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Montana Weather

Rainbow Over Ennis Lake in Montana
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Montana weather is, to say the least, diverse. The combination of its northern latitude, its location in the Great Plains, the dramatic elevation changes and the numerous tall mountain ranges in the state all combine to produce some wild, wacky and generally unpredictable weather.

For anyone who is venturing to Montana on a fly-fishing or pleasure trip, knowing what kind of weather to expect is very important. This is more so if you plan to do outdoor activities in the mountains. Thus, I’ve prepared a Montana weather guide to give Montana visitors an idea of what to expect, weather wise, when venturing to Montana. This guide will cover the basics of the weather here in Montana – which will hopefully allow you to figure out what to take on your trip here.

General Montana Weather

For starters, let’s just go into a quick summary of the weather in Montana. These are just general rules, and change at the whims of nature, of course. But, overall, eastern Montana (which is defined as that portion east of the Continental Divide) is drier than western Montana (which is defined as that part of Montana that is west of the Continental Divide).

During the summer, eastern Montana generally has warmer weather than the western half – frequently much warmer. During the winter months, though, western Montana usually has warmer temperatures than does the eastern half of the state – often times with remarkably substantial differences.

An additional Montana weather trait relates to clouds. The area of Montana that is west of the Continental Divide has much more cloud cover than eastern Montana. This is especially true during the winter months when inversions set in. During the winter, inversions often keep the western valleys of Montana under clouds for weeks at a time, while over in eastern Montana the sun shines continuously.

Thunderstorm Brewing in the Gravelly Range
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The reason for these general traits in Montana weather is due to the Continental Divide. The Continental Divide essentially divides Montana into two distinct big geographical zones that vastly influence the weather in Montana. The Continental Divide, for those not familiar with it, are towering mountain peaks that meander through the entire width of Montana. More than anything else, the Continental Divide shapes the weather in Montana, causing the often times dramatic differences in temperature and precipitation.

The Continental Divide has a dramatic effect on weather in Montana for two reasons. The first reason is that the divide hinders the flow of the often very cold air that invades eastern Montana from making its way over into the western part of the state. This occurs because that very cold air (you know, the stuff that is like -20 degrees) is usually just a “shallow pool” of air, often times not more than 2000 feet in height. Consequently, this cold air is frequently unable to make it over the mountains to invade the western half of the state. Because of this, during the winter months or during cold snaps that happen to occur during the rest of the year, it is not uncommon at all to have Great Falls shivering in 20 below weather (and windy) while Kalispell basks in 25 degrees above zero weather.

Yet, the Continental Divide doesn’t always prevent the cold air from invading western Montana. From time to time, these pools of cold air are “deep enough” (or are thick enough) to penetrate through the mountain passes along the Continental Divide. When this happens, conditions along the Continental Divide and near these passes, become less than pleasant as this very cold air blows through, resulting in cold, very windy conditions.

The second dramatic effect of the Continental Divide relates to moisture and sunshine in Montana. The Continental Divide is a wonderful rain and snow-gathering machine. The high peaks of the divide force moisture out of the weather systems that cross Montana, leaving very little for eastern and central Montana (or those areas that lie to the east of the Continental Divide). As such, most areas in western Montana receive decent amounts of moisture, while areas to the east have significantly less – particularly the vast swath of prairie in Montana. That said, it needs to be noted that the many mountain ranges that lie to the east of the Continental Divide are high enough to force significant amounts of moisture out of weather systems. This is why many central and southern Montana mountain ranges receive so much snow – their huge vertical rise (measured from base to summit, which can exceed 7000 feet in places) is able to force abundant moisture out of weather systems that leave the surrounding area completely dry.

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