textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Near record or record cold temperatures forecast for Saturday and possibly Sunday. Patchy clouds and/or easterly winds over the Divide/channeled terrain and points west could help temperatures from getting too cold in spots.

- Snow showers will persist along the Divide and points west through the weekend. Roadways and mountain passes could become snow-covered at times.

- Gradual warm-up with afternoon showers is expected Tuesday and Wednesday. Frosty mornings are possible.

- 70-80% chance for high temperatures reaching the 70s by Friday across north-central Idaho and western Montana lower valleys. 70-80% chance for low 80s through Hells Canyon and Whitebird by Friday.

An unseasonably cold airmass will remain in place through early next week. How often do we see the legit polar vortex, normally found in the Arctic latitudes or Hudson Bay, over southern Saskatchewan and Montana in late April? This system will continue to keep the upper levels cold keeping it unstable producing daily showers. Also with the coldest air found east of the Divide and lower pressure to our south and west, easterly winds will develop nightly. Snow showers could be persistent over the Divide and points west, including the Glacier Region.

Freeze warnings and frost advisories have been issued for many locations. We opted to leave out the higher valleys along the Divide as those have been cold already. Patchy clouds and easterly winds through channeled areas could help keep hard freezes from occurring. Outside of the wind and cloudy areas, temperatures may fall into the teens and lower 20s which could cause damage to unprotected outdoor plumbing and irrigation systems. We tried to give more detail in the warnings on the areas that have a higher probability for this to occur.

Previous daily record low temperatures for Saturday that could be tied or broken include: Eureka 18 degrees F. 2006 Kalispell 21 degrees F. 1924 Libby 22 degrees F. 2002 Missoula 24 degrees F. 1988 Butte 12 degrees F. 1960 Salmon, Idaho 12 degrees F. 2022 Fenn, Idaho 22 degrees F. 2013 Grangeville, Idaho 25 degrees F. 1984 Pierce, Idaho 22 degrees F. 2009

Models continue to show a secondary trough Sunday night situated over Idaho which could bring a period of precipitation to southern Idaho County and Lemhi County. This could bring snow-covered conditions at Lost Trail Pass and other passes in southwest Montana by Monday morning.

We'll be under cyclonic flow through Thursday. This means the air aloft is moving in a counter-clockwise direction over the region relative to the center of the polar vortex which will shift east over the Great Lakes region by mid-week. It also means that temperatures may be a little slower to moderate, afternoon instability clouds and showers are possible, and we could be looking at more frosty mornings.

A warm ridge of high pressure finally builds by the end of the week. Probabilities are high that 70s may return to western Montana valleys, and near 80 towards Hells Canyon in Idaho.

AVIATION

The warm spring sun and a near record cold air mass over the region is causing convection again today. The amount of water available in the atmosphere for precipitation today is less than yesterday, so we expect the showers to be much less intense today. The overall air mass is much colder and we expect near record low temperatures overnight tonight with lows in the low to mid 20s by Saturday morning.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Freeze Warning from midnight tonight to 8 AM MDT Sunday for Bitterroot/Sapphire Mountains...Flathead/Mission Valleys... Lower Clark Fork Region...Missoula/Bitterroot Valleys.

ID...Freeze Warning from midnight MDT /11 PM PDT/ tonight to 8 AM MDT /7 AM PDT/ Sunday for Eastern Lemhi County...Northern Clearwater Mountains...Southern Clearwater Mountains.

Winter Weather Advisory until 11 AM PDT this morning for Southern Clearwater Mountains.

Frost Advisory from midnight MDT /11 PM PDT/ tonight to 8 AM MDT /7 AM PDT/ Sunday for Orofino/Grangeville Region...Western Lemhi County.