Expect a quiet Sunday night with a few showers in southwest Montana. A thunderstorm or two is possible late Sunday night into Monday morning.
A strong low pressure system will arrive late Monday after lurking to the south on Sunday. Showers and thunderstorms will develop and move north-northwest in the afternoon. Highs will hit the 60s in southwest Montana and the 70s further northwest. There is a slight chance that one or two of these storms could become severe, though it is unlikely. Storms will be stronger the further southeast you travel in our region. The main threats from said storms will be frequent lightning and heavy rain that could lead to flash flooding of small creeks. 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 along a line from Salmon to Butte to Helena to Great Falls and Lewistown could see over 2 inches of rain.
There are new flood watches tonight in anticipation of the storm. 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 next Thursday as they try to send all the fresh rain downstream. Minor flood stage or action stage is possible on many rivers in the region due to the rain. Moderate to major flood stage is likely for parts of the Salmon and Jefferson Rivers. If you're in these areas, now is the time to plan for flooding.
The forecast beyond Tuesday is a lot sunnier with only occasional afternoon showers Thursday and Friday. 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.