The Compacting Trough

There are lots of strong cold front passages and airmass changes in Utah, but the biggest are associated with compacting upper-level troughs.

Compacting means that the wavelength of the trough is decreasing with time.  In other words, the trough is becoming "thinner."  That is the case for the trough that is moving through today.  Check out in the loop below how the trough, indicated by warm colors, gets thinner over time as it approaches the Pacific coast (click to enlarge).

This enables the warm air to remain over the Intermountain West while cooler air over the Pacific approaches.  As a result, the temperature contrast over Nevada and California gets larger before moving into Utah, and we get a strong frontal passage or large airmass change.

And if you are wondering where the cold air is, just look upstream.  At 1000 MDT its 75ºF here on the University of Utah campus and 46ºF at Wendover.