Stability Page 2
It is important to analyze the current state of the atmosphere before you analyze what is to be expected. Often times the anticipation lets us forget that what is going on now is just as important as what is expected to happen later. Let's take a possible scenario. You are analyzing a Skew-T diagram over Wichita, Kansas the morning that you plan to chase. You notice that the skies have not cleared out and you know from experience that you will need a little more moisture in order to get the deep convective cells that you seek. After reading the morning sounding, you view the forecast sounding for twelve hours later and there is a big difference in the variables. If things don't begin to change soon, you know that the forecast sounding will not verify (be proven correct). So if you still want to have a good chase day, it would be wise of you to monitor the data hourly so you can make any adjustments that may be needed.
Here are some of the variables that should be noted:
Lifted Index (LI)
The Lifted Index shows the stability of an air parcel. It is computed by lifting the parcel of air from
the surface to the 500mb level then comparing the temperature of the air parcel to that of the
environmental air temperature. Through the forces of nature explained above, the temperature of the
air parcel may be much higher than the surrounding air, causing it to be unstable in a sense that it
wants to be displaced vertically. As the air parcel becomes saturated, its physical state is
transformed from a gas to a liquid state, and in doing so (condensation) the energy absorbed is
called latent heat. Once this "latent" heat is added to the air parcel, it now becomes lighter and
warmer than its surrounding air. We look for negative numbers with this index. The more negative
the number, the more unstable the air is. Put another way, the more negative the number, the more
potential there is for a stronger thunderstorm. Values of zero or below are a good indicator of
general thunderstorms. Severe thunderstorms are possible when the values reach -4 or so. When
MESO tracked the tornadic supercell near Coldwater, Kansas on Memorial Day 1999, Lifted Indices
were in the -12 to -14 range, indicating a very unstable atmosphere.
Total Totals Index (TT)
The Total Totals Index combines the effect of the atmospheric lapse rate, and low level moisture. It
is computed by using the Cross Totals Index (the 850mb dewpoint minus the temperature at
500mb), and the Vertical Totals Index (the 850mb temperature minus the 500mb temperature). An
index of 50 is a good starting point for thunderstorms. Scattered thunderstorms may be prevalent
with values between 52-55, and anything above 56 will yield scattered severe thunderstorms with
the possibility of scattered tornadoes.
There is a false sense of security when using this index. It is not meant to be used for the sole purpose of severe thunderstorm forecasting. High lapse rates and cold mid level temperatures will yield a high Total Totals number, but it does not take into consideration the low-level moisture that is needed for deep convection.