Wednesday, March 20, 2013

Astronomical Spring, Meteorological Reality

Astronomical spring arrives today, but no matter what my daffodils say, meteorological reality will beg to differ in the coming days.

This morning we have some snow falling in the mountains and rain in the Salt Lake Valley.  The precipitation began with your classic "cloud storm" scenario.  An incredibly dry airmass was in place over the Salt Lake Valley this morning, so much of the early precipitation evaporated before reaching the ground.  In this morning's sounding, taken about 5 am MDT, there was a 34ºF dewpoint depression at the surface (which equates to a relative humidity of 25%) and even drier air just above the surface layer.

A cold front moves in tonight, followed by a prolonged period of northwesterly flow that will gradually bring in even colder air on Thursday and Friday.  By Saturday morning, our 700-mb temperatures are forecast to be about -16ºC, which is below average even for January.

The overall change in temperature is perhaps the easy part of the forecast.  Precipitation is far more difficult.  As we have discussed a couple of times this week, there are wide variations in the timing and intensity of precipitation forecast by the models.  Yesterday's NAM, especially the high-resolution nest, clearly blew the precip forecast for today (see Does Precision = Accuracy), providing a good illustration that high resolution is not necessarily a forecast panacea.  For Alta, here are some numbers produced by the 0600 UTC GFS, 1200 UTC NAM-12-km, and 1200 UTC NAM-Nest-4-km:

6 AM Wed – 6 PM Wed: 0.18" SWE, 1.3" Snow
6 PM Wed – 6 AM Thu: 0.31" SWE, 2.8" Snow
6 AM Thu – 6 PM Thu: .10" SWE, 1.4" Snow
Total: .59" SWE, 5.5" Snow

6 AM Wed – 6 PM Wed: 0.05" SWE, 0.5" Snow
6 PM Wed – 6 AM Thu: 0.13" SWE, 1.3" Snow
6 AM Thu – 6 PM Thu: .02" SWE, 0.4" Snow
Total: .20" SWE, 2.2" Snow

NAM-Nest-4-km (no snow algorithm applied)
6 AM Wed – 6 PM Wed: 0.23" SWE
6 PM Wed – 6 AM Thu: 0.87" SWE
6 AM Thu – 6 PM Thu: .09" SWE
Total: 1.19" SWE

So the SWE forecast produced by these three models varies from .20" to 1.20".  Good luck!  We don't run our snow algorithm on the 4-km NAM yet, but using something like a 10-to-1 ratio, which is close to what we are getting from the other models and consistent with the warm temperatures, gives us a range in snowfall of about 2–12 inches.  How's that for uncertainty.  

My read of this event is that something in the 4–8" range is the most likely scenario for the total accumulation in upper Little Cottonwood through tomorrow afternoon.  That accumulation will probably come in a few pieces, with this morning being the first.  Much of the snow will be high density, which we really need to stick to the bone rattling frozen granular that covers the upper elevations of the Wasatch.  

All of this being said, this is a situation where forecast uncertainty is high, as indicated by the model spread above.  

1 comment:

  1. It would be interesting to compute a "storm-minimum" 700-mb temperature climatology for significant (or precipitating) trough passages and see how it changes throughout the season. Along most of the west coast the coldest trough passages in general seem to be in March, and for much of California likely near the end of March, which is reasonable based on storm track patterns and ocean temperature climatology. It would be interesting to see how this parameter varies seasonally here in Utah.