[Updated July 20 for a more frequently updated map from the Climate Prediction Center]
People have asked me several times this week, “how much rain do we need to end the drought?”
There is no easy way to answer this. The normal rainfall per week in Illinois is about an inch. So we need that inch per week just to keep from slipping farther behind. Taking it a step farther, that means you need well over an inch per week to start recovering from drought. Of course, no amount of rain at this point will undo the damage done to crops already.
There is one product, based on the Palmer Hydrological Drought Index, that attempts to answer this question from the Climate Prediction Center. However, I would treat it as an estimate. Even so, it gives you an idea of how far we have to go for a recovery. They estimate that we would have needed 9 to 15 inches of rain across much of Illinois to end the drought. That is a tall order. The wettest July on record for Illinois is 8.03 inches in 1958 and the wettest August on record was 6.91 inches in 1977.
Personally, I’m not sure it would take record-breaking rainfall. And I’m not sure we want 9 to 15 inches over the course of one or two months because that could lead to all kinds of other problems like flooding and heavy soil erosion.
Based on past droughts in Illinois, a month with rainfall 50 percent above normal (around 6 inches) followed by several months with near-normal rainfall would be capable of turning things around without the more serious consequences of heavy rainfall.