Tuesday, March 28, 2017

A Remarkable Alta Snow Burst

Yesterday afternoon from noon to 1 PM MDT,  Alta was pounded with intense snowfall.  The automated Alta-Collins stations recorded 0.25 inches of water equivalent in that hour, and the interval snow-depth sensor reported an increase in snow depth of 7 inches.

Due to round off errors and other issues, perhaps the 1-hour snowfall wasn't 7 inches, but something closer to six, which is what I've heard reports of from friends in the vicinity.

In any event, it sounds like a very intense snowfall period.  The question is why.

And the answer is that I don't know!

Nothing jumps out from the radar loop for the period (Alta identified roughly with a red box near the center of the frame).  There is a brief period with relatively high radar reflectivities (yellow color fill indicates >35 dBZ) that would be consistent with strong, localized snowfall, afterwhich the echoes are actually non-existent.

Often, there is a very poor correlation between snowfall rates and radar reflectivity in Little Cottonwood Canyon for a variety of reasons.  One is that the radar beam is partially blocked.  Another is that the snow growth can sometimes occur at low levels below the unblocked portion of the beam.  Thus, we get little insight into the intensity of yesterday's snow from the radar loop.

A curious aspect of the burst is that temperatures at the time were relatively high.  At Alta-Collins (9670 ft) it was about -2 to -3ºC.  The snow that fell, however, appears to be of low density (6 inches of snow with 0.25" of water equates to a water content of about 4%).  Thus, the snow was likely comprised primarily of dendrites and dendritic aggregates (dendrites that have stuck together).  In layman's terms, "goose feathers." Dendrites typically form at temperatures of -12 to -18ºC, which means the flakes formed at about 4000-5000 meters above sea level based on yesterday afternoon's sounding.  When I think back to lower density events at Alta, I think of shallower storms, but that does not appear to be the case yesterday.

So, it was an interesting burst of snow the causes of which are unknown to me.  I wish I was there! Feel free to share your ideas and observations.


  1. Watson employee here; Snowed very hard ( at times, couldn't see angle station from the shelter) from about 11:30 to 1. Flakes were larger after 12:30. Very little wind. One of the best ski "moments" of the year, especially "Keyhole". Guessed about 10" on the Watson outdoor (2nd floor) deck, very fluffy on top of about 2 inches of denser stuff that fell earlier that morning.

  2. This burst also shows up well on the Snowbird SnowCam time lapse between 11:40 AM and 12:40 PM. My guess is that it is related to the high reflectivity feature between 1700 and 1715Z (11 to 11:15 AM) to the south of Snowbird and Alta, which is well before your loop starts. The KMTX radar beam width, if refracting normally, extends between 11000' and 17000' in this location, so into the dendritic growth zone for this event. The winds at Baldy are SE at this time with what appears to be SW flow in the radar echo region. Considering a snow terminal fall speed of ~1 m/s, and a possible region of upslope lift with the SE flow impinging on the Little Cottonwood ridge crest limiting the fall rate of the snow and causing rapid growth and aggregation, I think the snow could be carried all the way to the front side of Snowbird and Alta reaching the ground at mid-mountain in a half hour to hour time frame from when the radar observed the feature to the south.

    1. Adam:

      Nice, helpful analysis. Good points too with regards to fall speed and transport. Thanks for adding it.


  3. I was at Snowbird and what was a dust on crust day turned in to an amazing powder day in literally an hour. There were spots where it felt like over a foot fell during that time. What I found interesting was that for the first two hours the snow at the bottom was very very light. Around 3 pm that snow turned really more dense. I suspect it's because the temps were high at the base allowing the snow to set up quicker? Regardless it was a strange epic day.

    1. It's tough to hold back the caustic effects of solar radiation this time of year. Temperatures at 8500 feet yesterday reached about 36 degrees. That, with some solar radiation and long wave radiation from the low clouds is enough to do the damage.

  4. Was touring near Davenport at that time. Very intense snowfall and was light small crystals coming down as hard as it could. Even S facing high elevation was near blower returning to car at 2:30.