Wasatch Weather Weenies

Just another day in the paradise that is the Wasatch Front.  The latest west-facing image from the University of Utah shows a blanket of smog over downtown Salt Lake City.   Source: https://horel.chpc.utah.edu/camera_pages/wbbw.html It's tough to get above it without going to the mid elevations.  I hiked this morning to the top of the Avenues foothills and it was unclear if I was completely out of it even at 5700 feet.   Recent station (filled squares) and mobile (filled circles) PM2.5 observations show values in the valley generally between about 28 and 45 ug/m3.   Source: https://utahaq.chpc.utah.edu/ The threshold for unhealthy for sensitive groups is 35.5 ug/m3, so we are seeing values that are near or above that.  The DAQ sensor at Hawthorne Elementary is at 41.3 ug/m3 as of 2 PM.   Is there hope for a mix out?  The best chance is probably on Thursday and Friday.  On Thursday there is a weak upper-level trou...

For the most part, the news during prolonged inversion events is bad, but I did want to share one "silver lining" with regards to the current event, which is the cloud cover and what it is doing to increase mixing in the Salt Lake Valley, especially overnight. The air is still polluted, but these clouds are actually a help rather than a hindrance If the solution to pollution is dilution, then the development of cloud cover over the Salt Lake Valley is an improvement over the cloud-free inversion that we had prior to yesterday.  Let me show you why. Below is a loop of upper-air soundings collected each morning at the Salt Lake City airport over the past week.  These soundings are plotted as a "skew-t" so a line of constant temperature slopes upward to the right.  Note the two-stage evolution of the inversion event.  First, the upper levels warm dramatically while the morning surface temperatures remain relatively steady.  This occurs in the first 5 fram...

Source: http://www.keepcalm-o-matic.co.uk After a 2nd winter of hideous air quality, we are now approaching the end of the inversion season.   As shown in the graph below, days with PM2.5 concentrations > 17.5 (blue) and > 35 ug/m3 (red) are most common during the heart of the winter and drop off dramatically after the 7th week of the year (Feb 12-19).   Source: Dave Whiteman, University of Utah Thus we are almost out of the woods.  There are, however, some PM2.5 > 35 days all the way to the 8th week of the year (Feb 26-Mar 4) and an elevated frequency of PM2.5 > 17.5 through the 10th week of the year (March 5-12) (events after that are probably due to blowing dust).  For that reason, I won't stick a fork in it just yet.   Nevertheless, poor air quality events later in the cool season (e.g., late Feb and early March) typically require a good snowstorm followed by a long-lived ridge.  We do have a ridge forecast to build ...

We remain mired in the inversion, but there are some curious oddities with the PM2.5 concentrations that warrant analysis by people who know more about emissions and air chemistry than me. From a stability perspective, there's been little change over the past couple of days.  The inversion remains very strong, with a temperature increase of about 13ºC sitting on top of the valley cold pool. During this period, the 24-h average PM2.5 concentrations at the DAQ sites in Logan and North Provo  climbed fairly steadily.  Utah County is now approaching a putrid 100 ug/m3. Source: Utah DAQ In contrast, the 24-h average PM2.5 concentrations in Ogden and Salt Lake have declined somewhat.  Source: Utah DAQ Perhaps someone can chime in and explain these declines.  Yes, there are differences in the magnitude and timing of weekday and weekend emissions, but why would Logan and North Provo keep climbing steadily while Salt Lake and Ogden decline?  ...

The changes are subtle, but over the past three days, the inversion has gradually lowered and strengthened, reducing the depth of the valley cold pool through in which pollution is trapped. A quick look at the air quality data shows we have exceeded the National Ambient Air Quality Standard for 24-hour average PM2.5 in several counties including Utah, Salt Lake, Davis, Weber, Cache, and Tooele.  This is a regional scale event with a large fraction of Utah's population impacted. In Salt Lake, our 24-hour average PM2.5 presently sits at about 60 ug/m3.  Despite a drop in hourly PM2.5 overnight, there seems to be little reason not to expect the longer term upward trend to continue. Source: Utah DAQ Our first opportunity for a bit of stirring will be mid week.  The models still lack consensus, however, concerning how much of a mixout we may get.  Keep your fingers crossed.

Strong northerly flow is ushering in much warmer air aloft as a high amplitude ridge builds over the western United States. This is not good news for northern Utah. As Jim “Head Blogger” Steenburgh suggested a few days ago , this massive warm up aloft, which is already apparent in the morning sounding, will isolate a very cold pre-existing air-mass in the valleys of the intermountain west setting up for a persistent cold-air pool… or as Jim put it, “the mother of all inversions.” KSLC 12 UTC sounding. Note the strong northerly flow aloft above the inversion. The warming aloft, which often accompanies the initiation of a persistent cold-air pool, is the result of both subsidence and warm air advection. These two processes occur in concert today along an upper level front , which marks the departing edge of the cold trough that impacted Utah over the weekend. The slowly descending frontal zone will result in a continued increase of 700 hPa (or near ridge top) temperatures ...