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I don't understand why a lot of meteorologists are stating that we need anywhere from 12 to 20 inches of rain to get out of this drought, when the rain fall deficit is about 6 inches. I don't understand the disconnect between what rain we need to get out of the drought and the actual rainfall deficit.

Posted October 30, 2007

MIKE MOSS SAYS:      Jim,       Those numbers are associated with a certain time span - for example, the 24 inch figure that keeps coming up applies to the minimum end of the rainfall required over the course of 5 to 6 months to end the drought for our climate division (and much of surrounding NC). The most recent available calculation for this quantity is from the end of September, and new updates are produced by the National Climatic Data Center at the end of each month.

When you consider that normal rainfall over that Oct-Feb 5-month period would be about 16-17 inches, what the "drought-ending" figure really says is that we need to run about 7-8 inches above normal or more across that time span to cancel out recent deficits, recharge groundwater, re-establish consistent streamflows and re-fill surface water impoundments.

Generally speaking, for longer time periods the drought conditions can be overcome with rainfall amounts that are closer and closer to (and a greater percentage of) normal, while to completely erase the drought over short time spans requires much higher than normal rainfall. For example, the September figures showed a need for 12-15 inches to end the drought in one month, compared to normal one-month rainfall of 3.2 inches, while to end it in 3 months would require 18 inches (see included map), just about double the normal 9.1 inch normal value for that period.


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