Good Grief!...Frost Points off the Charts in Colorado
Before I went to bed last evening (January 22), I was checking obs across the country when I startled my wife with a fairly loud "Good grief!" My outburst was prompted by the 00Z temperature and dew-point soundings at Denver's Stapleton Airport (see the 00Z skew-T below). No excuses... I simply lost it when I saw that dew points were literally "off the charts" from roughly 650 mb to 250 mb. As a side note, I confess that it's probably better to use frost points instead of dew points in this context, but old habits are hard to break.
The 00Z temperature (red) and dew-point (green) soundings at Denver's Stapleton Airport on January 23, 2013 (the late afternoon of January 22). note that dew points (frost points) were "off the charts" between roughly 650 mb and 250 mb. Courtesy of UCAR.
With my interest piqued, I started to look more closely at weather observations in the region and found these data at Broomfield, Colorado, which lies just to the north of Denver (map). Scroll down to the 2:47 P.M. observation on January 22. Good grief! A frost point of minus 42 degrees and a temperature of 63 degrees. That's a dew-point depression of 105 degrees! Yep, the relative humidity was a desiccating 1%. Note that the wind was sustained at 33 mph (gusts to 41 mph). No doubt that the westerly wind was downsloping into Broomfield. And the visibility was 70 miles (on a clear, pristine day in the Rockies, you can't see forever). For reference, I include the meteogram from 12Z on the 22nd to 12Z on the 23rd (below) so you can see the trace of relative humidity (focus your attention on the bottom plot of the topmost graph; you can read percentages off the vertical axis on your right).
The meteogram for Broomfield, Colorado, from 12Z on January 22, 2013, to 12Z on January 23, 2013. Courtesy of the University of Wyoming.
The weather pattern along the Front Range of the Rockies was dominated by a prominent 500-mb ridge to the west (check out the 00Z NAM 500-mb analysis on January 23).
The lack of moisture and the super-low relative humidity were not localized. Nor were they confined to the surface. To make my point, I took a cross section (vertical slice) through the atmosphere from Trinidad, Colorado (KTAD), to Cheyenne, Wyoming (KCYS...check out this map). The cross section I took displays the 00Z NAM model analysis of relative humidity (image below; larger image). To get your bearings, I point out that the hatched area along the bottom represents the lay of the local terrain. Also note that pressure, in millibars, lies along the cross section's vertical axis.
A cross section of relative humidity from Trinidad, Colorado (TAD), to Cheyenne, Wyoming (CYS), at 00Z on January 23, 2013. Larger image. Courtesy of Penn State.
Wow! The relative humidity along the Colorado Front Range was less than 20% all the way from 300 mb to the earth's surface (the unlabeled contours closest to the ground correspond to 10%, so that the relative humidity below these contours was less than 10% (consistent with conditions at Broomfield, for example). Moreover, this cross section was consistent with the 00Z skew-T at Denver's Stapleton Airport (shown earlier)...the wide separation between the temperature sounding (red) and dew-point sounding (lime green) indicates very low relative humidity.
To confirm the pattern of downsloping winds along the Front Range, check out the 00Z NAM model analysis of vertical motion at 700 mb (3000 meters), in microbars per second (image below; larger image). The resolution of this version of the NAM is 12 kilometers, which is small enough to capture the complexity of the vertical motion field over and east of the Rockies. At any rate, the positive values (solid green contours) indicates that there was subsidence over the Front Range and other portions of eastern Colorado.
The 00Z NAM model analysis of vertical motion at 700 mb, in microbars per second, on January 23, 2013. The resolution of the model is 12 kilometers. Note the downward motion (postive values indicated by solid contours) along and east of the Colorado Front Range. Larger image. Courtesy of Penn State.
As you already know, sinking air parcels warm by compression on descent, thereby lowering relative humidity.
After my outburst, it took a while for my wife to get to sleep. Sorry Kathie.