On April 27, 2011 I was monitoring the severe threat. I was looking at the SPC Mesoanalysis and noticed the extremely high helicity values. Typically with a tornado outbreak, you won't get a number higher than 300-500 or so. With this situation the values were near 1000. Before you speculate and assume that is acceptable, consider what they had to say about the Tornado Watch issued near the landfall of Hurricane Katrina.
The following is a direct quote from the Storm Prediction Center from a convective outlook that was issued on August 29, 2005.
HURRICANE KATRINA WAS MOVING ASHORE OVER FAR SERN LA AT 12Z AND IS
FORECAST TO TRACK GENERALLY NWD ACROSS SRN/ERN MS WITH CENTER OF
SYSTEM LOCATED NEAR THE TN-MS-AL TRI STATE BORDER REGION BY THE END
OF THE PERIOD. /REFERENCE LATEST FORECASTS AND BULLETINS FROM NHC
FOR FURTHER INFO ON KATRINA'S TRACK AND SPECIFIC HURRICANE
INFORMATION./ WIND FIELD ASSOCIATED WITH THIS SYSTEM REMAINS QUITE
EXPANSIVE WITH 90 KT WINDS AT 1 KM OBSERVED AT MOB YIELDING 0-1 KM
SRH IN EXCESS OF 1000 M2/S2. 0-1 KM SRH IN EXCESS OF 250 M2/S2 WAS
OBSERVED AS FAR EAST AS TLH THIS MORNING AS WELL. IN
ADDITION...LATEST RADAR IMAGES INDICATE DISCRETE DEVELOPMENT
CONTINUING TO REGENERATE NORTH-NORTHEAST OF PRIMARY RAIN BANDS FROM
THE WRN FL PANHANDLE INTO SRN AL. SHOULD THESE CONVECTIVE TRENDS
CONTINUE AS SYSTEM LIFTS NWD THROUGH THE PERIOD...THERE WILL BE AN
INCREASED THREAT OF TORNADOES ONCE HEATING COMMENCES GIVEN WEAKENING
CINH AND LOW LCLS. THE THREAT OF ISOLATED TORNADOES WILL LIKELY
ACCOMPANY ANY THUNDERSTORM DEVELOPING WITHIN THIS BROAD REGION OF
EXTREME LOW LEVEL SHEAR.
