K Index and Lifted Index for Soaring Forecasts

K index (Unisys) - An index of moisture and instability - see table below. For Soaring K values in the mid 20's to low 30's are optimal. -5 is the lower threshold, Overdevelopment at +28. After a flight try to correlate performance to the K value.
Unisys' K Index Stability Contour 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 less than 10 indicate areas of stable weather where skies are generally clear. (Purdue University) Basically double the KI value to calculate the chance of thunderstorms.
Some of the feedback I have gotten is that the K Index is a more reliable predictor of thunderstorms (which it was invented for) than lift, so your mileage may vary. So this is just one of the many indicators.

General Soaring Lift Performance
"K" value Convective Activity Western US Eastern US
<-10 None < 300 fpm ?
-10 to +5 Blue Thermals* 300-600 fpm ?
+5 to +10 Increasing Convection 500-700 fpm ?
+10 to +15 Isolated strong vertical extent 600-800 fpm ?
+15 to +20 20% coverage thunderstorms 700-900 fpm 17.5=300-500fpm
in July LaGrange
+20 to +25 20% to 40% coverage thunderstorms 800-1000 fpm ?
+25 to +30 40% to 60% coverage thunderstorms 900-1100 fpm ?
+30 to +35 60% to 80% coverage thunderstorms 1000-1200 fpm 31.8=500-700fpm w/
hi pressure system in June LaGrange
33.1=400-600fpm, July LaGrange
+35 and higher Greater than 80% coverage thunderstorms 1100-1300 fpm

*Blue thermal conditions can deliver strong lift rates at times, especially with unusually clear skies intensifying heating for thermals. . . . Above K value data from "Soar Sierra" edited by John Joss Copyright John Joss 1976, the Soaring Press. Chapter 1 "Mountain Meteorology, know before you go!" by Doug Armstrong & Chris Hill. An excellent book available from the SSA.

* ETA Relative Humidity/Lifted Index
The Relative Humidity/Lifted Index (4 panel Forecast) chart depicts two fields:
1 Lifted Index (in white line contours).
The LI field shows instability in the atmosphere. where LIs are <0 (also highlighted in a gray shading), thunderstorms are possible. the lower the number, the more unstable the atmosphere is and as a result, the stronger the thunderstorms (and Thermals) 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.

2 Integrated Relative Humidity (in color contours) from 850 to 500 mb
The RH field is a good predictor of cloud location and thickness. Areas of RH <60% generally are clear or have partly cloud skies. areas of 60-80% are generally overcast or mostly cloudy. areas greater than 80% are overcast with a high likelihood of precipitation as rh approaches 100%. This can show you if the thermal is likely to be capped by a cloud.
Lifted Index
Current Lifted Index (Unisys) Contour plot 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 (lift) could become. Values > 10 indicate areas of stable weather where skies are generally clear.
     LI > 2    No significant activity
 0 < LI < 2    Showers probable, isolated thunderstorms possible
-2 < LI < 0    Thunderstorms probable
-4 < LI < -2   Severe thunderstorms possible
     LI < -4   Severe thunderstorms probable, tornadoes possible

Lifted Index - is calculated by lifting (frontal, orographic, upper air dynamics, etc.) a parcel of air dry adiabatically while conserving moisture until it reaches saturation. At that point the parcel is lifted moist adiabatically up to 500 mb. The Lifted Index is the ambient air temperature minus the lifted parcel temperature at 500 mb. If the parcel is warmer than the environment (negative L.I.), it has positive buoyancy, and will tend to continue to rise, favoring convection. L.I. values less than -5 C indicate very unstable conditions. A positive L.I. value indicates negative parcel buoyancy, and the parcel will tend to sink. This is representative of stable conditions where convection is unlikely. Increasingly negative numbers correspond to increasing instability and likelihood of severe weather. At times, very high (stable) lifted index values in cold air are indicative of frozen precipitation verses rain during warm advection events. The extreme stability results in cold air "damming", which restricts the advance of warm air at the surface. .. Forecast Product Development Team (FPDT) NOAA - 1997


K Index and Lifted Index Information Courtesy of: Southern Eagles Soaring (adapted).