BergenSkywarn Convective Indicators Information Page

BergenSkywarn Key Convective Activity Indicators
Totals Total Index	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	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.
CAPE	CAPE stands for the 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. Values < 1000 denote a relatively stable atmosphere.
Lifted Index	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.
Helicity	Helicity 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.
Dewpoint Temp	Dewpoints above 65 reflect humid conditions. Summer dewpoints can reach the low 80s. Dewpoints above 50 mean that there is enough moisture in the atmosphere to generate thunderstorms. The higher the dewpoint, the easier storms form and the more potent they can become. An additional factor of low level wind convergence is also needed to help storms form. Intense storms can form at dewpoints of 55 if the right conditions are in place including high convergence values.
Precipitable Water	This is precipitable water in inches. Precipitable water reflects the amount of water contained in a vertical column above the surface if it were all precipitated out. This is a good indicator of how much rain or snow might fall as a result of a thunderstorm or low pressure system.
Skew T Plot NWS Upton NY (KOKX)	A Skew T plot is a standard plot used by meteorologists to analyze data from a balloon sounding. This is a plot of temperature with height as denoted by pressure. The pressure lines are plotted horizontally in blue and are also on an inverse log scale. The concept of Skew T means that the temperature is not plotted vertically but angles off to the right at a 45 degree angle. The temperature lines of the Skew T are in blue. The green lines are called dry adiabats. The light blue dashed lines are saturation adiabats. The yellow dashed lines are lines of constant mixing ratio. The sounding is plotted as two white lines. The right line is the temperature profile. The left line is the dewpoint profile. The winds are plotted as wind barbs with height (see below) on the right edge of the plot.