STABILITY INDEXES AS AN INDICATION OF INSTABILITY

The overall stability or instability of a rawinsonde sounding is sometimes conveniently expressed in the form of a single numerical value called the stability index. Such indexes have been introduced mainly as aids in connection with particular forecasting techniques. Most of the indexes take the form of a difference in temperature, dewpoint, wet-bulb temperature, or potential temperature in height or pressure between two arbitrarily chosen surfaces. These indexes are generally useful only when combined, either objectively or subjectively, with other data and synoptic considerations. Used alone, they are less valuable than when plotted on stability index charts and analyzed for large areas. In this respect they have the value of alerting the forecaster to those soundings, routes, or areas that should be more closely examined by other procedures.

Figure 5-6.-Example of anticyclonic shear and curvature at 850 hPa preventing thunderstorms, with otherwise favorable air mass conditions present in the Little Rock area.

There are a number of methods that maybe used for determining stability or instability. Among these methods are the Showalter Stability Index (SSI), the Lifted Index (LI), the Fawbush-Miller Stability Index (FMI), and the Martin Index (MI). Only the SSI method will be discussed in this TRAMAN. The National Weather Service currently uses the (LI) method for producing their facsimile products. All current methods are discussed in detail in NAVAIR 50- IP-5, Use of the Skew T, Log P Diagram in Analysis and Forecasting.

For more information on the SSI, the forecasting of peak wind gusts, and the forecasting of hail, refer to the AG2 TRAMAN, volume 2, unit 6. These subjects are covered in an abbreviated form in this chapter. The SSI is the most widely used of the various types of indexes.

Figure 5-7 shows the computation of the SSI. For forecasting purposes, the significance of the index values to forecasting is as follows: 

. When the index is +3C or less, showers are probable, and some thunderstorms may be expected in the area.

. The chance of thunderstorms increases rapidly for index values in the range of +1C to -2C.

. Index values of -3C or less are associated with severe thunderstorms.

. When the value of the index is below 6C, the forecaster should consider the possibility of tornado occurrence. However, the forecasting value of all index categories must, in each case, be evaluated in the light of the moisture content of the air and of other synoptic conditions.

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