Friday, May 11, 2018

May Powder?

It's never over until it is over.  At 1200 UTC (0600 MDT) this morning, and upper-level trough was centered just northwest of northern Utah. 

Local radar shows precipitation filling in across the northern half of the state.  Most of this precipitation is still light and scattered, but there is a band of heavier precipitation across the northern Great Salt Lake and a few stronger showers scattered about.

If you think it feels humid and sticky out there, that's because it is (by Utah standards).  Dewpoints at the airport this morning were near 50ºF. 

Turning out attention to the mountains, it's balmy up there, with a 700-mb temperature of about +3˚C.

Current surface temperatures include 39˚F at Alta Base, 36˚F at Alta-Collins, and 32˚F at the summit of Mt. Baldy (thank you Alta Ski Patrol for keeping the observations going in the off season).  That gives us a freezing level of about 11,000 feet and a snow level of perhaps 10,000 feet.

The models call for the upper-level trough to move southward into Nevada today and tonight.  Expect unsettled weather through at least tomorrow, with some thunderstorms.  The valley will see rain. 

Our Upper Cottonwoods meteogram from the 0600 UTC NAM forecast illustrates the situation for the mountains.  The forecast is holding up well so far with a temperature of 32˚F on Mt. Baldy, similar to observed.  Temperatures decline slowly through this evening, rise a bit overnight, and decline again tomorrow morning through early afternoon.  This complex temperature evolution reflects the fact that the trough is both digging to our west and intensifying, which results in a situation whereby we transition back into warmer southeasterly or southerly flow early tomorrow.  Basically, it's a mess. 

Thus, I expect snow levels to drop through this evening, perhaps to about 8000 feet, but rise again late tonight or early tomorrow morning.  There are large error bars on this, however, and here's why.

Model soundings for Alta such as the one below, valid 0800 UTC (0200 MDT) tonight, show a very deep layer in which the temperature is 0ºC.  This is the sort of thing that can happen in the Cascades when the precipitation rates are high.  Think of it as the ice cube effect.  Mother Nature dumps a ton of snow very quickly, and the atmosphere cools to 0ºC, much like throwing ice cubes into your cocktail.  I'm never sure whether or not to count on these sorts of details in the model forecast as they are dependent on many factors.  However, if it were to happen, the snow level will lower perhaps more than expected.  If it doesn't, well it will be high.  Regardless, the snow in such a layer tends to be wet and sticky .  
The NAM is quite generous with water totals, producing 2 inches through noon Saturday.  Our algorithms suggest that equates to about 12 inches of snow at 9700 feet.  Cascade concrete for sure.

This is, however, a very difficult precipitation amount forecast for several reasons.  One is that it is dependent on the track and intensity of the wobbly trough.  Another is that we're going to see a lot of pop-up showers and thunderstorms which can't be pin pointed.  As a result, the SREF shows remarkable forecast plume spread at Alta-Collins. 

Thus, this might be the most wishy-washy post I've done.  I expect precipitation, possibly heavy at times, in the central Wasatch.  I also expect the snow to be high density.  I'm not sure about precipitation totals, other than ruling out low amounts, or snow levels.  The best option in a pattern like this is to be ready to go in the morning, set the alarm, pull up the Alta-Collins ob ( when it goes off, and either go back to sleep or jump in the car depending on what you see.  May powder?  I don't think so.  Creamy crud?  Maybe, if the trough delivers enough snow tonight.  Otherwise, there's always yardwork.

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