Friday, January 11, 2013

Powder Perversion

Check out these snow totals through 4–6 am this morning:

East Millcreek: 15 inches
Upper Avenues: 12 inches
Snowbird Base: 11 inches
Alta Base: 8 inches
Alta-Collins: 6 inches
Snowbird Gad-II Snow Cam: 4 inches

You have got to be kidding me.  We do get storms from time to time that dump more in the valley than the mountains, but I didn't think this would be it.  

Last nights storm featured two phases.  The first was the frontal passage, when it is not uncommon for mountain and lowland precipitation to be comparable.  The second was the post-frontal period after the front had moved downstream and the flow became northwesterly.  This is typically a period when there is strong orographic enhancement and more precipitation in the mountains than the lowlands.

However, this wasn't the case last night.  Check out the radar loop below, which covers the late night hours (0815–1300 UTC; 0115–0600 MST) when the front was through and the flow northwesterly.   Over the Salt Lake Valley, the strongest and most persistent echoes are located over the eastern half of the valley and over the Wasatch mountains and canyons immediately to the east.  There are very few radar echoes in the upper Cottonwoods (although there was light snow falling at times, it wasn't detected by the radar).  

This is the type of scenario that one sees often in the European Alps, which are so high and wide that many storms dump their loads over the alpine foothills, with less precipitation falling over the higher interior.  In fact, this can be seen quite clearly in the annual precipitation climatology for the central and eastern Alps.  Note how bands of higher precipitation surround an area of lower precipitation in southeast Switzerland, northeast Italy, and southwest Austria, which includes the Valais Alps, Otztal Alps, and Dolomites.  

Alpine mean annual precipitation.  Source: Christoph Frei, ETH
The situation last night was one in which the high terrain of the Cottonwoods may have been a curse rather than a blessing.  There are a couple of reasons why this might be the case.  The instability, which may have been enhanced over the lake, was relatively shallow.  The morning sounding showed a weakly stable layer above about 750 mb.  This may have prevented the storm from penetrating over the highest portions of the Wasatch around the Cottonwoods (it is interesting that this effect was not as pronounced over the lower Oquirrh Mountains and northern Wasatch).  We have observed this before (including during a field program last fall), but to have it happen in a deep post-frontal airmass if pretty rare.  The second, which was proposed by a National Weather Service meteorologist, is that the temperatures were more favorable for the growth of dendritic snow crystals at lower elevations, which leads to lower density snow and greater snowfall amounts.   Perhaps both processes are contributing.

In any event, it will be a weird first chair today with the deepest snow at the base and less up high.

Addendum 9:52 AM:

These two photos pretty much sum it up.

Olympus Cove.  18" new.  Courtesy Steve Krueger
Snowbird G-II cam.  4" new. Source:

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