Saturday, April 7, 2018

Cascadian Day Ahead

Humid and overcast this morning, with a puddle of haze along the valley floor
Saturday has dawned humid and overcast with a more Cascadian-like feel than we are used to around here.  The morning sounding from the airport showed a "juicy" environment, with near-saturated conditions through the troposphere and a precipitable water of 0.77 inches, which ties the highest observed in the record at KSLC for the month of April.

Source: SPC
Note in the photo above the puddle of haze along the valley floor.  As can be seen in the sounding, there is a shallow inversion present with high humidity, which leads to haze as some of that water vapor condenses on aerosols (i.e. pollution) from the valley floor.

The dewpoint (blue line below) has climbed steadily over the past three days and from yesterday through this morning has hovered around 45˚F.

Source: MesoWest
With so much moisture around, you might be wondering why it is not raining right now (about 9 am). The answer is that right now there's no dynamics, meaning something to lift the air so that it cools, resulting in condensation and the formation of precipitation.

That all changes late this morning with the approach of the cold front, which at 1500 UTC (0800 MDT) was draped across northern Nevada.

In fact, if you want to see a really juicy sounding, check out the one below from Elko, which has a precipitable water of 0.84".  That is the highest ever observed in the period of record at that site between November and April.

And that is what is coming to Salt Lake and environs today.

Turning our attention to the mountains, total precipitation (water equivalent) at Alta Collins and Snowbasin Middle Bowl from 6 AM MDT yesterday through 8 AM MDT this morning was about 0.85 and 1.2 inches, respectively.  Most of that precipitation fell as rain in the low-to-mid elevations or as very wet snow at upper elevations. 

This morning, the Utah Avalanche Center reported a 9300 foot snow level in the central Wasatch.  At 8 AM MDT the freezing level was sitting near about 9700 feet with temperatures of 32˚ at Alta-Collins (9662 ft) and Solitude Summit (9641 ft).

Two things will cause the freezing (and snow) levels to climb this morning, one is daytime heating, the other is the transport of warm, humid air into Utah by southwesterly flow ahead of the cold front.  The NAM shows 700-mb (10000 ft) temperatures climbing from -0.8˚C at 6 AM MDT to 3.9˚C by 3 PM MDT.  Thus, if you go out this morning thinking you'll be skiing in the snow at upper elevations, think again.  Snow levels look to peak near 11,000 feet this afternoon during the warmest part of the storm.

The numbers below from the 12Z NAM for the Alta area pretty much tell the story.  Note how the wet-bulb zero level (i.e., the height at which the wet-bulb temperature is 0ÂșC) rises from 9500 feet this morning to 12000 feet at 2 PM.  Snow level is typically about 1000 feet below this level.  You do the math.  It ain't good.  It remains above 10000 ft through 8 PM, when it begins to drop, along with precipitation rates.  Sad!

Our estimated temperatures for Mt. Baldy (11,000 ft) peak at 36˚F.  Simply incredible.  Total water equivalent from 8 AM to midnight is 1.02", with one measly inch of snow at Alta-Collins (9700 feet).

This afternoon will be highly unusual by historical ski-season standards, with incredible warmth and rain to remarkable elevations.  

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