Saturday, November 27, 2010

Stingy inversion

I'm quite impressed with the intensity of the inversion that is presently in place over the Salt Lake Valley.  This morning's sounding shows a roughly 10C inversion extending form the surface to about 775 mb where the temperature was about +2C.  

MesoWest obs this afternoon (~3:30 PM MST) show that fully inverted conditions persist over the valley.  At KSLC, it's only 27F, whereas is it in the 30s in the Park City area and at a few locations in the Wasatch Mountains.  I watched the car thermometer drop from 35F to 29F descending from the base of Mineral Fork to the bottom of Big Cottonwood Canyon this afternoon.  

The flow aloft is certainly quite strong as Alta-Mt. Baldy has had several hours with gusts >60 mph and a peak gust of 72 mph.  My experience, however, is that strong flow at the ridge tops is pretty ineffective at removing an inversion/cold pool.  Instead, one needs some sort of large-scale pressure gradient to drive strong southerly flow within the valleys and we don't have that today.  In fact, there is a north wind presently at Point of the Mountain.  

PCAPers may want to keep a close eye on this one as we could be dealing with a frontal passage late tonight or early tomorrow morning with minimal pre-frontal southerlies, which contrasts with many cold-pool events.  Thus, cold pool removal may not occur ahead of the front, but may depend instead on post-frontal northwesterly flow and cooling aloft.  The NAM brings us down to -16C at 700 mb by mid day Monday, which is not quite cold enough to mix out the current cold pool thermodynamically, so at issue is whether or not there will be sufficient flow to do it mechanically, or if a lens of cold air might persist over the lake and along the valley floor.  

1 comment:

  1. Thanks for the analysis Jim, the "PCAPers" will be watching and I think Neil will probably be picking your brain about this case since it is IOP1.