Classic Intermountain Cold Front

The computer models are forecasting a classic Intermountain cold front to develop tomorrow (Wednesday) and come through Salt Lake City in the late afternoon or early evening.  The event contains all the hallmarks of a great Intermountain cold front, including discrete propagation and rapid development over northeastern Nevada.

The 0600 UTC 12 Apr initialized NAM model shows a cold front about to make landfall on the Pacific coast at 6 AM tomorrow morning.  The weather is "peaceful" over the Intermountain West, but note the confluence over north-central Nevada and weak temperature gradient over western Nevada, which leads to new frontal development ahead of the landfalling cold front.

0600 UTC 12 Apr NAM model forecast of sea level pressure
(black contours), 925 mb temperature (red contours), 10-m wind
(barbs), and 3-h accumulated precipitation) valid 1200 UTC 13 Apr.

By 6 PM tomorrow afternoon, an intense cold front has developed over northern Nevada and is steamrolling into northwest Utah.  What a monster.  The Pacific cold front has dissipated in its wake.  In other words, the front over northwest Utah is a new cold front that formed in advance of the Pacific cold front.  We refer to this as discrete propagation.

0600 UTC 12 Apr NAM model forecast of sea level pressure

(black contours), 925 mb temperature (red contours), 10-m wind

(barbs), and 3-h accumulated precipitation) valid 0000 UTC 14 Apr.

How about a video instead, which shows it even better.  The gap between the old and new front is about the largest I have seen.

The evolution of this event is remarkably similar to the 2006 Saints and Sinners storm, which we described in Steenburgh et al. (2009).  In that paper, we show that frontal development in the Saints and Sinners storm occurred in response to troughing, confluence, and convergence downstream of the Sierra Nevada, which concentrated a pre-existing temperature gradient over Nevada.  Low-level evaporative cooling from post-frontal precipitation, combined with pre-frontal surface heating, further intensifies the front.  Based on the NAM forecast above, these processes will likely contribute to the development of tomorrow's front, which comes through northern Utah at a time that it should produce a whopper of a temperature fall.

Curiously, Greg West and I have done a number of numerical simulations of the 2006 Saints and Sinners storm and have found that although the Sierra strengthen the cross-front temperature contrast, a decent cold front forms even if we remove the upper portion (>1500 m) of the Sierra Nevada.  Conventional wisdom suggest that trough and cyclone development over Nevada is a consequence of vortex stretching to the lee of the Sierra, but there appears to be more to the story.  Better understanding these events would be a good topic for a motivated graduate student.