Thursday, April 14, 2016

Frontal Update, Part Deux

It's not quite the equivalent of Groundhog Day The Movie this morning, but it's close.

As was the case yesterday, a frontal boundary and precipitation band are flirting with northern Utah.  The main difference is that they are a bit further north this morning than yesterday.

The large-scale analysis from 1400 UTC shows a pronounced 850 mb (~1500 m/5000 ft) wind shift extending across central Nevada and northern Utah, including the Great Salt Lake, with a band of precipitation just to the north.

MesoWest surface observations similarly show the wind shift extending through Dugway Proving Ground, across the Great Salt Lake, to north Ogden.

Although we are south of the front this morning, temperatures are pretty similar to yesterday morning and in the mid 50s.  If you are outside and not paying attention to which way the wind is blowing, you will hardly notice a difference.

The 1200 UTC initialize HRRR model  calls for the front and precipitation to push into the Salt Lake Valley this morning with showers and possibly a thunderstorm today.  I think this forecast is a bit fast on the precipitation as it has the band over us by 8 AM this morning, but it will get here eventually.

What's the scoop on mountain snow?  Well, yesterday afternoon and overnight the Alta-Collins precip gauge picked up 0.33" of water, but the snow depth sensor maxed at only 1".  Temperatures ranged from 38ºF yesterday afternoon to 33ºF over night.  Ick.  Temperatures and snow levels will lower some with the frontal passage today, but I'll stick with my forecast from yesterday which called for a total of 3-6" of upper elevation snow from yesterday through this afternoon.  That means 2-5" today.

Over the past couple of days I've seen some big snow numbers forecast for storm totals through Friday.  I'm less enthused.  We never get into a good period of moist northwesterly flow and the frontal passage for today is a relatively warm event, leading to less snow per unit of water.  The NAM calls for a storm total today and tomorrow of about 0.6" of water, which our algorithms convert to about 6" of snow (10% water content overall, although it will be higher water content to start and lower later).  The NCAR ensemble has most members between 0.35 and 0.75" of water, although there is one with only about 0.08" and another with closer to an inch.

In addition, this is April.  Climatologically, the enhancement of precipitation by the mountains this time of year is quite a bit lower than it is during mid winter.  In January, the average precipitation at Alta is more than five times what it is in Salt Lake City, but in April, it's only about 2.5 times that.  Typically frontal and convective storms of the type we are experiencing today produce less enhancement of precipitation in the mountains, so jacking up the model totals by a lot, as one might do in winter, doesn't make sense in this pattern.

Thus, for a big dump, we will need the front or the post-frontal environment to be bigger producers than advertised presently by the NAM and NCAR ensemble.  While I can't rule that out, it's simply not a good bet and expecting more modest totals of 2-5 today and perhaps a couple inches more in snow showers tonight and tomorrow makes the most sense.

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