lightning. READ MORE: Met Office thunderstorm warning: Tropical 97F heatwave to sweep in. For example, if the rising parcel has an temperature of -5°C when it reaches 500 millibars, but the actual temperature at 500 millibars is -11°C, then the lifted index is -6 Kelvin (or K) indicating the potential for strong thunderstorms. only approximate and should not be used for safety applications. If you are under a storm surge watch, check for evacuation orders from your local officials. 1.7.0.2612 . to our detector, generating very strong signals, and there is a The current heatwave conditions in southern and eastern England are expected to last until at least Thursday. 500 millibars level (at about 5,500m or 18,000 feet asl), subtracted from the actual temperature of the environmental air at 500 millibars.