Warm and Wet

Posted Tuesday, March 20th, 2018 by Lucas Bohne

I don’t know about you, but I had a fantastic weekend at Snowbird – it seemed like it didn’t stop snowing all weekend! Periods of fluffy flakes and periods of graupel made for some fun skiing. We ended up with a good amount of snow water, and more is coming to the Wasatch.

As mentioned in previous posts, we are in for quite the atmospheric river event later this week. So far, we are looking at a strong tropical moisture tap that will feed into northern Utah beginning Wednesday, and warm temperatures will impact the snow levels in an unfavorable way. Let’s take a look at the midday NAM 12 km time height section from weather.utah.edu.

3202018

This time-height looks pretty similar to others from several previous model runs and can give us a good idea of what to expect.

Round 1: Some moisture will move into the area beginning Wednesday morning. Combined with moderate southerly/southwesterly winds, this looks like it will mostly be confined to the mountains until afternoon hours. It’ll be fairly warm, so lower elevation resorts will see rain at the bases – especially as the day progresses. Warming temperatures and a rising freezing level (blue line) into Thursday will mean rain at low elevations. Note that the freezing level isn’t exactly where the snow turns to rain, as snowflakes can fall through warmer layers and survive to accumulate, especially in heavier precipitation. For this first round, don’t expect much for accumulation. I’d guess up to 2 inches of snow at upper elevations/higher parts of resorts and up to an inch of wet snow at lower portions of resorts with some rain mixing in.

Round 2: This will be the slug of moisture associated with what is hyped up as the atmospheric river. While a good portion of moisture is usually intercepted by California’s High Sierra, atmospheric rivers can penetrate into Utah and provide good amounts of moisture if the trajectory isn’t directly intercepting the highest parts of California. Today’s 12Z GFS forecast for Thursday morning is just that, as water vapor transport is strong and on a path from extreme southern California into Utah.  the following shows IVT (Integrated Vapor Transport) 48 hours out from the model run that happened at 12Z this morning.

Integrated Vapor transport for Thursday morning from NOAA's Earth Systems Research Laboratory. (https://www.esrl.noaa.gov/psd/psd2/coastal/satres/data/html/ardt_ivt_gfs.php)
Integrated Vapor transport for Thursday morning from NOAA’s Earth Systems Research Laboratory. (https://www.esrl.noaa.gov/psd/psd2/coastal/satres/data/html/ardt_ivt_gfs.php).

For a lot more atmospheric river guidance, check out this site: http://cw3e.ucsd.edu/iwv-and-ivt-forecasts/

Alright, so continuing with Round 2… Note how the freezing level is ~700 mb for most of the larger green blob in the time-height section. This is roughly mountain top height for  our highest northern Utah peaks. So, even the higher resorts will likely have rain/wet snow at their bases for a good chunk of the event. If you’re planning to ski make sure you have your waterproof gear! Snow amounts are somewhat difficult to forecast for this second round considering how impactful the warm temperatures will be for accumulations. Also, there is still a wide range of possibilities – even for higher elevations. Just know that this we’ll be wet and quite warm later this week. To hit that point home, here is the NAEFS plume forecast for this week for Alta. It sure is wet, but sure has some uncertainty!

NAEFS plumes from weather.utah.edu for Alta. Forecast water amounts on top panel, snow on bottom panel.
NAEFS plumes from weather.utah.edu for Alta. Forecast water amounts on top panel, snow on bottom panel.

Check tomorrow as models hone in on some of the details.

-Lucas

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