textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Rivers, streams, and creeks continue to run high across northwest Montana from prolonged rain and ongoing snowmelt.

- Showers and thunderstorms return late Wednesday into Thursday morning.

- Active weather pattern this weekend into next week.

The low pressure system that brought widespread heavy rain to western Montana the past several days is finally moving off to the east. Latest satellite and radar images show the heaviest precipitation east of the Continental Divide. While northwest Montana is starting to see some breaks in the clouds with only a few isolated showers producing a hundredth or two of rainfall per hour. Rivers and streams continue to be running high across northwest Montana so areal flood watches and warnings remain in effect for areas along the Mission and Swan mountain ranges up into Glacier National Park.

The next disturbance is expected to push across the Northern Rockies late Wednesday into Thursday morning. This feature will cause scattered to numerous showers and isolated thunderstorms starting off in north central Idaho by Wednesday afternoon and then western Montana during the evening. The shower activity is expected to linger over the area throughout the overnight hours and diminish Thursday afternoon. The bulk of the valleys should receive between 0.05 to 0.15 inches of precipitation with the higher terrain getting 0.25 to 1.0 inch of rain. A few areas could receive higher amounts under heavier thunderstorm activity. Snow levels appear to be hovering around 9000 feet during the passage from this disturbance.

The upper level flow becomes southwesterly on Saturday as a trough of low pressure moves into the Pacific Northwest. This flow pattern will allow temperatures to be near to above seasonal average with the potential of showers and thunderstorm development, especially across Lemhi county into southwest Montana. The long term models are still struggling on how this low pressure system will impact the Northern Rockies and develop during the first part of next week. The far majority of the models do show part of the trough making its way through the Northern Rockies on Sunday causing cooler temperatures along with scattered shower activity.

AVIATION

The upper level low that has brought sensible weather to the area for the past week is finally pulling off to the east. In its wake residual clouds are found with a few lingering showers generally over the Divide and generally over I-90 from Clinton eastwards. For the terminals, expect mid-cloud through this evening with either variable or northerly winds less than 15 knots. Sky cover will begin to break up tonight as a transient ridge moves into the region. With the recent moisture, there could be patchy fog that develops, possibly at KGPI, KMSO and KBTM. On Wednesday, a shortwave will bring showers and a few thunderstorms across north-central Idaho before 6 PM MDT, then into western Montana thereafter. Impacts from storms include lightning, gusty winds to 35 knots, heavy rain and small hail.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Flood Watch until 6 PM MDT this evening for West Glacier Region.

ID...None.