Friday, May 22, 2020

Soaking Rain on Tap

Forecasts are looking good for a few tenths of an inch of soaking rain tonight.

The latest 12Z NAM forecast valid 6 PM MDT (0000 UTC) this afternoon shows the surface cold front pushing into the Salt Lake Valley.  If this forecast holds, the surface front, marked by the abrupt wind shift in the image below, will precede the precipitation band, so the frontal passage will be dry.

However, the lagging frontal precipitation band slides in over the next few hours.  The period from 2100-0000 MDT (0300-0600 UTC) looks to feature a good soaking rain, with the NAM producing more than 0.25 inches at the airport for the three hour period.  Gardens rejoice!

Rain will then taper off to showers by morning.  In total, the 12Z NAM pumps out 0.69 inches of precipitation for the airport, and what a godsend that would be for our crusty soils.  The 6Z GFS (the 12Z isn't in yet) is a bit less bullish on precipitatoin, but still produces 0.42".  A look at the SREF shows a range of about 0.17 inches to 0.7 inches, with a median of just over 0.35 inches, so we will certainly get something.

Precipitation in the upper elevations will fall as snow (snow levels could drop down to about 6000 feet by tomorrow morning).   On average, precipitation increases with elevation and under the right conditions, perhaps one would expect this to produce a big dump, but I'm not too excited about that happening in this one for a few reasons.  First, the primary mechanism for precipitation development is the frontal lifting and under those conditions, we don't usually see as strong of an increase of precipitation with elevation.  Second, the flow direction at crest level never really comes around to northwesterly.  Third, it's a fairly stable event during the frontal passage.

These storm characteristics can be seen in the time-height section below.  There's only a brief period of shallow northwesterly flow at 6Z (0000 MDT) and the increase with height in a variable known as equivalent potential temperature (black contours) above 650 mb (about 11,000 feet) is indicative of the stability.

Thus, the mountains will get something, but perhaps not a lot.  Even the NAM generates less precipitation in the mountains than in the valley, with 0.27 inches at Alta.  The GFS is a bit more bullish with 0.45 inches.  I'll call it 3-6 inches, with an upper end of 8 inches if things come together and a bottom of 2 inches if it falls apart.  If you feel the need to scratch the itch, the best option will probably be low-angle terrain with a smooth underlying surface.  That might actually ski quite well in the creamy snow if we can get 6 inches or more.

Pete Townshend's birthday was earlier this week, so we conclude today with Love Reign O'er Me as a prelude to tonight's rain.

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