Weekend Weather Blog: What happened to Spring?

Published: Apr. 9, 2022 at 3:57 PM MDT
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TWIN FALLS, Idaho (KMVT/KSVT) — On Friday April 8, Twin Falls hit a record high of 80°. The next day? 40s and strong wind. Those who live this far north in the continental U.S. know that this isn’t all out of the ordinary, but people new to the area may be wondering how it can drop almost 40 degrees in only a matter of 24 hours.

In this week’s weekend weather blog, I’m going to show you the anatomy of a storm system - from the very top of the troposphere (the layer of the atmosphere that weather happens in) to the surface of the earth.

It all starts with a gradient in temperatures at the surface, signaling two different airmasses. One airmass is cold and dense, with low pressure throughout the entire profile of the atmosphere. The other airmass is warm and much less dense, having high pressure throughout the majority of the atmosphere. The larger the difference in airmasses (i.e. the temperature difference), the stronger the system will be.

Looking higher up in the atmosphere, a very fast area of wind, known as a jet streak develops.

USA Jet(Nicholas Snider)

As stated earlier, the larger the difference in temperature, the stronger the jet streak will become. The next thing we look at are those black lines drawn on the map, known as isohypses. This essentially tells us the direction of the winds, with the winds running parallel to these lines from left to right on the map. Notice at the east end of the jet streak, over Wyoming and Montana, the black lines spread farther and farther apart. This is called diffluence.

In response to the diffluence aloft, winds in the lower levels start accelerating toward the region of diffluence. This causes warm air to be moved to the north, and cold air to move to the south. This forms the area of low pressure, and the cold front at the surface.

Temp advection
Temp advection(d-maps.com)

The movement of this cold air south and this warm air north is what causes the large contrasts in temperature from day to day in the northern portions of the country. Over the far southern portions of the country, near I-10, these systems have a tough time reaching that far south, especially in the summer, as the Coriolis force is much weaker down there.

Coastal areas, especially along the Pacific, also don’t see as much of a contrast in temperatures. This all has to do with how much more effort it takes to heat up water vs land. While land heats up and cools off much more rapidly, it’s a much slower process for water. This moderates everything out pretty well along the oceans. Because Idaho is obviously landlocked, temperature contrasts are much more common; and as low pressure systems enter the picture, things can rapidly change from day to day.

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