WeatherWeather Science


Meteorologist Alan Rose explains "the warm before the storm"

Downslope Winds
The effects of downslope winds
Posted at 5:10 PM, Nov 19, 2019

You've heard our very own Mike Daniels say it, "the warm before the storm".

It's an expression used to explain how the warm sector of a mid-latitude cyclone precedes the cold and snowy part of the storm.

In the mid-latitudes, the jet stream is typically found about 30,000 to 40,000 feet above sea level. The mean flow of the jet is from west to east in the northern hemisphere, but varies at times as storms tend to amplify or strengthen the jet stream.

Think of the jet stream as being like a river of upper level winds, or more simply put, it's the storm track.

When strong upper level winds move down the eastern side of mountain slopes, this is considered downslope wind. The downslope flow carries colder, high-density air down the mountains under the force of gravity.

This process is known as subsidence, and can lead to dry, windy and warmer conditions. It's the same process that can fuel devastating wildfires across the Front Range and throughout the western U.S.

Long periods of subsidence or sinking air can lead to poor air quality. This is common here in the winter months, when a high pressure system with subsidence can sit over the Rockies, leading to a brown cloud of pollution. A new storm is what's needed to clear away those polluted skies.