Wednesday, March 6, 2013

A Freaky Frontal Forecast

The forecast for tonight and tomorrow is challenging.  As of 1200 UTC (0500 MST) this morning, a closed upper-level low was setup off the coast of the Pacific Northwest.  Two smaller-scale disturbances, called short-wave troughs by meteorologists, were circling the closed low and are annotated on the image below with their expected tracks.

1200 UTC 6 March 2013 IR satelite image and 500-mb
geopotential height analysis
The first short-wave trough made landfall in California overnight and will push into Idaho late today.  Accompanying this feature is a developing cold front that is expected to sag into northern Utah and pass through Salt Lake City tonight.  The second short-wave trough is currently located on the back side of the trough, but is expected to dig southward before turning eastward and moving across the Pacific coast on Friday.  

For northern Utah, the forecast for tonight and tomorrow is very dependent on the precise positioning and intensity of the cold front, which is expected to stall and weaken as the second swings southward and eastward.

The 0600 UTC GFS produces the greatest precipitation through 1200 UTC (0500 MST) tomorrow along a narrow band that runs from approximately Ely through Salt Lake City to the Cache Valley.  This band is about 100 km wide, so the forecast is very sensitive to it's exact position and intensity.

0600 6 March 2013 GFS sea level pressure, surface wind, and
6-h accumulated precipitation forecast valid 1200 UTC 7 March 2013
The frontal band then stalls due to the influence of that second upper-level trough.  So rather than progressing across Utah, it just sort of peters out.

0600 6 March 2013 GFS sea level pressure, surface wind, and
6-h accumulated precipitation forecast valid 1800 UTC 7 March 2013
Thus, this isn't a case where a cold front and trough sweeps across northern Utah and everyone gets some precipitation.  Much depends on the positioning of the frontal band and its intensity where it stalls.

Rather than rely on a single model, another option is to look at the 21 model forecasts that are part of the Short Range Ensemble Forecast System (SREF).  The weighted-mean precipitation produced by the SREF for the 24-hour period ending at 1800 UTC (11 am) tomorrow shows something roughly similar to the GFS solution, although the precip maximum is shifted slightly southward and extends from about Big Cottonwood Canyon to Snowbasin (note: the resolution of this model is unable to really resolve the terrain).

0900 6 March 2013 SREF weighted mean 700-mb wind and 24-h
accumulated precipitation forecast valid 1800 UTC 7 March 2013
The probability of >0.25 inches of snow water equivalent derived from these 21 forecasts is also greatest in that region, reaching about 60%.

0900 6 March 2013 SREF weighted mean 700-mb wind and probability 24-h
accumulated precipitation > 0.25 inches forecast valid 1800 UTC 7 March 2013.
Probability color fill every 20% beginning at 20%.  
So, if you are looking for a few inches of snow tomorrow to cover up the melt-freeze crust that will likely result from today's warmth, your best bet is to hope the frontal band delivers and to get up and see where it has been most productive.  

Finally, this is an especially difficult forecast period.  Over the past few days I've seen some really ridiculous icon-based forecasts for the next few days.  This is a good time to remind everyone that icon based forecasts are bogus, and if you use them, then you'll be bogus too.  

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