Mesoscale analysis: Tornado swarm in New Orleans area

The Storm Prediction Center's 12Z outlook last Tuesday morning was not unlike a handful we've seen this year -- a "slight" risk over a wide area. Severe thunderstorms were expected, but New Orleans was barely within the Slight Risk area. The SPC believed there was a 5 percent tornado threat but twisters were most likely in Alabama and Mississippi and New Orleans was on the border of the lower threat risk (2 percent). Our forecast was similar. The 13Z outlook added an "enhanced area" of risk from New Orleans northeastward into southern Mississippi.
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By 10 a.m. Central time, a storm near Killian, Louisiana presented very severe on radar. After two hours had passed, more supercell thunderstorms had spawned half a dozen tornadoes which crawled across the Cajun countryside including three major EF-3 twisters, one that struck New Orleans proper. All told, there were 23 tornado reports that day, and at least five were outside of the Slight Risk area. It was a rare forecast miss for the SPC.
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Fifty-three Tornado Warnings were issued that Tuesday across the South, with 35 of those coming out of the New Orleans NWS office. Almost all the warnings were between Baton Rouge and New Orleans. The NWS has a detailed report on each of the tornadoes:

• Killian, LA to Madisonville, LA EF-3 Tornado

• Elmwood to Old Jefferson, LA EF-0 Tornado

• New Orleans East, LA EF-3 Tornado

• Donaldsonville, LA EF-1 Tornado

• Watson, LA EF-3 Tornado

• Near Montpelier, LA EF-1 Tornado