Back To A Tough Forecast Situation

Yesterday was totally suppressed over much of southeast Arizona and definitely throughout the Tucson area, where there was no deep convection, much less any shower activity. This morning there is midlle-level instability present and some shallow buildups are floating around. See the 7:15 am MST visible satellite image above - heavy cloud to the east in New Mexico with the cold front - see previous post.

The morning Tucson sounding, from Univ. of Wyoming  upper-air page, is above. Things of note: current PW is quite low (around an inch) and 850 mb Td remains low at 6C; there is some hint of cooling and moistening at low levels; 500 mb T is quite cool at -7 or -8C; winds remain light and variable below 400 mb; finally, seriously dry and warm air from the Pacific dominates the upper-troposphere. The 500 mb anticyclone is nearly overhead. Low-level winds will have a downslope component during the day. The upper-air winds appear confluent through next 12 to 18 hours. More moist air will advect in from the east behind the backdoor front, and increased moisture will also try to come in on the diurnal circulations from the west to southwest. So, a serious mix of positives and negatives today. The current inversion above 500 mb appears too strong of a cap to allow much in the way of deep convection - except perhaps over some favored higher terrain.

The early run of the Atmo WRF-GFS model provides some interesting forecasts. Forecast sounding right at Tucson can never overcome the upper-level warm air. However, the model does get some convection going in southeast arizona - above is model-forecasted composite radar image for 4 pm MST this afternoon. Below is the model forecast of accumulated rainfall through midnight tonight - which conveys a considerably different picture. Very little rain forecast to reach the ground except over to the east in Cochise county. Considering that cloud bases will remain near 600 mb, little rain is reaching the ground in the model forecasts - NAM forecast this morning is quite simlar with an indication of light, high elevation showers. The WRF-GFS forecast does bring a strong outflow from the south across the metro area by late afternoon. So, it appears that best guess for now is that there'll be some deep convection battling through the warm air aloft, but here in Tucson area only virga, or sprinkles, and strong, dusty outflows. There are enough different features in the mix that I'll definitely take a look at the new WRF runs when they become available.