Tuesday, January 29, 2013

Another Look at the Mountain Effects

The image below shows the radar reflectivity (purple = high) this morning at 1300 UTC (0600 MST) and the dramatic effects of the mountains on the precipitation during this event.  Note the higher returns over the Stansbury, Oquirrh, and most of the Wasatch Mountains (see also previous post).  I have also annotated the outlines of the ski areas on this map in red.  There are issues with radar sampling over and downstream of the Wasatch range (for example, the lack of returns over Snowbasin is due solely to beam blockage by the terrain - it was snowing hard there during this period).  Nevertheless, this image illustrates that in the central Wasatch, the heavier precipitation was found over/near the western slopes, with a dramatic dropoff in radar reflectivity as one moved eastward to the Park City ridge line and the Wasatch Back.    

In addition, the storm was quite shallow in the Cottonwood Canyons.  I just got an informal report from Snowbird and was told there were a few inches on top and a foot and a half near the base.  Apparently Baby Thunder was the place to be.  Wow.

Also evident in the image above is the dramatic enhancement of precipitation near Ben Lomond Peak northeast of Snowbasin.  At 11 am, Liberty, which is in the valley immediately east of Ben Lomond, reported a storm total of 28 inches.


  1. While I believe less snow at the top than the bottom, a few inches seems really low. It is pretty windy on the ridges, so I would expect a lot of variability up high. I would also point out in the radar images that the radar beam is often above mountain top in the Central Wasatch. With snow falling at ~1 m/s (~15 minutes to fall 3000 feet), a substantial amount of snow is carried downstream of the radar echo before hitting the ground. In this scenario, I would imagine that you would be getting dendritic growth beneath the radar beam as well. Those would be good reasons for why you don't see a decent echo over Alta or Snowbird in the image even though Collins was recording an inch an hour at that time.

  2. You are right about the transport and possible growth, and the need for caution in interpreting the radar images. During the period examined, however, automated snow sensors along the LCC highway did receive more snow than Collins, and far more than observed in the lee. Thus, one needs to be cautious in interpreting the radar, but the general picture of a decrease in snowfall from west to east as one moves across upper BCC, the PC ridgeline, and into the Wasatch Back seems reasonable.

    1. Definitely. I agree with you on all points. My point is that I doubt it is snowing much harder over Broads Fork Twin Peaks or Dromedary than at Snowbird or Alta. I think a fair bit of that snow reaches Alta and Snowbird but beneath the radar beam when there are decent NW winds aloft. Beyond the PC ridge line, things are more complicated and I agree with your lee subsidence analysis in this case.

    2. Ah yes, that makes sense. Thanks for the clarification. BTW, we have some really great DOW data from winter storms last year. You should have a looksee sometime.