Total Totals Index is actually a compound index designed to more accurately predict the occurrence of severe weather. It is used for both air mass and frontal thunderstorm activity and should be calculated whenever SSI or LI indicate that thunderstorms may occur. To calculate the Total Totals index, you must first algebraically compute two values: the vertical total (VT) and the cross total (CT). Each of these values is used along with the Total Totals to assess the probability of severe thunderstorm occurrence. VT is a measure of vertical stability without regard for moisture. It is found by subtracting the 500-millibar temperature from the 850-millibar temperature.

CT is a measure of stability that includes moisture. It is found by subtracting the 500-millibar temperature from the 850-millibar dew point temperature.

The Total Totals (TT) index is simply the sum of VT and CT.

The forecaster will evaluate thunderstorm potential according to the general guidelines presented in table 6-2-1. While it is advisable to consider the VT and CT values, in common practice the TT index is the more reliable single predictor of severe activity in both warm- and cold-air situations. During 1964 and 1965, 92 percent of all reported tornadoes occurred with a TT of 50 or greater, with most family-type tornadic outbreaks occurring with a TT of 55 or greater. However, TT must be used with careful attention to either the CT value or the actual low-level moisture, since it is possible to have a large TT value with insufficient low-level moisture to support thunderstorms.

There are numerous other severe weather indices in use. Many of these are used at the National Severe Storms Forecast Center, by forecasters who specialize in severe weather forecasting. The indices presented here are those you should routinely evaluate when analyzing a Skew T diagram. If your calculated indices lead to a forecast of thunderstorm activity, your forecaster may ask you to calculate some additional parameters for wind gusts, convective turbulence, hail occurrence, and hail size. The procedures used to find these guides are presented in the next section.

Learning Objective: Describe the computa-tion procedure for convective activity Diagram.

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