Sunday, September 02, 2007

A Big Storm Day in Southern Arizona and California

Yesterday afternoon , Saturday September 1st, turned out to be a big storm and severe storm day across southern Arizona and into southern California. There were several small tornadoes in California, and events in Arizona were mostly from hybrid downbursts, although there was one hail report. Most notable was a measured gust to 84 mph out at Yuma, Arizona. I have not yet chased down many photos, but will try to post some damage shots tomorrow.

Strong, drying easterly winds today have led to mostly suppressed conditions across southern Arizona, with significant storm activity focused in southern California.

Data from TWC yesterday provide a good illustration of how difficult it is to use RRS soundings with suspect data in making a forecast.

In my previous post I noted that the 12 UTC (1 Sept. 2007) sounding at TWC was suspect and that it was difficult to estimate what the CAPE might actually be in the afternoon. I guessed that a well-mixed afternoon BL might have a mixing ratio of ~9 g/kg. But I also mentioned that 1 g/kg either way would have substantial implications for the evolution of afternoon storms. The TWC sounding at 00 UTC (2 Sept. 2007) apparently entered a storm updraft - see Figure 1. The data are mostly bad or suspect, except for the temperature trace from the surface to about 650 mb. However, the updraft data from 500 to a bit above 400 mb have theta w values of 24 to 25C. This implies that the BL air at cloud base had a mixing ratio of 10.5 to 11.5 g/kg.

My morning estimate, using the sounding with the suspect Td trace, was clearly way too low. It is fairly obvious that moisture advection, given the winds below 650 mb for the morning and evening soundings, was not a major player during the day. Figure 2 shows the morning sounding with lines added showing q = 10 g/kg and theta w = 23.5C. The Sippican hygrister appears not to have responded well at all in the residual layer and indicated conditions that were too dry.

It is likely that the sounding problems yesterday contributed to forecasts that significantly under-estimated the extent of the severe storm outbreak. And so it goes in the new era of NWS RRS soundings - a serious crap shoot for forecasters that sometimes has public safety implications.

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