Showalter index is a measure of thunderstorm potential and severity. In other
words, it gives a good indication where the atmosphere is unstable and where
convective development may be expected. Showalter index between 1 and 3 gives
the indication that thunderstorms are possible but a strong trigger is still
needed. For the values between -2 and 1 chance of thunderstorm increases. Values
between -3 and -2 are a sign of very unstable atmosphere with a good potential
for thunderstorms. Values under -3 are associated to heavy thunderstorms.
In this chapter forecast fields of Showalter index (ALADIN/HR model) are compared with Meteosat 8 IR 10.8 µm satellite images. In some cases
the
discrepancy between the values of stability indices and the real situation (satellite image) can be noticed.
Since this might be confusing, it must be emphasized that Showalter index predicts
the conditions for instability over some area.
CAPE (Convective Available Potential Energy) - a measure of the amount of energy available for convection. CAPE is directly related
to the maximum potential vertical speed within an updraft; thus, higher values indicate greater potential for severe weather. Observed
values in thunderstorm environments often may exceed 1,000 Joules per kilogram (J/kg), and in
the extreme cases even 5,000 J/kg.
In this chapter forecast fields of CAPE (ALADIN/HR model) are compared with Meteosat 8 IR 10.8 µm satellite images.
It should be remembered that CAPE represents potential energy, and will only be used should a parcel be lifted to the level of free convection.