Severe Thunderstorm and Tornadic Activity


Since 1988, NASP has observed a definite pattern to the severe thunderstorms that produce tornadoes. The general scenario begins with an area or line of thunderstorms oriented northeast to southwest, moving eastward with cell movement south to north. This scenario will produce tornadic activity from large cells moving inland from the Gulf. 

The increase in friction overland may enable the thunderstorm to produce the turning vortex that is necessary for tornadoes. In the northern section of Perdido Bay, this same scenario will produce waterspouts that will move inland. Saufley Field, located along the northeast shoreline of Perdido Bay, should be watched closely during this type of thunderstorm activity. 

On 10 September 1996, a funnel cloud formed over the runways at Sherman Field from a cumulonimbus cloud with a 3,000' base. Although the funnel cloud never reached the ground, the rotating system remained over the field for more than 15 minutes, but caused no damage. 

The following indices should be used to evaluate thunderstorm development, severity and potential tornadic activity: 

Convective Available Potential Energy (CAPE) 
< 0 Stable 
0 1000 Marginally unstable 
1000 2500 Moderately unstable 
2500 3500 Very unstable 
3500 4000 Extremely unstable 

Storm Relative Environmental Helicity (SREH) 
150 The approximate threshold for supercell development 
150 - 299 Weak Tornadoes (F0 and F1) possible 
300 - 449 Strong tornadoes (F2 and F3) possible 
>450 Violent tornadoes (F4 and F5) possible 

Lifted Index (LI) 
3 or less unstable to very unstable, strong thunderstorm potential 

Showalter Index (SI) 
3- 1 Thunderstorm possible, strong trigger needed 
0 (-3) Unstable, Thunderstorms probable 
(-4) (-6) Very Unstable, heavy thunderstorm potential 
< (-6) Extremely unstable, heavy to strong thunderstorm potential 

K Index (KI) 
20 - 25 Isolated thunderstorms 
26 - 30 Widely scattered thunderstorms 
31 - 35 Scattered thunderstorms 
35 and > Numerous thunderstorms 

Total Totals (TT) 
48 Scattered thunderstorms, isolated severe thunderstorms 
50 Scattered thunderstorms, few severe thunderstorms, isolated tornadoes 
52 Scattered to numerous thunderstorms, few to scattered severe thunderstorms, few tornadoes 
56 Numerous thunderstorms, scattered severe thunderstorms, scattered tornadoes 

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