textproduct: Missoula

DISCUSSION

KEY MESSAGES:

- Localized heavy rainfall, gusty outflow wind, lightning and small hail threat today with showers and storms.

- Breezy winds 15 to 25 MPH on Wednesday could cause choppy lakes. A few thunderstorms are possible.

- Warmest temperatures of the summer this weekend through early next week.

A 5 millibar pressure gradient between Missoula and Helena has resulted in easterly winds gusting to around 25 MPH across western Montana late this morning.

Latest high-resolution models depict the focus of thunderstorms occurring from southeastern Idaho County to points east and south over Lemhi County this afternoon. The early nature of the shortwave has brought in a lot of cloud cover which could initially limit instability. Current satellite imagery shows some breaks in the clouds located south of the Salmon River corridor. The greatest probability for storms producing heavy rain is over the Bitterroot and Sapphire mountain ranges. Near the old Daly burn scar or near Skalkaho Pass, the probability is around 55%. If a storm were to hit this particular spot, then there could be some runoff issues. The storm motion of the storms will be fairly quick, moving off to the northeast between 20 and 25 MPH which would limit the amount of rain. There could be enough shear and instability that a few storms could produce 0.25 to 0.75 inch hail.

Models have trended down with shower activity on Tuesday. A departing shortwave with building ridge usually doesn't bode well for thunderstorms. There is still a slight chance for storms over the Divide.

A westerly wave will bring breezy conditions across the region on Wednesday, along with a chance for thunderstorms across southwest Montana. Westerly winds between 15 and 25 mph could bring choppy conditions to area lakes. Zonal flow continues on Thursday and could bring similar winds to the region.

The heat ridge starts to build across the Four Corners of the country starting Friday. We'll be under mostly dry, southwest flow with temperatures climbing into the 80s and 90s, and even up to 102 in the Hells Canyon and Salmon River Valley in north-central Idaho. There could be just enough instability for a few showers or thunderstorms to form over the terrain in southwest Montana.

The hotter air from Utah and southern Idaho pushes up here over the weekend. The greater probabilities(70-80+%) for 100 to 105 degrees are situated in the lower valleys of north- central Idaho and Lemhi County through Monday. During this period, make sure you drink plenty of water, take frequent breaks in the afternoon hours and do not leave children or pets in vehicles.

AVIATION

Some clearing is occurring at midday on the south side of the high/mid level cloud shield that has kept down heating this morning for areas I-90 southward. Shower/thunderstorm activity is expected to increase due to the differential heating near the clearing and dynamic forcing ahead of a decent shortwave. Several rounds of showers/thunderstorms are likely across southwest/west-central Montana and north-central Idaho into early evening with activity shifting into northwest Montana overnight (mainly along and east of US-93). Terminals directly impacted by these storms can expect brief MVFR/IFR conditions in heavy downpours, along with frequent lightning and terrain obscuration. Erratic convective outflow gusts of 25 to 35 knots are possible. There is a potential for a southerly wind surge tracking north through the Bitterroot and Missoula valleys (directly impacting KMSO) between 23Z and 01Z with gusts of 25-30 kts but high resolution guidance is less confident compared to earlier runs. Showers and isolated thunderstorms will linger early Tuesday morning, particularly across northwest Montana (KGPI) and along the Continental Divide. The lower levels of the atmosphere dry out on Tuesday behind the departing disturbance but isolated afternoon thunderstorms are still expected, focusing primarily near the Continental Divide.

MSO WATCHES/WARNINGS/ADVISORIES

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