Add Showalter and Modified Showalter Index

[jrleeman]

Index used to assess the possibility of thunderstorms or severe convective activity.

From COD (http://weather.cod.edu/sirvatka/si.html)

Showalter Index

SI = T500 - Tp500;
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.

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.