Weekly Weather Event -Week of October 28November 1, 2019
This Halloween was a surprisingly cold and snowy one for large swaths of the Lower 48, with some places recording temperatures 10 to 25 degrees below average. The National Weather Service in Milwaukee put out a preliminary estimate of 5.7 inches of snow accumulation in Madison on October 31st. This incredibly early snowfall also marks the 4th straight day that Madison has seen snowfall totals of 1 inch or greater, something that has only been recorded three other times.
This unseasonable cold is the result of a strong high-pressure region over Alaska, sometimes referred to as a ridge, and a large trough in the jet stream that created a low-pressure region over large portions of the United States. As the jet stream dips lower, it moves cold air further south than normal for this time of year. All these factors combine into a strong cold front that has plunged temperatures more typical of early December than late October and early November.
Fronts represent the boundaries between two air masses, with the front being named for whichever air mass is moving into an area and which air mass is being pushed out. Cold fronts tend to have a sharper temperature gradient than warm fronts, often resulting in the warm air ahead of the front to rapidly rise. This fast-rising motion can create quick forming showers and thunderstorms, as was seen on Thursday evening in portions of the eastern United States.