Friday, March 16, 2012

Amazing Intermountain Frontal Evolution

Looking at the model forecasts for Saturday night and Sunday, I've been trying to figure out two things.  The first is why there is an apparent jump in the precipitation band from Nevada to the Wasatch and Oquirrh Mountains.  The second is the mechanism contributing to the development of a stationary band of precipitation over those ranges, especially the Wasatch Mountains.  The easy answer to the stationarity is that the precipitation is orographic (i.e., generated by flow over the mountains), but that answer would be wrong.

Instead, this is a remarkable case of frontal evolution, which I've tried to summarize below using analyses of 700 mb (crest-level) wind and equivalent potential temperature (a.k.a., theta-e), as well as 3-hour accumulated precipitation.  I've also annotated frontal boundaries with thick yellow lines.  For those of you not familiar with theta-e, it combines temperature and moisture into a single variable and is often used in frontal analysis.  Areas with a high concentration of theta-e contours typically delineate frontal zones (but not always).

The forecast valid 1800 UTC (1200 MDT) tomorrow (Saturday) shows a well defined front extending through central Nevada, across the Sierra Nevada, and over the eastern Pacific south of southern California.  Note, however, that strong flow circumscribes the southern Sierra Nevada and then extends northeastward across southern Nevada and western Utah.

As a result, low theta-e air subsequently moves rapidly around the Sierra Nevada and northeastward into southern Nevada and western Utah.  This occurs ahead of the pre-existing frontal boundary in central Nevada, which is held fixed by the pronounced trough and wind shift directly downstream of the Sierra Nevada.

Eventually, the leading edge of the low theta-e air that swept around the Sierra Nevada becomes the dominant frontal boundary of this system, and does so over the high terrain that runs up central Utah in the NAM model (note: this terrain does not resemble the real terrain very well).  This airmass boundary then becomes the locus for precipitation over the next several hours, as the one over Nevada migrates slowly eastward and dissipates.

While the Sierra Nevada appear to play an important role in this event, the contribution of the Wasatch Mountains and other high terrain that runs down the center of Utah is unknown.  It's not clear if they help control where the front sets up, or if it is simply coincidental that it happens there.

We will have to see if Mother Nature does things in the same fashion as the models.  If she does, this will be a very nice event to examine in detail to better understand frontal evolution over the Intermountain West.  In addition, how much snow falls in the central Wasatch on Saturday night could depend on the location and intensity of this frontal boundary.  Time will tell if the models are on the right track or out to lunch.

1 comment:

  1. The Sierra Range has really been squeezing the moisture out of this system so far. Blue Canyon has had at least 8" of rainfall over the last few days (and another 5" forecast within 24 hours) while 60 miles away Reno has reported nothing. The high clouds we have been getting this week are all that is left over after dropping up to 10" of rain in northern California.