Last Evening's Flagstaff Sounding - Worth A Look

The Flagstaff sounding was quite interesting last evening, exhibiting a very cold (-38C) temperature at 500 mb and with the tropopause also at 500 mb. It is my feeling that this an extremely unusual sounding for Flagstaff, although I don't have a historical data base at hand to check this in detail. The nearly adiabatic conditions from the surface to 500 mb appear a bit strange, but they are similar to other very cold soundings in the West. Thus, it doesn't appear to me that this sounding is obviously bad - but it certainly seems unusual. The figures above are from the Wyoming upper-air site. Flagstaff skew-T (top), Flagstaff Stueve diagram (middle), and 500 mb chart (bottom). Note that the very cold, -35 to -38C, air at 500 mb has wrapped around the western side of the upper low in a banana-shaped arc, that extends from Spokane to Flagstaff. Can anyone tell me how often such cold 500 mb temps are observed at Flagstaff (Winslow)?