Atmospheric Stability Indicies

Standard Pressure Level and Approximate Heights

Lifted Index Stability Contour ( LI )

This is a contour plot of lifted index with a contour interval of 2 degrees Celsius. The LI field shows instability in the atmosphere by lifting a parcel of air from the surface to 500 mb and comparing its temperature to that of the environment. Where LIs are less than 0, thunderstorms are possible. The lower the number, the more unstable the atmosphere is and as a result, the stronger the thunderstorms could become. Values of -4 or lower indicate areas where severe thunderstorms are possible. Values > 10 indicate areas of stable weather where skies are generally clear.

Showalter Index ( SI )

Total Totals Index Stability Contour ( TT )

This is a contour plot of total totals index with a contour interval of 2. The TT field shows instability in the atmosphere based on the lapse rate from 850 to 500 mb plus dewpoint at 850 mb. Where TTs are greater than 45, thunderstorms are possible. The higher the number, the more unstable the atmosphere is and as a result, the stronger the thunderstorms could become. Values of 52 or higher indicate areas where severe thunderstorms are possible. Values < 40 indicate areas of stable weather where skies are generally clear.

K Index Stability Contour ( K I )

This is a contour plot of K index with a contour interval of 4. The KI field shows instability in the atmosphere as it relates to the development of air mass thunderstorms. It is based on the 850 to 500 mb lapse rate plus 850 mb dewpoint minus the 700 mb dewpoint depression. Strong Springtime thunderstorms often require dry air at mid levels to cap the convection. On the other hand, summer air mass thunderstorms need a very moist atmosphere at mid levels to prevent evaporation through entrainment. Where KIs are greater than 35, air mass thunderstorms are likely. the higher the number, the higher the probability. Values < 10 indicate areas of stable weather where skies are generally clear.

SWEAT (Severe Weather Threat ) Index ( SW )

CAPE Contour

This is a contour plot of CAPE or Convective Available Potential Energy. CAPE represents the amount of energy a parcel might have if it were lifted. Often this reflects the strength of updrafts within a thunderstorm. CAPE values of greater than 2000 represent enough energy to produce thunderstorms. A value greater than 3000 represents enough energy to produce strong thunderstorms

Light Blue

Lifted Index 6 or Greater, Very Stable Conditions

Green

Lifted Index Between 1 and 6 : Stable Conditions, Thunderstorms Not Likely

Yellow

Lifted Index Between 0 and -2 : Slightly Unstable, Thunderstorms Possible, With Lifting Mechanism (i.e., cold front, daytime heating)

Orange

Lifted Index Between -2 and -6 : Unstable, Thunderstorms Likely, Some Severe With Lifting Mechanism (i.e., cold front, daytime heating)

Red

Lifted Index Between -6 and -10: Highly Unstable, Severe Thunderstorms Likely With Lifting Mechanism (i.e., cold front, daytime heating)

Violet

Lifted Index Less Than -10: Extremely Unstable (i.e., cold front, daytime heating)

Helicity Contour

This is a contour plot of helicity which is the amount of storm relative rotation/shear in the atmosphere. Helicity is used to indicate where rotation/shear is high enough to allow thunderstorms to organize into severe or supercell storms. In the lack of helicity, storms develop vertically and the precipitation will snuff out the updraft killing the thunderstorm. Severe storms need helicity to maintain an organized structure allowing hte storm to develop to severe limits. A value of 400-500 is often needed to produce severe storms. Often this is used in conjunction with CAPE to determine severe storm location.