Tuesday, January 9, 2018

Lessons in Snow Level Forecasting

It's worth taking a look at temperatures this morning as the overnight southwesterly flow has brought in mild air.

At about 1440 UTC (0740 MST) temperatures at Snowbasin were around 35ºF at the base, 35ºF at the Middle Bowl observing site, and 29ºF at the top of Mt. Ogden.  This puts the freezing level, the level at which temperatures are 32ºF/0ºC at about 9000 feet. 

Source: MesoWest
In the central Wasatch, temperatures are above freezing at the base of all the ski areas.  It's currently 38ºF in town at Park City, 36ºF at the base of Alta, and 36ºF at the bases of Solitude and Brighton.  Here, the freezing level sits at right around 9000-9500 feet depending on the local topography. 

The snow level is typically lower than the freezing level for a number of reasons.  As snow falls and begins to melt, it extracts heat from the atmosphere.  This often results in a layer of constant temperature that is near 0ºC.  The evaporation of snow also can cool the atmosphere some or slow the melting of snow.  Eventually, the snow turns into a mixture of snow and wet snow (or slush) and eventually wet snow and rain, before becoming all rain.  The layer in which this occurs is called the transition zone

Because of the effects of melting and evaporation, the snow level (and freezing level) can yo-you depending on precipitation rate, lowering when precipitation rates are high, and there is greater cooling due to evaporation and melting, and rising when precipitation rates are low, and there is less cooling due to evaporation and melting.  This may be noticeable if you elect to don the garbage-bag look and are skiing today. 

To estimate snow levels, meteorologists use soundings extracted from numerical forecast models.  In addition to temperature, humidity (or dewpoint) are also used, along with estimates of precipitation intensity.  Typically, one uses the temperature and humidity profiles to estimate the height of what is known as the wet-bulb-zero level, the height at which the wet-bulb temperature is 0ºC.  The wet-bulb temperature is the temperature of the air if it were cooled by evaporation to saturation and helps to account for some of the cooling effects noted above.  In Utah, one often lowers this level by 1000 feet for an estimate of the snow level, although there are times when one might fudge it by less or more.  For example, in very high precipitation rates, the one might lower it more.

We extract the hight of the wet-bulb-zero level from the NAM and provide them from the latest forecast at http://weather.utah.edu/text/COTTONWOODS.txt.  Below is an example of the tabular output from last night 0600 UTC initialized NAM forecast.  The wet-bulb-zero level was forecast to fluctuate between 8700 and 9000 feet through 8 AM this morning.  It remains between 8700 and 9300 feet through 5 PM this afternoon.  Thus, expect snow levels to be near or around 8000 feet today. 

Lowering of the wet-bulb zero begins slowly after about 10 PM tonight, with a more sudden drop very early Wednesday morning with the approach of the cold front.

Also available in that table are temperatures and winds for Mt. Baldy, estimates of snow-to-liquid water ratio/water content based on an algorithm developed by Trevor Alcott and myself, water equivalents produced by the NAM, and estimates of snowfall based on the water equivalent and the snow ratio estimates.  Handy, if you keep in mind it is just guidance from the NAM model.  Through 7 AM this morning, this NAM run produced 0.35" of water and 2.5" of snow at Alta-Collins, which compares fairly well to the 0.34" and 4" observed.  I can assure you that most forecasts aren't that good!


  1. Jim - I've heard that parts of Northern Italy and Eastern France just experienced a 100 year snow event with 2+ meters in 24hrs, any chance you can break down what came into play to make that such a significant event?

  2. Also, I am guessing the time (or fall distance) that snow takes to melt is somewhat proportional to air density? So the melt zone could be much larger up at 650 or 700 than at sea level... I don't know exactly to what degree this is true.