Dry Aloft But Not As Dry As TWC Sounding Indicates

The Tucson sounding this morning (Sunday, May 8th - top) indicates a bone-dry atmosphere with only 1.2 mm of precipitable water (PW). However, when RH is very low the RRS sonde's humidty sensor tends to be too dry. This is illustrated at bottom where GPS PW is shown (blue) vs TWC sounding PWs (green squares), which have been consistenly too dry for past few days. This morning the GPS data indicate that there's almost 3 mm more PW than indicated by the sounding - so it is VERY dry, but not quite as dry as the sounding data indicate. The real amount of PW present in southeast Arizona will be an issue of interest from now all the way through the summer thunderstorm season.