There’s as much variety in Montana’s weather as there is in the state’s topography. The Continental Divide splits Montana into two broad climatic regions. West of the divide, the climate is influenced strongly by mild marine air from the Pacific; to the east, harsher continental patterns prevail.
Stories of extreme weather abound in Montana. It can get cold; the lowest temperature in the lower 48 states was recorded at Rogers Pass, northwest of Helena, in 1954: –70°F. Hot summers are common, with temperatures of 117°F recorded in both Glendive and Medicine Lake.
During the spring, the winds shift, and moisture comes up from the Gulf of Mexico. These storms sweep through the Midwest and hit the east slopes of the Rockies. May and June are the wettest months over most of the state; the exception is in the northwest, which gets most of its moisture from winter Pacific storms.
July and August are usually the warmest months. It does rain during the summer, but brief thunderstorms are the norm. By September, the weather may start to change for the colder and wetter, but there is frequently a lovely Indian summer in October. Winter storms can begin anytime, but roads are often clear through early November.
Warm, dry winter winds coming off the east slopes of the Rockies and across the plains are called “Chinooks.” As the Pacific air passes over the mountains, it unloads its moisture and is warmed on its eastern downslope run. Chinook winds carry an almost mythological force. They can bring incredibly rapid relief from frigid weather; in Great Falls, the temperature once rose from –32°F to 15°F in seven minutes, almost seven degrees a minute.
Montana’s dry spells can be as striking as its extremes in heat and cold. Average rainfall for the western part of the state is 18 inches per year, 13 inches for the east. Studies of almost 300 years’ worth of tree rings have shown that, every 20-some years, the western United States experiences a drought. A five-year drought that started in 1917 in eastern Montana drove many homesteaders away from agriculture. This drought was coupled with particularly harsh winters. The 1930s saw not only economic depression, but (according to tree rings) the most severe drought since 1700. The 1980s proved to be another era of drought for much of the state. When it’s too dry for prairie grasses to survive, ranchers are forced to buy expensive feed, or sell their herds and wait for rain.
© W.C. McRae & Judy Jewell from Moon Montana, 7th Edition