textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Mountain snow and difficult conditions in the back country continue through tomorrow morning.

- Frost/freeze concerns remain Monday and Tuesday mornings.

- Northwest flow becomes established for the rest of the week with unsettled but mild conditions.

A cold trough is moving across the region today causing snow showers at higher elevations. Due to the higher springtime sun angle, it warms up considerably during the day causing showers. Adding the uncertainty in the snow levels due to the warm season and wide range of changes during the day to the spotty nature of showers leaves us with a wide range of potential snow amounts in the mountains. On the low end of the forecast (10th percentile) the mountains above 5,000 feet will pick up 2-4 inches across the region. On the high end (90th percentile) those same areas pick up 5-12 inches of new snow. This is all by 0600 MDT Monday morning, and of course the snow amounts will depend strongly on elevation. We have issued winter weather advisories to cover the impacted areas.

At the lower elevations, showers will be mostly rain, but graupel is also a possibility. Yesterday evening there were reports of graupel showers that covered the road and caused intermittent slick roads. These types of showers are likely again this evening.

With the cold air and north flow behind this system, frost and freeze concerns remain for the major valleys of western Montana including the Mission, Flathead, Missoula, and Bitterroot valleys through tomorrow morning. Most of these valleys are nearing their climatological average last date for a frost, so this isn't surprising. However, this represents a big change from the warm start we've had to Spring and many may have been lulled into a false sense of safety for their cold sensitive plants. They'll need to take precautions to protect those plants for another day or two, so we'll continue with the frost and freeze warnings we have out. The air mass starts to moderate and warm some by Tuesday morning, but it will still be on the cold side. We'll evaluate if we need to continue the frost advisories into Tuesday tomorrow morning.

After this late spring shot of cold air, the flow remains northwesterly the rest of the week, with a slow trend towards westerly flow next weekend. Overall, this will lead to a gradual warm up with temperatures near average for the rest of the week. Intermittent shortwaves will cause showers from time to time through the week, but most of that activity will be in the mountains. When it does reach the valleys, the impacts will be minor with just light showers.

AVIATION

A cold air mass over the region with a high spring sun angle will cause another round of convection this afternoon and evening. Area terminals will mainly be impacted by rain, but some graupel and occasional lightning are also possible again this evening. Wind gusts around 30 kts are possible with showers.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Winter Weather Advisory until midnight MDT tonight for Bitterroot/Sapphire Mountains...Butte/Blackfoot Region... Potomac/Seeley Lake Region...West Glacier Region.

Frost Advisory from 8 PM this evening to 9 AM MDT Monday for Flathead/Mission Valleys...Missoula/Bitterroot Valleys.

ID...Winter Weather Advisory until 11 PM PDT this evening for Northern Clearwater Mountains...Southern Clearwater Mountains.