Summer turns up the heat on Monday out ahead of some Monsoon moisture from the Southwest. Morning cloud cover will keep lows mostly in the 50s. Highs will push into the 90s around Missoula, Thompson Falls, Libby, and Bozeman, with mid to upper 80s elsewhere.
Moisture moving north around a large high pressure system over the Rockies arrives first Monday morning with high clouds, and then a few weak thunderstorms during the afternoon. Without nearby weather features to help with lift, it will be primarily the heat of the day that kicks off storms near some of the bigger mountain ranges. The Southern Bitterroot Valley, The Pintlers, the Helena area, and Yellowstone Park have the best chance to see a thunderstorm. Expect very little in the way of rain or lightning with the few storms that do pop up.
Another surge of moisture comes early Tuesday, with a slight chance of thundershowers south of Butte. Most of the action will be south of Dillon, near the southern state line near Yellowstone. Temperatures Tuesday will be slightly cooler overall, but areas around Libby will be close to triple digits during the afternoon.
Wednesday continues an unsettled forecast. A small area of low pressure will head north from the Oregon coast and skirt Northwest Montana before fizzling out in Canada on Friday. Forecast models are doing a poor job with figuring out the strength of this system. As the system approaches, it is likely that there will be more widespread thunderstorms ,including the Missoula and Kalispell area, later in the week. Beyond that is still to be determined.
Temperatures won't change much across the region later in the week. Highs will range from the low 90s to mid 80s. Lows will be in the mid 50s to around 60. Do not expect much in the way of rain during the week.