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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
SZ
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Alex Lamers
WiscWx Lead Forecaster -