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The last 7 days across Southwest Montana has been milder than normal for most areas due to a lack of valley ground snow and high pressure producing mostly sunny days. The exception to this trend has been the higher elevated valleys with plenty of valley snow like Wisdom and West Yellowstone.

Last week and the first half of this week morning low temperatures for Wisdom and West Yellowstone have been dropping well below zero and down to an extremely cold -24 below zero on the 5th for Wisdom and -18 below zero on the 6th for West Yellowstone. Why are they colder than rest of Southwest Montana?

Under a dominate High pressure ridge weather pattern where conditions are dry with very little to no wind along with mostly clear conditions produce strong radiational cooling and with ground snow and higher elevation and surrounding mountains all contribute to the pooling of very cold air at the surface. At night the colder air, which is dense or has weight, will pool up in the valley. These two locations generate their own localized Arctic air-masses under a strong High pressure weather pattern in the winter months and they are frequently the coldest places in the continental U.S. on most given days.

More extreme cold nights are likely for both cities until the weather pattern changes which could be early next week.