave passes this morning, that will change and snow levels will come back down to the valley floors again region wide.

Another shortwave is forecast to move through on Thursday. With the snow levels much lower, this will be the best chance for some snow in the valleys. However, this next wave will have much less moisture to work with, so snow amounts will be lower than the current system. Even in the valleys, most locations will get less than an inch of new snow. The most valley snow will be in northwest Montana again, and even then a high end forecast (90th percentile) is 1.5 inches.

Friday night into Saturday a ridge starts building into the region. This will cause some warming in the terrain, but with the recent snow and stable conditions it will also cause cold pools to form in the valleys. There will be plenty of moisture moving through the ridge, so expect some mid and high cloud cover most days under the ridge. This complicates the temperature forecast in the extended, because the timing of cloud cover can have a big impact on the actual temperatures. If it clears out for a while at night, it will cool off more. And if it's cloudy during the day, it will warm up less. In any case, conditions will be very stable and we expect stagnant air in the valleys by mid next week.

AVIATION

A plume of moisture and a shortwave all embedded in strong zonal flow are moving through the region this morning. That's causing a lot of mountain snow. So far this morning the plume has moved south of terminal KGPI. Terminals KMSO and KHRF are better aligned, but the strong westerly winds are causing a rain shadow effect in the valleys around the terminals, so the main impact is mid-level ceilings. However, as the shortwave gets here around 07/12Z, expect that to be able to overcome the down-slope flow and cause some showers. Currently we are not expecting conditions worse than IFR at any area terminals as this system moves through.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Storm Warning until 11 AM MST this morning for Lower Clark Fork Region.

Winter Weather Advisory until 11 AM MST this morning for Bitterroot/Sapphire Mountains...Butte/Blackfoot Region... Kootenai/Cabinet Region...Potomac/Seeley Lake Region...West Glacier Region.

ID...Winter Storm Warning until 10 AM PST this morning for Northern Clearwater Mountains.

Winter Weather Advisory until 10 AM PST this morning for Southern Clearwater Mountains.