Update: 8:30 pm: Heavy rain this evening has prompted a Flood Advisory for Granite, Powell, Deer Lodge, and Silver Bow counties through Tuesday. Heavy rain over the past few hours will cause creeks to be close to their banks and cause standing water in low-lying areas. Avoid flooded roadways.
A strong storm system remains on track to move across Montana tonight and Tuesday. Strong thunderstorms have already developed in southwest Montana during the early afternoon with rainfall rates of a half-inch to one inch per hour.
The main threats from said storms will be frequent lightning and heavy rain that could lead to flash flooding of small creeks. There's a chance one or two of these storms could become severe in southwest Montana. Stay alert this afternoon for possible severe thunderstorm warnings and flash flood warnings. Storms will head north across all of southwest Montana and will reach Missoula Monday evening. Kalispell will have a slight chance for a shower this evening but will most likely have to wait for rain until early Tuesday.
Storms transition to broad areas of moderate rain overnight and continue through most of Tuesday. Snow levels drop to around 7000 feet. Temperatures in the valleys will only reach the mid 50s to around 60 degrees on Tuesday. By the end of Tuesday areas from Missoula to Butte and Helena to Bozeman and Dillon will see 1 to 2 inches of moisture. The heaviest rain will be in the Bitterroot Range and from the Continental Divide between Butte and Helena northeast to Great Falls with over 2 inches of rain possible in these areas.
All of southwest Montana along with Great Falls, Lewistown, and parts of Granite, Powell, Ravalli, and Missoula counties will be under a flood watch until early Wednesday morning. People in areas that could be subject to flooding should closely monitor conditions and look out for any flood warnings issued in the next couple of days.
River levels will jump 1 to 2 feet by Thursday. Minor flood stage or action stage is possible on many rivers in the region due to the rain. Moderate to major flood stage is possible for parts of the Salmon and Jefferson Rivers. The Clark Fork, Big Hole, and Bitterroot Rivers will also be ones to watch. If you're in a flood-prone area, now is the time to get prepared.
The forecast beyond Tuesday is a lot sunnier with only occasional afternoon showers from Thursday into the beginning of next weekend. Highs will be in the 70s with lows in the 40s through Sunday with the exception of Friday when highs should only be in the 60s.