Tuesday, March 15, 2011

Curious Cold Front Evolution

Over the next few days, the weather over Utah will be strongly affected by a cold front that makes landfall on the Pacific coast today, but undergoes dramatic transformation as it slides slowly across the Intermountain West tomorrow.  The good news is that it will bring some snow to the Wasatch Mountains, although the timing and amount remain somewhat uncertain given variations in position, speed, and intensity of the front and related precipitation between the forecast models.

What I find most interesting is how the frontal intensity varies diurnally.  I'll concentrate on the NAM model forecast of 12-h accumulated precipitation, 850-mb temperature (850-mb is roughly the elevation of the basins and valleys of the Intermountain West), and surface wind from 1200 UTC 15 March (0600 MDT this morning).

By this afternoon (0000 UTC 16 March), the Pacific cold front has been shredded and lacks a well organized structure.  There is simply an area of precipitation and cold advection penetrating into western Nevada.

1200 UTC 15 March NAM precipitation (color fill), 850-hPa temperature
(red contours), and 10-m wind forecast valid 0000 UTC 16 March
This lack of a coherent frontal structure persists through tomorrow (Wednesday) morning (1200 UTC 16 March) when precipitation has spread into central Nevada Idaho, and extreme northern Utah.

Same as above except valid 1200 UTC 16 March.
This is where things get interesting as the cold front undergoes rapid intensification during the day tomorrow, becoming quite sharp over central Nevada and northern Utah.  Note the intense wind shift and temperature gradient/contrast that has developed in the image below.

Same as above except valid 0000 UTC 17 March.
Differential diabatic heating and cooling are likely critical for the frontal intensification in this case.  Diabatic processes are those that involve a transfer of heat to or from a parcel of air.  In this case, the pre-frontal environment is being strongly heated by the sun, whereas the post-frontal environment is less strongly heated because of extensive cloud cover and precipitation.  The precipitation also results in evaporation that further cools the post-frontal airmass.  Likely also contributing to the frontal development is the development of pressure troughing (not shown) and confluence (see above) over the Intermountain West downstream of the Sierra Nevada (see West and Steenburgh 2010).

The importance of daytime frontal sharpening is, however, underscored by the evolution over the subsequent 24-hours.  In particular, note how the front weakens by Thursday morning and then strengthens again by Thursday afternoon.

Same as above except valid 1200 UTC 17 March.
Same as above except valid 0000 UTC 18 March.
How much snow the Wasatch Mountains get will be dependent on the frontal position and speed. This model simulation suggests the heaviest accumulations will be from the Cottonwoods south, but it's too soon to hang our hat on that.  Stay tuned.

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