CONVECTIVE STABILITY INDICES

LIFTED INDEX:

LI = T500 - T500 (parcel)

a) Determine the mean T and the mean Td in the lowest 50mb of the sounding. These values are considered representative of the level 25mb above the surface.

b) From a level approximately 25mb above the surface. lift the estimated mean T along the dry adiabat until it intersects with the mixing ratio line which passes through the mean dew point. (This level is the LIFTING CONDENSATION LEVEL LCL). Now lift this parcel along a moist adiabat until reaching 500mb. Read off the T500 (parcel)

c) Subtract T500 (parcel) from the actual T500. This is the LI.


LI = 0 to -2 Thunderstorms are possible but good "trigger" needed

= -3 to -5 Unstable - Thunderstorms possible

= < -6 Very unstable - Heavy to severe thunderstorm potential


BEST LIFTED INDEX:

BLI = same as above but computed at 50mb intervals between the surface and 850mb. The most negative lifted index from all of the 50mb intervals is the BLI.

SHOWALTER INDEX:

Same as LI and BLI except you use the T and Td at 850 to determine the LCL then lift that parcel moist adiabatically to 500mb and compare with the actual T500.

SI = 3 to 1 Thunderstorms possible but need a strong trigger

= 0 to -3 Unstable - Thunderstorms possible

= -4 to -6 Very unstable - good heavy thunderstorm potential

= < -6 Extremely unstable - good severe thunderstorm potential



K - INDEX:

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

The K-Index is primarily applicable in the prediction of air mass thunderstorms. Term 1 measures the vertical lapse rate. Term 2 measures the low-level moisture availability. Term 3 measures the depth of the moist layer.

K value Airmass T-storm probability
< 15 near 0%
15 - 20 20%
21 - 25 20-40%
26 - 30 40-60%
31 - 35 60-80%
36 - 40 80-90%
> 40 near 100%

The K-value does not indicate likelihood of severe T-storms. Very low values of K in the presence of other "Strong T-storm" indicators may suggest sever t-storm potential.

TOTAL TOTALS INDEX:

TT = (T850 + Td850) - 2*(T500)

where (T850 - T500) = Vertical Totals (vertical lapse rate)
and (Td850 - T500) = Cross Totals (lifted low level moisture)

TT = 48 Isolated to few thunderstorms
= 52 Scattered T-storms; few moderate in strength
= 55 Scattered T-storms; few moderate; isolated severe
= 58 Scattered moderate T-storms; few severe; isolated tornadic activity
= 61 Numerous moderate T-storms; few to scattered severe; few tornadoes
= 64 Numerous moderate to severe T-storms; scattered tornadoes

Beware...in a cold deep trough (T500 < -28C), TT values will be high without corresponding low level convective support.


SWEAT INDEX:

SWI = (12*Td850) + 20*(TT-49) + 2*(FF850) + (FF500) + 125 (S + 0.2)

Term 1 is the low-level moisture. Set this term =0 id Td850 < 0. Term 2 is the convective potential as measured by the computed Total Totals Index. Terms 3 and 4 are the low and mid-level winds measured in knots. Term 5 is a measure of wind shear and is determined as follows:

S= sin (DD500-DD850) where DD is the wind direction in degrees.

Set S = 0 if all of the conditions below are met.
a) the DD850 is between 130-250 (clockwise)
b) the DD500 is between 210-310 (clockwise)
c) (DD500 - DD850) < 0
d) FF500 or FF850 < 15 knots

SWI = 300-400 Chance of severe T-storms
= 400 - 500 Severe T-storms likely; Chance of tornadoes
= 500 - 600 Severe Thunderstorms and tornadoes likely
= 600 - 800 Tornadoes nearly always occur
= > 800 No severe weather - mid level winds too strong (too much vertical shear)

The above statistics were derived form severe weather cases. SWI only gives severe weather potential in the presence of trigger mechanisms. Should not be used to predict airmass type T-storms.

DYNAMIC INDEX:

DI = same as the lifted index, but compute as follows...

a) determine the mean mixing ratio for the lowest 100mb of the sounding. (sometimes the surface, or lowest 50mb mean mixing ratio is used)

b) Then follow the mean mixing ratio line until it intersects the actual temperature curve. (This is the CONVECTIVE CONDENSATION LEVEL CCL) From the CCL follow the moist adiabat to 500mb. This is T500 (parcel)

c) subtract T500(parcel) from the actual T500.

If DI > 0 , the atmosphere is considered stable and no T-storms are likely. If DI < 0, conditional instability is present. The condition necessary to produce T-storms is that Tsfc must rise high enough so that the Tsfc parcel lifted dry adiabatically from the surface can reach the CCL before intersecting the sounding T-curve.