An objective method is suggested which will enable forecasters to determine a direct probability-of-occurrence factor to be included with thunderstorm warnings. The method is based on radar observations and employs a simple grid overlay. The fractional area of the radar scope covered or traversed by thunderstorm echoes is used to determine the probability factor.
1 This is a contribution from the Dept. of Oceanog. and Meteor., Oceanog. and Meteor. Series No. 152.