Current (7-8 AM) temperatures in the central Wasatch are 37 at Spruces, 39 at the base of Park City, 38 at the base of Alta, and 33 at Alta-Collins, and 31 at Germania Pass. That puts the freezing level at about 10,000 feet.
In addition, dense, mid-level overcast is draped over the mountains, with some west snow at upper elevations, rain at mid elevations, and the transition zone in between. The Alta webcam below summarizes the dreary conditions quite well.
For the most part, snow on north facing aspects will survive just fine this time of year under clear, dry conditions, even when temperatures are above 32ºF. Without direct sunlight, you simply don't have enough energy to melt snow.
However, if you can add humidity and cloud cover to the mix, things change. You lose the cooling effect of snow sublimation and gain the energy input of infrared radiation from the clouds.
The eastern U.S. gets these conditions in spades at times, with fog and rain doing it's number on snow frequently during the cool season.
We don't really get such a snow-eating extreme in Utah. Today is about as close as it gets. Above 9000 feet, everything will be fine. Below 8000 feet, we're going to see snow losses. In between there may be a net loss, but it probably not a huge one.
Fall continues it's grip on the Wasatch, with no desire to let go and let winter take control....
The Tuesday weather you describe in Utah was in the Sierra Monday. I skied the aftermath Tuesday/Wednesday. It was not surprising that the mist/drizzle left a hard surface behind. The silver lining was that Tuesday/Wednesday were unseasonably warm, probably hitting 50F on top of Mammoth at 11,000.
ReplyDeleteSo here's the interesting question for a snow scientist. At this time of year the sun is generally too low angle to melt/freeze anything even moderately north facing (2/3 of terrain)at Mammoth. Yet this time the frozen surface layer did soften to pleasant spring conditions with a couple hours of sun exposure, even though it was low angle. Perhaps Jim can enlighten us here.
FYI last week's storm was a warm one. Mammoth's patrol snow plot at 9,000 feet recorded 27 inches of snow but also 6.25 inches of rain. The top of the mountain was above the rain line and got 5-6 feet of very high density snow. Perhaps the high density snow is also more easily melt/frozen in subsequent warm weather than low density snow would be.
It takes energy to raise snow to 0ºC and then melt it. It is the total energy balance of the snow that matters and that is dependent on not just solar radiation, but long-wave radiation from the atmosphere and clouds, heat transfer in the snowpack, heat transfer to the atmosphere, and vapor exchange with the atmosphere (sublimation cools the snow, condensation warms). And I've probably missed something. The type of snow can matter. New snow reflects more sunlight than old, for example. Humidity can make a big difference. A few clouds, etc. Tough to say without data.
DeleteJim