Forecast of the Day • A National Weather Forecasting Contest

A very cold surface air mass will cover all of Montana to the east of the Continental Divide on Wednesday. Dillon, Montana is on the western side of the Divide, and the two main U.S. models have not always been in agreement about whether it will also be in the cold air or not, which could lead to a large forecast error. So let's forecast Wednesday's Tmin for Dillon, and see what happens. Here's a model plot showing the strong variation in temperature, and how close the edge of the cold air is to Dillon, in the southwest corner of the state.

Question: Why is it a common situation that cold air masses dropping southward from Canada stay on the eastern side of the Continental Divide?