textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- A few strong thunderstorms are possible this afternoon and evening.

- Significant rain event for much of the Northern Rockies Sunday through Tuesday morning.

- Gradual drying trend develops mid-week alongside morning valley fog.

- 72% of ensemble clusters depict another weather-maker to impact the Northern Rockies next weekend.

Very impressive thunderstorms occurred overnight across western Montana! Besides the lightning show, these were heavy rain producers with reports of close to 0.40" of rain falling in only 15 minutes in Florence in the Bitterroot Valley. The rainfall has fallen in a narrow swath from Lemhi County northwards across Missoula County then northeast towards Powell County and beyond.

This activity was in response to shortwaves that have been embedded in the southerly flow, connected with the large upper level low over Washington and Oregon State. There could be a brief break in the showers later this morning into the early afternoon before another round of storms will develop. The latest convection-allowing models depict a few stronger storms developing over the Bitterroot Valley, Granite County, even extending northwards towards Flathead County this afternoon and evening. The latest HRRR model is showing that a few of these storms could produce one inch hail. Also with the atmospheric moisture content being 150% of normal for this time of year, the storms will be capable of bringing very heavy rain rates on the order of 0.50" to 1" per hour. The heavy rain in particular could cause localized flooding issues, particularly if a flood prone area or burn scar takes a direct hit.

The upper low continues to be on track to move from near Boise, Idaho tonight to near central Montana by Monday afternoon. This will set the stage for widespread moderate to heavy rainfall across western Montana. NOAA's Weather Prediction Center has placed a "slight" risk for excessive rainfall for northwest Montana including Glacier National Park on Monday. This means that there is at least a 15% risk of rainfall exceeding flash flood guidance within that area. Much of the northwest Montana area could see 0.10 to 0.20" per hour rainrates, but locally higher rates are possible. Locations that receive the higher end totals may see rapid rises on small streams and creeks, rock and debris slides onto roads, and minor flooding in poor drainage areas. Total rain amounts will range between 1 and 3 inches across Flathead County, and some mountain ranges like the Mission, Swan and Lewis Ranges have a 30% probability in exceeding 3 inches.

Showers will persist on Tuesday mainly across northwest Montana while other areas will start to moderate. Lingering boundary-layer moisture will support overnight and early morning fog and low stratus clouds on Wednesday and possibly Thursday.

Temperatures will rebound Wednesday through Friday, getting back to near seasonal levels for this time of year...70s and 80s for the lower elevations of Idaho.

For the longer range, including next weekend, a lot hinges on what evolves well upstream of us! One of the deepest troughs so far this season across eastern Russia will support a very amplified ridge over Alaska later this week. This will in turn support a deepening trough over western Canada. This kind of setup can lead to another trough digging over the Pacific Northwest which could bring unsettled and cooler weather for us. Some of the guidance is depicting a colder trough scenario which could bring lower snow levels. In fact, the NBM is starting to show 20 to 30% chance for snow above 7,000 feet along the Divide next Saturday!

AVIATION

Scattered showers and thunderstorms are possible across western Montana this afternoon and evening and could impact KGPI, KMSO and KHRF. Main threats will be brief heavy rain, wind gusts to 30 kts and small hail. A significant rain event will develop tonight, lasting through Tuesday morning. Several inches of rain is expected to fall from central Idaho to northwest Montana. Widespread terrain obscurations are anticipated.

MSO WATCHES/WARNINGS/ADVISORIES

MT...Red Flag Warning from noon today to 9 PM MDT this evening for East Beaverhead.

ID...None.