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Downslope Winds Warm the Plains

15 Dec 2013, 10:38 am

Temperatures are increasing across the eastern side of the Rocky Mountains, but the heat is moving in from the… north? This phenomenon is known as downslope flow, and it allows temperatures to fluctuate rapidly near mountain ranges.

It all begins with wind streaming up and over the mountain range:

Notice the wind direction comes straight down the side of the mountain, and at a pretty good clip. Sustained winds are above 20 mph for both Cut Bank and Livingston.

Wind warnings are in effect for these regions due to this brisk downslope flow. Gusts can top out near 60 mph, which is equal to the threshold for wind damage in severe thunderstorms. We’ve even seen some reports of wind gusts over 80 mph!

As these brisk winds come across the Rocky Mountains, air is rising to move over the higher terrain. As the air rises, the moisture is squeezed out of the parcel of air, which leaves it very dry at the summit. This dry, cooled air then sinks down the lee side of the mountains and warms adiabatically. This means that the temperature of the air parcel increases due to the increase in pressure as it approaches the ground (air near Earth’s surface is under more pressure than at high altitudes).

So thanks to this sinking air, temperatures today and tomorrow will be significantly above average in the Central Plains:

These warm conditions are eating away at the snow pack. Look at the change in snow depth between Friday and Sunday!

 

This phenomenon is not uncommon during this time of the year. When the Lower 48 has cold air in place during the winter months, it is easy for a warm downsloping event to occur and create a large temperature change. Below is a chart depicting the months with most frequent wind storm activity in Boulder, CO and the Gaspe Peninsula of Canada due to similar circumstances.

Enjoy the warmth for now! More arctic air is on the way, just in time for Christmas. It’s what you were hoping for, right? [Image below courtesy: WeatherBell.com]

 

Thanks for checking in, and have a great week ahead! -Meteorologist Miranda Hilgers

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