textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Showers with a few thunderstorms(20-50% chance) today through Wednesday, especially across northwest Montana and over the Continental Divide.

- Frost/freeze concerns remain for Tuesday morning.

- Mountain snow above 6000 feet with raw/cold backcountry conditions later Wednesday into Thursday morning over the Divide, Glacier National Park, extending west towards the Idaho border.

- Return to summer-like conditions as moderating temperatures are expected late week into next weekend.

Temperatures this morning were quite cold across the region. Whenever you get such a cold trough originating from the northern latitudes like Alaska, you almost have to expect possible frosty mornings. The Ravalli County Airport located northeast of Hamilton saw a low of 21 degrees! Looking back at climatology, its really not that uncommon to get these types of systems in May where highs struggle to get out of the 30s and 40s. Looking at the climatology for the Hamilton area for the second half of May the coldest low temperature recorded was 24 degrees set on May 22, 2010. The Stevensville COOP recorded a 15 degrees on May 30, 2001. Even other locations, like the higher 3000 feet valleys in Idaho were in the upper 20s this morning. With the chance for frost continuing into Tuesday morning, we issued more frost advisories. We listed the percentile ranges of low temperatures for select locations in the "additional details" section to help people make decisions on if they need to cover things or not. For those who are unaware, we typically will not issue frost/freeze advisories for the high valleys(i.e. southwest Montana/Divide) that will continue to have sub- freezing nights for the remainder of May until the climatological last frost/freeze day. If we get a cold system thereafter, we may choose to issues advisories then.

For the next two days, we'll be under cyclonic flow with disturbances dropping out of southern British Columbia. There is generally a 20 to 50 percent chance for lightning over the mountains of northwest Montana this afternoon and evening. Other impacts from the showers will be gusty winds to 35 mph in the stronger cells.

Then shortwave energy from the Gulf of Alaska will carve a larger upper level trough which digs over Montana later Wednesday into Thursday. With the colder temperatures aloft, this will drive the snow levels down to around 6000 feet. This could bring additional snowfall to the mountains over the Divide, including Glacier National Park. Also the associated cold front will shift south and west with time just after peak heating which will allow for possible showers and thunderstorms. The latest probabilities show the best instability present across northwest Montana. This system will basically move off into the Northern Plains by Thursday into Friday while we will have rising heights (moderating temperatures).

A general ridge will build by next weekend and temperatures will rise back into the 70s and 80s across the region. There is a hint that a trough may either bring mild and windy conditions on Sunday, or dig across the region by early next week.

AVIATION

Scattered showers and thunderstorms will be possible today across the region. The more intense cells will be found across northwest Montana, especially initiated over the mountains. They will have a general storm motion from northwest to southeast around 23 knots. Main impacts will be isolated lightning, brief visibility reduction and gusty winds to 30 knots. A few lingering showers are possible tonight across northwest Montana thanks to a shortwave dropping out of British Columbia. There could be patchy fog along river bottoms elsewhere Tuesday morning. Another round of showers with a few thunderstorms are expected Tuesday afternoon and evening with the focus over the Divide and points west.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Frost Advisory from midnight tonight to 9 AM MDT Tuesday for Bitterroot/Sapphire Mountains...Flathead/Mission Valleys... Lower Clark Fork Region...Missoula/Bitterroot Valleys.

ID...Frost Advisory from midnight MDT /11 PM PDT/ tonight to 9 AM MDT /8 AM PDT/ Tuesday for Eastern Lemhi County...Northern Clearwater Mountains...Southern Clearwater Mountains.

Frost Advisory from 11 PM this evening to 9 AM PDT Tuesday for Orofino/Grangeville Region.