Saturday, March 1, 2014

Musings on the Storms

Orographic clouds over Lone Peak, Saturday, 1 Mar 2014
It's been a great storm cycle thus far.  We are now looking at a total of 21 inches of snow at Alta-Collins since Thursday morning with 2.78 inches of water.  The stuff that fell last night and this morning was especially dense with 10" of snow containing 1.42" of water for a 14.2% water content.  One look at the highly scientific sample of snow collected by my open door's arm rest this morning at Snowbird pretty much shows why.  Those nasty looking orographic clouds in the photo above were laying down nothing but graupel.

You can also see the small graupel in the real-time data (12:05 PM MST) collected by the University of Utah MultiAngle Snowflake Camera (MASC), provided by Alta on their Snowflake Showcase.

Source: University of Utah/Alta Ski Area
This collaborative effort between the U and Alta provides real-time photos of snowflakes (or in this case snow pellets) up at Alta.  

A few people have commented that it seems like upper Little Cottonwood has gotten an unusual amount of snow during these two "southwesterly flow" events.  That generalization that Alta/Snowbird doesn't get a lot of snow in southwesterly flow has always bothered me.  There does seem to be a subset of events during which that's true, but they sometimes get it pretty good in southwesterly flow.  Further, a portion of the Thursday storm featured northwesterly flow, so this isn't necessarily the best case to consider for the generalization (although I suspect a good fraction of the precip has been generated during southwesterly flow.  

Although there is some value in using wind direction when forecasting mountain precipitation, here are some of the dangers:

1. Snowfall at Snowbasin, Ben Lomond, and Sundance have a fairly strong directional dependence for snowfall (heavy events tend to be W-SW flow, and they tend to get far less in NW flow), but this dependence is weaker in the central Wasatch.  In part, this reflects the more three-dimensional nature of the terrain in the central Wasatch, which exposes the upper Cottonwoods to storms featuring a wide range of flow directions.  

2. The precipitation processes that operate over the central Wasatch very dramatically from event to event and can be highly sensitive to temperature, wind shear (directional and speed), stability, and the vertical distribution of moisture).  Unfortunately, we don't really understand all these relationships yet and they are easier to diagnose in hindsight than they are to predict.  

3. Because of 2, all "southwesterly flow" storms in the central Wasatch are not created equal.  Some have southwesterly flow at low levels, others southeasterly.  Some have low stability, allowing the low level flow to move over the terrain, others have high stability, forcing the air to move around the terrain.  These effects all influence where precipitation is generated and where it falls out.  

I'd add a few more things to the list, but it's the weekend...

Addendum @ 12:55:

The small graupel shows up beautifully in the Snowbird cam.  Nothing but tiny balls of styrofoam, both on the snowpack and falling in the photo.  I'm surprised they haven't wiped it.  They must be going for maximum effect.

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