Thursday, January 14, 2016

Sandwiched Between Clouds and Gunk

The Alta-Collins site is reporting 3 inches of snow from only .13" of water as of 8 am.  That's cold smoke worth of the dust-on-crust designation in bounds, but should be aesthetically pleasing where the underlying surface is soft and in untracked areas in the backcountry.  We may squeeze another 1-3 inches of cold smoke out in the upper Cottonwoods this morning.

The view of the Salt Lake Valley from the University of Utah at 0815 MST showed that we were sandwiched between overcast and air pollution.

Looking southwest across the Salt Lake Valley and Oquirrh Mountains from the University of Utah
This is a common occurrence with the passage of a weak storm system that lacks the punch to quickly mix out the inversion.  Instead, we had a top down mix out overnight with the valley cold pool and associated air pollution pollution being scoured out aloft, resulting in cleaner air on the upper benches, while the coldest air and a thin lens of air pollution persisted on the valley floor.

This can be seen quite well in the PM2.5 measurements from the University of Utah on the east bench and Neil Armstrong Academy on the valley floor in West Valley City.  At the University of Utah, note two dramatic downward spikes in PM2.5 concentration at about 0000 MST and 0400 MST, followed by a sustained drop at about 0600 MST.

In contrast, PM2.5 levels at Neil Armstrong Academy remained elevated throughout this period and currently sit very near the "Unhealthy for Sensitive Groups" threshold.  

The coldest upper-level temperatures associated with today's storm won't be here, however, until later this afternoon.  Thus, I suspect we'll see further scouring of the remaining cold pool and air pollution today, with some assistance from surface heating.  As this occurs, we may see a return of some pollution along the benches as the shallow cold pool sloshes around, mixes out, and is transported by northwesterly flow.

Additional News:

Weather and science geeks might want to check out some of the articles in the new NSF special report on the Science of Winter.

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