Wednesday, April 8, 2015

A Couple Degrees Makes a Big Difference

The good news is it's snowing in the mountains.  The bad news is that the system has come in a bit warmer than forecast and it is raining on campus and in the Salt Lake Valley.

Snowfall forecasting is particularly problematic when the forecast places you anticipate being in the melting layer where precipitation is transitioning from dry snow to wet snow to slush and then finally rain.

Source: Secrets of the Greatest Snow on Earth
In winter storms in Utah, a temperature increase of 1ºC raises the melting layer about 150 meters (almost 500 feet).  Therefore, if you anticipate being in the melting layer, as I thought the University and Salt Lake Valley would be this morning, a small error in temperature can be the difference between snow or rain.

Indeed that is what has happened. Yesterday's NAM forecast for 1500 UTC (9 AM MDT) this morning called for a 700-mb temperature of -9ºC or so, cold enough to put the U in the melting layer with the anticipation of wet snow.

Instead, we're about 2ºC warmer in this morning's NAM forecast for 1500 UTC (9 AM MDT) this morning.  That shifts the melting layer up nearly 1000 feet and turns our flakes into rain.

If we get a burst of very intense precipitation, perhaps we can turn over to snow, graupel, or ice pellets but that will likely be short lived.  Oh how I miss winter!

No comments:

Post a Comment