Isaac Will Lessen Drought, but Only in Some Places

By Michael D. Lemonick

The U.S. Drought Monitor released on Thursday was different from most such reports, in that it was essentially out of date before it was published. As always, the monitor was based on dryness readings as of 7 A.M. the previous Tuesday, and it’s unusual for conditions to change drastically in such a short time. In fact, the newest drought map showed little change in the percentage of the lower 48 states feeling some level of drought: it was 52.63 percent, compared with 52.89 percent in the report released on August 23.

1-5 Day map of expected rainfall (inches) from Tropical Storm Isaac. Valid August 30-September 4.
Credit: NOAA

The map showed 35.42 percent of the nation in severe drought or worse, compared with 36.83 percent a week earlier; 19.38 percent in extreme drought or worse, compared with 19.24 percent the week before; and 5.05 percent in exceptional drought, compared with 5.27 percent the preceding week.

This week, however, Hurricane Isaac made for just such a drastic change in a much shorter period. Isaac has already dumped torrential rains on Louisiana, Mississippi and Alabama, where drought hasn’t been a major issue. But its path is forecast to take it through parched Arkansas and Missouri, where some of the worst drought has been lingering for weeks, and then on into Illinois and Indiana, where the drought has been less intense lately, but which is still suffering badly. Up to seven inches of rain are forecast for some areas along Isaac’s path. “If the forecasted storm track of anywhere from four to seven inches of rain materializes, that could change the landscape of drought in a hurry,” said Brian Fuchs of the National Drought Mitigation Center, and the author of the Drought Monitor report, in a press release.

While Thursday’s report isn’t dramatically different from the one issued August 23, the drought did ease somewhat in Kansas, Oklahoma, Iowa and parts of Nebraska and Illinois, while it worsened slightly in western Nevada, parts of

Isaac’s moisture won’t do much for Oklahoma, Kansas, Nebraska and eastern Colorado, however, where exceptional drought remained firmly entrenched.

And even in areas where heavy rains are expected, they will be unable to make up for the excruciatingly dry conditions that haunted farmers for most of the growing season. “The rain is going to come too late for many crops,” Fuchs said. “It’ll help with pasture greenup and soil recharge, but with too much there could be problems as harvest starts up.” Those problems could include fields too muddy for farm machinery to navigate, and some crops knocked flat by winds and torrential rains before they can be harvested

The combination of extreme heat and dry weather during July, especially, has taken a heavy toll on the nation’s agricultural sector. July was the warmest month on record in the contiguous U.S., beating out the Dust Bowl-era year of 1936 for that dubious distinction. Excessive heat increases the transport of moisture out of soils and vegetation, and into the atmosphere, thereby drying the environment faster than would occur in cooler conditions.