Forecast Tools: Severe Indices

BRN Shear

Bulk Richardson's Number. A BRN below 10: Strong vertical wind shear and weak CAPE. The shear may be too strong given the weak buoyancy to develop sustained convective updrafts. However, given sufficient forcing, thunderstorms may still develop; if so, rotating supercells could evolve given the high shear. BRN = 10 to 45: Associated with supercell development. BRN over 50: Relatively weak vertical wind shear and high CAPE which suggests multicellular thunderstorm development is most likely..

MUCAPE

MUCAPE represents potential energy in the atmosphere. The greater the value reached, the greater the likelihood of the formation of a storm. Values less than 300 are low, between 300 to 1000 are weak, 1000 to 2000 are moderate, and over 2000 are high, when the possibility of the occurrence of strong storms is highly likely.

Helicity

SRH (Storm Relative Helicity) SRH=150:The approximate threshold for supercell development. SRH= 150 to 299: Weak tornadoes (F0 and F1) possible. SRH= 300 to 449:Strong tornadoes (F2 and F3) possible. SRH over 450: Violent tornadoes (F4 and F5) possible.

Lifted Index

The lifted index (LI) is the temperature difference between an air parcel when it reaches the 500 hPa level and the actual temperature of the environmental air at 500 hPa. When the value is positive, the atmosphere is stable and when the value is negative, the atmosphere is unstable. Values below 0 indicating instability and an increasing chance of thunderstorms. Lifted Index is generally scaled as follows: 6 or greater: very stable conditions (without thunderstorms), between 1 and 6: stable conditions, (thunderstorms not likely), between 0 and -2: slightly unstable (thunderstorms possible), between -2 and -6 : unstable (thunderstorms likely), less than -6: very unstable (severe thunderstorms likely).

Precip Water

Supercell

K Index

The K index is a measure of thunderstorm potential based on the vertical temperature lapse rate, and the amount and vertical extent of low-level moisture in the atmosphere. K's below 30:Thunderstorms with heavy rain or severe weather possible; K over 30:Better potential for thunderstorms with heavy rain; K = 40: Best potential for thunderstorms with very heavy rain.

Total Totals Index

40-45 - Thunderstorms are possible; 45-50 - Severe thunderstorms possible; 55 or greater - Severe thunderstorms likely and possible tornado

Tornado Helicity Index

Values greater than 2 or 3 have been correlated to cyclonic supercells with increased tornadic potential. Negative values are indicative of an environment favorable for anti-cyclonic (left moving) supercells, but tornadoes associated with anti-cyclonic supercells are extremely rare.