Sunday, March 3, 2013

A Spring Storm

Rain!  Yup, it's been a while since we saw a prolonged period of the wet stuff in the Salt Lake Valley, but we have it this morning.  I confess that I find the pitter patter quite comforting and that I enjoyed pulling on my hard shell before heading into the office this morning.  Six years of living in Seattle has never fully rubbed of on me and I still enjoy the wet stuff.

This is an interesting storm from a few perspectives.  First, we still had a strong inversion over the Salt Lake Valley as the precipitation moved in this morning.

I think because of the strong stability of low levels, we're seeing stronger refraction (i.e., bending) of the radar than usual and this may be leading to more ground clutter in the radar imagery.  Note the high radar reflectivities near the higher topographic features in northern Utah, which complicates the radar interpretation a bit.  We know from the observations that it is snowing in the mountains, but the high returns likely reflect contributions from terrain features.  

One way to confirm this is to examine the Doppler velocity detected by the radar.  This shows flow away from the radar near the Wasatch Mountains (indicated by red color fill below), which is consistent with the large-scale westerly flow, but near the mountains, where the clutter exists, the Doppler velocity is near zero (indicated by grey color fill).

This is fully consistent with ground clutter, which is produced by topographic features that are not moving.  As a result, with ground clutter you get a high radar reflectivity, but no Doppler velocity.

Second, the northern Wasatch Front and northern Wasatch Mountains have thus far gotten the brunt of the storm, as can be inferred by the imagery above.  It must be a serious hard-shell day at Snowbasin.  Check out how temperatures at the Middle Bowl Observing site (7400 ft) dropped and then flatlined at 32ºF with the development of precipitation.

The snow-depth sensor suggests about 2.1 inches of snow with 0.28 inches of snow-water equivalent, for a water content of about 13%.  At 830 am, the temperature at the base was 33ºF, so at open this morning Snowbasin skiers will be skiing through the so-called transition zone.  My guess is that the gondolas will be appreciated, at least until the snow level lowers a bit more as the day goes on.

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