A Rainy Night for Santa

A warm storm is on tap for the Night Before Christmas, one that will not make Santa or his reindeer happy.   

To illustrate this, let's take a look at the forecast from the High Resolution Rapid Refresh (HRRR) for Little Cottonwood Canyon (click to enlarge).  The HRRR calls for periods of precipitation overnight (green box below) adding up to 1.5" of snow water equivalent at Alta-Collins through 7 AM Christmas morning.  

That's the good news.  The bad news is that the overnight period is remarkably warm, with the wet-bulb-zero level, the altitude where the wet-bulb temperature reaches 0ºC and roughly the top of the melting layer, reaching as high as 10,700 feet (the snow line is usually several hundred feet below the wet bulb-zero level).  Our algorithms suggest this period, highlighted with green shading above, may produce rain even at the 9600 foot altitude of Alta-Collins.  

Snow during other periods at Alta-Collins adds up to 8.4" by 7 AM, although we don't account for losses due to rain, so that might be optimistic.  That 8.4" will probably be wet and dense.  Even by 7 am tomorrow, the HRRR has the wet-bulb-zero level at 9300 feet, which suggests that the precipitation will be probably be all rain below 8000 feet overnight.  

The HRRR is a relatively wet model.  Others, such as the RRFS ensemble below, are calling for less precipitation.  Through 7 AM tomorrow (14z 25 Feb), the RRFS members produce 0.45 to 1.2" of water.  Some of those members also produce rain to near the Alta-Collins level during the warmest part of the storm.  Snowfall is in the 3-8" range by 7 am, with amounts varying depending on water equivalent, snow level, and snow-to-liquid ratio.  

So, Santa will likely be delivering presents in the rain below 8000 feet.  He might also encounter rain or a rain-snow mix up to 9500 feet or even a bit higher depending on his delivery schedule.  Christmas skiers probably won't find The Greatest Snow on Earth under the tree tomorrow morning, but perhaps 4-8" of wet, dense snow at 9500 feet. Expect a strong gradient in snowfall amount with altitude between about 8500 and 10000 feet and the most snow above 10000 feet.  

This is one forecast that I hope is totally wrong or out to lunch.  There is a wide range of forecast water equivalent being produced by the models, so maybe we can get lucky and we can have the storm come in colder than anticipated and produce a lot of water, with more substantial accumulations over 8000 ft.  It is Christmas after all.  

There will be a cooling trend over the next few days with some snowshowers on the 25th and 26th and a snowstorm Friday night and Saturday.