DISCUSSION: On Friday, September 20, the National Weather Service (NWS) offices in Riverton, WY, Great Falls, MT, Billings, MT, and Missoula, MT issued their first winter weather advisories and winter storm warnings of the 2019-2020 snow season. The advisories and warnings are the first of this season, as a trough of low pressure moved in from Canada bringing in a cold front to much of the northern Rockies including Yellowstone and Grand Teton National Parks. The warnings and advisories were mainly for the mountainous regions of Montana, Wyoming and parts of eastern Idaho.
One of the biggest factors for there to be snow with this front is that much of the air over the Yellowstone region was moist due to a southerly flow that was occurring in the higher levels of the atmosphere due to the wind flow of the trough over the past few days. The front edge of the trough was responsible for the southerly flow bringing moisture from the Gulf of Mexico to the colder area of the upper mountain west. However, the cold air that came from the north on the western side of the trough interacted with the moisture as air holds less water as it gets colder. Due to the amount of the moisture that was present in the atmosphere and the lower amount of vapor the colder air from the north could hold, the moisture condenses and forms larger droplets which in turn precipitate when large enough due to the effects of gravity. Normally, these droplets would dissipate as some of the lower levels of the atmosphere are warmer and not as saturated.
In addition to the snow in the Tetons and Yellowstone area, the cold front continued to move south and east bringing severe thunderstorms to the Dakotas, northwestern Nebraska and eastern Wyoming as well as below freezing temperatures into Colorado. The front moved across the Mississippi River Valley over the weekend bringing more thunderstorms to the area which brought heavy rains to the area. The concern of heavy rains has resulted in many NWS offices along the northern portion of Mississippi River to issue flash flood watches and warnings along the river and the tributaries.
This snow event was followed by another trough of low pressure on the very next weekend of September 28 through September 30 which brought even more snow to the upper Rockies and the Yellowstone area. This second trough also moved east bringing heavy rains to the Mississippi River Valley which led to the reissuance of flood warnings along the Mississippi River especially in Iowa, Minnesota, Illinois, and Missouri.
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©2019 Meteorologist JP Kalb