A Spring Storm Is A Comin'

Mother Nature just won't turn the spigot off this spring and she certainly has no intention of turning it off over the next few days.

Over the past couple of days, a high-pressure system has been parked out over the subtropical eastern Pacific Ocean, with southeasterly geostrophic flow on its western flank extending from roughly Hawaii to about 40N.

0600 UTC 17 Apr GFS Analysis of sea level pressure (contours)
and 925-mb potential temperature

This flow has been pumping subtropical moisture around the high-pressure system and into the mid latitudes off the coast of California, priming the pump for a spring storm in Utah over the next two days.

Three-day loop of integrated precipitable water.
Source: SSEC

The NAM model taps into this moisture and brings the plume into the western United States today and tomorrow.  700-mb temperatures (near 10,000 feet) are quite high and near -2C, so this is truly a spring storm with high snow levels probably near 8000 feet (possibly higher during the day).

0600 UTC 17 Apr initialized NAM forecast valid 6 PM
MDT 17 Apr

0600 UTC 17 Apr initialized NAM forecast valid 6 AM
MDT 18 Apr

 Below are some time series based on the NAM forecast for Alta and algorithms that Trevor Alcott has developed for snow density.  The NAM model produces about 1.5 inches of snow water equivalent from this afternoon through mid-morning Tuesday (and precipitation is already falling in portions of northern Utah).  Given how warm this storm is, and its subtropical origins, this won't be the Greatest Snow on Earth.  The density of the snow based on Trevor's algorithms is 12-17% through Monday Night.  This snow is going to be west, thick, and heavy.  In other words, Cascade Concrete.  Bring your best hard shell.   

Courtesy Trevor Alcott

There are concerns, and rightfully so, about flooding in lower elevation basins where rain might fall.  The National Weather Service has issued a flood watch, which means there is the potential for flooding based on current forecasts, for the Cache Valley, Wasatch Mountain Valleys, and the Wasatch Mountains from I-80 north.

Those of you looking for better powder might consider waiting until Tuesday.  At that time, temperatures are expected to fall and we may get some lighter snow on top of the heavy stuff.  It's far enough out that I'm not banking on it yet, but am keeping an eye on it.  You can see the decrease in forecast snow density in the plot above.