textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Scattered strong to severe thunderstorms this afternoon and evening, with strong winds, large hail, and heavy rain.

- Thunderstorms in forecast through the rest of the week with active North American Monsoon flow pattern.

The large scale flow pattern will remain near stationary for the next 7-10 days across the western US. Global ensembles are in strong agreement for high pressure to be anchored over the central Rockies, with a trough of low pressure over the Pacific Northwest and southern British Columbia. This places the Northern Rockies under a south-southflow pattern, with abundant subtropical moisture streaming northward associated with the North American Monsoon. Furthermore, this flow pattern will keep a warm airmass overhead, with temperatures running above normal through this weekend into next week.

The threat for strong to severe thunderstorms is the primary focus today and tomorrow. Satellite observations this morning suggest several hours of sunshine on the way through the early afternoon, with atmospheric instability peaking by late afternoon. The Storm Prediction Center has the entire area under a marginal to a slight risk of severe thunderstorms (5-15% probability). Forecast models suggest MLCAPE of 1000-2000 J/kg, along with 30-40kts of vertical wind shear, as south-southwest winds increase aloft as Pacific disturbance moves into eastern Oregon and Washington. This combination of instability and shear will bring the risk for large hail (>1 inch in diameter), strong winds (60+ mph or greater), and heavy rainfall. High-resolution models suggest a few supercells may form, where wind shear is maximized along the divide in western Montana and across the Camas Prairie in Idaho. Thunderstorms will initiate across the higher terrain in central Idaho and northeast Oregon by early afternoon, gradually expanding in coverage and moving northward through western Montana through the evening. Please stay weather aware this evening, especially if you have outdoor plans!

Another round of strong to severe thunderstorms is on tap for Friday, with the Storm Prediction Center placing the region under a marginal threat. This threat will once again be driven by high instability for our area, with CAPE values of 1000-1500 J/kg.

Looking ahead to the weekend, the focus for showers and thunderstorms shift into Lemhi County, ID and southwest Montana. The rest of the area will remain warm and dry. This change will be driven by a westerly flow aloft, with atmospheric instability decreasing for many areas.

AVIATION

A monsoon surge from the south-southwest has atmospheric moisture running well above normal across the region. Clear skies this morning into the afternoon, will set the stage for plenty of atmospheric instability. Thunderstorms will initiate off the terrain of central Idaho and northeast Oregon between 16/1800-2000Z, with storms moving northward through the evening. Increasing south-southwest winds aloft will provide sufficient wind shear to organize thunderstorms, with a 5-15% chance for severe thunderstorms across the region. Primary storm threats include large hail (1 inch in diameter or greater), strong winds (50-60kts), and heavy rain.

MSO WATCHES/WARNINGS/ADVISORIES

MT...None. ID...None.