Wednesday, September 05, 2012

Postmortem For 4 September, 2012

All of the forecasts for yesterday afternoon and evening busted seriously across Pima and much of Santa Cruz and Cochise Counties. A postmortem is needed, and this is my brief attempt at same.

First, the sunrise area of storms did produce substantial rains for several hours - over far south and southwest portions of the ALERT network12 stations had rainfall and 3 of these had over an inch. However, those rains were over by 11 am MST or so. The rest of the day the three southeastern counties were completely suppressed, even though all the forecasts called for substantial afternoon and evening storms - see previous post.

The afternoon sounding taken for Tucson (above from U of Wyoming upper-air page) has no CAPE and also almost no winds aloft. No CAPE = no storms, but it's not as simple as that. The GPS vs RAOB time series for U of Arizona (below from ESRL) indicates that the actual atmosphere at 00 UTC probably had 2/10" more PW than indicated by the RRS sonde data. Thus, there probably was some CAPE present. However, the sounding plot also indicates that there was a deep layer with Convective INhibition (CIN, from about 750 mb to 600 mb). Without substantial outflows, the surface layer had no chance of realizing its level-of-free-convection (LFC) - note also that the high temperature at TUS reached only to 91F yesterday, because of the morning cloud cover.

Above is the forecast (from WRF-GFS 12 UTC run) for Tucson area surface conditions at 3 pm. These temperatures were 5 to 8F too warm wrt observations. Below is the forecast for the Tucson sounding valid at 5 pm MST. It captures the shape of the actual sounding, but didn't forecast the substantial CIN layer (there is some CIN in the forecast sounding). The forecast was also too warm and possibly too moist, so much higher CAPE indicated than was actually present. The model forecast developed widespread storms due to a strong outflow predicted to move into the metro area around dusk. There were no storms along the Borderlands to generate that outflow. So, the net result was errant forecasts from all sources. Smaller scale details at play once again, impacting the 6 to 12 hour forecast negatively. Certainly glad I don't have to forecast the grid-point details out through seven days as some do!

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