Tuesday, April 18, 2017

A Spring Airmass Best Described as "Juicy"

I got some odd stares this morning carrying my umbrella on the bus, but I know better.  A juicy airmass is in place over the Great Basin and, with good dynamics later today, we're in for a wet period later today and tonight.

First, let's take a look at the moisture that we have in place upstream.  The 1200 UTC (0600 MDT) sounding from Elko shows a near saturated atmosphere through most of the troposphere, with the exception of a dry layer just above 500 mb.

Source: SPC
The total precipitable water in this sounding is 15 mm (0.6 inches).  The sounding climatology shows values of 0.6" are near, but not quite at the upper-limit of prior observations at Elko.  There are a few days in mid-to-late April with values reaching about 0.65-0.67".  Nevertheless, 0.6" is pretty high for April.

Source: SPC

If we look at the return interval of the precipitable water forecast produced by the North American Ensemble Forecast System for 1200 UTC (0600 MDT) this morning, we're looking at something that maximizes between 1 day every 5 years and 1 day every 10 years in a strip extending across central Nevada and northwest Utah.

Source: NWS
By 0000 UTC (1800 MDT) this afternoon, that strip is parked over the Salt Lake Valley.

Source: NWS
A juicy airmass isn't enough, however, to ensure precipitation.  One also needs lift.  In other words, something to initiate clouds and precipitation,  Better yet if the stability is weak.  We have both today along a developing trough.  The HRRR forecast shows this trough centerd over northwest Utah with extensive precipitation, some heavy, over much of the area.

  We will have to see how this all plays out, but rain and thunderstorms look likely to develop during the day today.  Below is a summary issued by the National Weather Service.


I'll add that the Storm Prediction Center has our region in a marginal risk category for severe thunderstorms this afternoon and evening.

Bottom line: Keep and eye on the sky and the radar.  Things might just get interesting...
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4 comments:

  1. UnknownApril 18, 2017 at 9:53 AM

    Im not sure how accurate our local news precip midels are, however the one I looked at early this morning showed a pretty definitive bow echo over the northern Wasatch. I'm excited to see how this all plays out as I love a good spring cold front. Hopefully we'll see a nice shelf cloud moving over the GSL this afternoon.

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  2. UnknownApril 18, 2017 at 9:54 AM

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  3. BasilApril 18, 2017 at 10:34 AM

    Hi Jim,

    I saw that the sounding profile at 700 mb was between 0C and -1C...with models keeping the Wasatch in that range thru midnight, before lowering to -2C to -4C by morning. What are your thoughts for snow levels and amounts in the Big Cottonwood, for those of us with Snowbird passes. Gad 2 Chair range is 8600-9800 ft, while Little Cloud Chair runs between 9500-10800 ft. Thanks.

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    1. Jim SApril 18, 2017 at 10:58 AM

      Snow levels later today look to be quite high. 700-mb temperatures reach 3ºC, whcih would proably yield a snow level near 10000 feet. After 6 PM, the snow levels look to drop and perhaps reach 7500-8000 feet by 9 PM. Snowfall accumulations below 9500 feet are going to strongly depend on the timing of that drop and the precipitation. Regardless, most of the snow is going to be of the high-density variety.

      Jim

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