Approaching squall

openWRF forecasts include two variables that indicate atmospheric instability and hence the probability of thunderstorms / squalls. These indicators are:

  1. CAPE (Convective Available Potential Energy) which is measured in Jouls per Kg. This indicate the amount of energy available for convection (the upward movement of air which is a major process in the creation of weather). A larger value of CAPE indicates the possibility of faster and more violent convection updrafts.
  2. LI (Lifted Index) is a measure of the atmosphere's stability. It helps in estimating the potential of the atmosphere to produce thunderstorms. The values of Lifted Index are the difference between the temperature of the environmental air at 500mb altitude (~5,450m) and the temperature of a rising parcel of air when it reaches the same altitude. If the rising parcel is warmer (shown by a negative differencial value) then it will continue to rise and possibly fuel a thunderstorm. Lifted Index is expressed in degrees Kelvin. Positive values indicate atmospheric stability and negative values indicate instability.

Where do I find the thunderstorm indicators?

Go to "Charts" in the main menu then select the model covering your area of interest. Last select "Thunderstorm Indicators"

What do the numbers tell me?

Consider both LI and CAPE indicators. LI is considered a good indicator except during winter (cold) storms. Both indicators show the level of instability and thus the potential for thunderstorms. Instablility on it's own is not enough to ensure thunderstorms. There needs to be a lifting factor due to orography or frontal activity. Check for other hints such as high humidity and a forecast for showers in the area.

Critical values of LI (should not be used in winter)

> 0 : stable atmosphere - thunderstorms unlikely

0 to -2 : weak potential, but with lifting trigger (front, topography) thunderstorms possible

-3 to -5 : moderate potential - thunderstorms probable

< -5 : strong potential - severe thunderstorms probable

 

Critical Values of CAPE

< 300 : little or no convective potential

300 to 1000 : weak convective potential

1000 to 2500 : moderate convective potential

> 2500 : strong convective potential

 

Sample Chart

This example shows extreme instability in the Western Mediterranean near Ibiza. The shaded colours represent CAPE and the contour lines represent Lifted Index (LI). Dotted contour lines show negative values of LI.

 cape li sample

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