Severe Thunderstorms

Severe Thunderstorms

Pre-storm conditions

Pre-Storm Conditions

Lifted Index (LI) – The lifted index provides an estimate of the thermal instability in the atmosphere due to the difference between the 500mb temp and the temperature an air parcel would acquire once it reached that height.

Less than 0 = slight instability

Between –3 & -6 = moderately unstable

Between –6 & -9 = very unstable

Less than –9 = extremely unstable

*Lifted Indices even lower are more likely to exist under a capped environment.

Other indices

Showalter Index (SI) – It is very similar to the lifted index except it measures the difference between the 850mb temp and the temp a parcel would acquire being lifted to 500mb. It is a better representation during cooler weather.

Between 3 & 1 = thunderstorms possible

Between 0 & -3 = unstable–thunderstorms possible

Between –4 & -6 = very unstable-very strong storms

Less than –6 = extremely unstable-intense storms

Total Totals Index (TT) – This index is an index of atmospheric instability composed of two indices: the cross total and the vertical total. The cross total is a measure of how buoyant the air parcel is due to less dense, moist air in the lower levels. It is the difference between the 850mb dew point and the 500mb temperature. The vertical total is a measure of how buoyant an air parcel is due to warm air at lower levels. It is defined as the difference between the 850mb temperature and the 500mb temperature.

Above 44 = isolated or few thunderstorms

Above 46 = scattered thunderstorms

Above 48 = scattered thunderstorms, isolated severe

Above 50 = scattered thunderstorms, few severe, isolated tornadoes

Above 52 = scattered to numerous storms, few to scattered severe, isolated tornadoes

Above 56 = numerous thunderstorms, scattered severe, scattered tornadoes

K Index (KI) – The K Index was composed for forecasting air mass thunderstorms.

Less than 15 = 0% probability of thunderstorms

15 – 20 = 20% probability of storms

21 – 25 = 20 – 40% probability of storms

26 – 30 = 40 – 60% probability of storms

31 – 35 = 60 – 80% probability of storms

36 – 40 = 80 – 90% probability of storms

More than 40 = near 100% chance of storms

Severe weather threat index (SWEAT)

Less than 272 = storms unlikely

273 – 299 = general storms

300 – 400 = moderate risk, almost severe

401 – 600 = strong risk, few severe storms, isolated tornadoes

601 – 800 = High risk of severe storms, scattered tornadoes

801+ = High wind damage, not favorable for severe weather.

Moisture – Moisture is a key component in thunderstorm development as it is needed for convection.

Surface dew point more than 60 degrees
850mb dew point more than 12*C
Precipable Water 1.5" or more

Convective Available Potential Energy (CAPE) – Also known as buoyant energy, this is the area enclosed by the environmental temperature profile and the moist adiabat connecting the level of free convection (LFC) to the equilibrium level (EL). Cape is measured in joules per kilogram (j/kg). The greater the CAPE level, the greater the difference will be between the environmental lapse rate and the temperature of the parcel. If the difference (CAPE) is great the acceleration upward will be strong. Therefore, the greater the CAPE, the better chance for severe weather.

0 = Stable

0 – 1000 = Marginally Unstable

1000 – 2500 = Moderately Unstable

2500 – 3500 = Very Unstable

3501+ = Extremely Unstable (capped?)

Convective Inhibition (CINS) - A measure of the amount of energy needed in order to initiate convection or how unlikely thunderstorm development may be. Values of CIN typically reflect the strength of the cap.

Less than 15 the entire day = fair wx cumulus

15-50 = Strong Storms if CINS weakens

50-199 = Strong Lines of storms if CINS weakens

200+ = strong cap, no storms

Capping Inversion or cap – A layer of relatively warm air aloft (usually several thousand feet above the ground) which suppresses or delays the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further. As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability. However if the cap is removed or weakened, then explosive thunderstorm development can occur.

700mb temp + 10*C = cap

2*C or higher = strong cap

Bulk Richardson Number (BRN) – A good overall indicator of convective storm types.

<10 = Strong vertical wind shear coupled with weak buoyancy indicate there may not be many sustained updrafts. Rotating supercells could evolve (?)

(I1, M1)

10 – 45 = Associated with supercell development

(M3, P3, H3)

>50 = Weak vertical wind shear coupled with strong buoyancy indicates that pulse storms or multi-

cell storms may evolve.

(N3, K3, B2, C2, D2)

Helicity - A property of a moving fluid which represents the potential for helical flow (i.e. flow which follows the pattern of a corkscrew) to evolve. Helicity is proportional to the strength of the flow, the amount of vertical wind shear, and the amount of turning in the flow (I.e. vorticity). Atmospheric helicity is computed from the vertical wind profile in the lower part of the atmosphere (usually from the surface up to 3 km), and is measured relative to storm motion. Higher values of helicity (generally, around 150 m2/s2 or more) favor the development of mid-level rotation (i.e. mesocyclones). Extreme values can exceed 600 m2/s2.

150 = Lowest # possible for supercells

150-299 = F0-F1 tornadoes

300-449 = F2-F3 tornadoes

450+ = F4-F5 tornadoes

Energy Helicity Index (EHI) – This index is a good indicator of rotation and/or type of tornado formed.

<1 = supercells and tornadoes unlikely

1-2 = supercells and weak tornadoes possible

2-2.4 = supercells more likely and mesocyclone induced tornadoes possible.

2.5-2.9 = mesocyclone induced supercell tornadoes likely.

3-3.9 = F2/F3 tornadoes possible

4+ = F4-F5 tornadoes possible

Wet Bulb Zero (WBZ) – This is a good indicator of hail potential.

5kft – 12kft = hail possible

7kft – 10kft = large hail possible

Helicity and CAPE – Below are the ranges that when both fall into the range, supercells are probable (maybe not widespread, but maybe)

Helicity = 250-400

CAPE = 1500-3000