where Tp500 is the temperature of a parcel lifted dry adiabatically from 850 mb to its condensation level and moist adiabatically to 500 mb.

SI values +3 indicate possible showers or thunderstorms.
SI values -3 indicate possible severe convective activity.

The MODIFIED SHOWALTER INDEX (1958) is similar to the Showalter Index except it uses the mean mixing ratio of the layer from 850 mb to 500 mb for the initial value of the parcel. Values of this index +5 indicate potential thunderstorms; whereas numbers greater than 11 indicate little chance for convective activity.

K-INDEX K = (T850 - T500) + Td850 - (T700 - Td700)

The K index is a poor indicator of severe thunderstorms since dry air at 700 mb may indicate convective instability. Dry air at 700 mb will give a low value to the K-Index.

K values +20 indicate some potential for air mass thunderstorms.
K values +40 indicate almost 100% chance for air mass thunderstorms,
with values over +30 to indicate potential MCC's.

The MODIFIED K-INDEX is similar to that of the K-Index except that instead of the T and Td at 850 mb, the average temperature and dew point between 850 mb and the surface are used.

LIFTED INDEX LI = T500 - Tp500;

where the mean mixing ratio of the lowest 3000' and the potential temperature of the predicted afternoon high are used. The LI can be used as a diagnostic tool as well if the current surface temperature is used. The lower the value (i.e. the greater the negative number), the better the chance for thunderstorms and the greater the threat for severe weather. Note that the Lifted Index differs from the Showalter Index by the initial location of the lifted parcel.

TOTAL TOTALS INDEX TT = T850 + Td850 - 2T500

TT values +60 indicate probable moderate thunderstorms,
with a possibility of scattered severe t-storms.

The total totals index is actually a combination of the vertical totals, VT = T850 - T500, and the cross totals, CT = Td850 - T500, so that the sum of the two products is the total totals.

SWEAT = 12Td850 + 20(TT - 49) + 2f850 + f500 + 125(s + 0.2);

where the first term is set to zero if the 850 mb Td (°C) is negative; TT is the Total Totals Index (if TT < 49, the term is set to zero); f is the wind speed in knots; and s = sin (500 mb wind direction - 850 mb wind direction). The last term is set to zero if any of the following is not met:

1) the 850 mb wind is between 130°-250°;
2) the 500 mb wind is between 210°-310°;
3) (the 500 mb wind direction - the 850 mb wind direction) is greater than zero; or 4) both the wind speeds are greater than or equal to 15 kts.

SWEAT values +250 indicate a potential for strong convection.
SWEAT values +300 indicate the threshold for severe thunderstorms.
SWEAT values +400 indicate the threshold for tornadoes.

Keep in mind that these indices are empirical only, that is, they are not governed by any physical laws. They are used by meteorologists to give a quick estimate of the atmosphere's condition. Whether the indices are high or low, a good forecaster needs to answer the question "What is the condition that this index is measuring?" Remember, thunderstorms need lift moisture and instability. These indices are nothing more than estimated tools for identifying some of these ingredients.

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