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Posted at 03:40 PM ET, 07/15/2011

Extreme heat wave to spread across U.S., drought conditions to worsen


NOAA hazards assessment for period spanning July 18-29.

Already during July, 882 record high temperatures have been tied or set across the U.S. At the same time, drought is more extensive than any time since at least 2000. Over the weekend and next week, the drought will worsen in many areas while a remarkable burst of humid heat surges north and then east.

The U.S. Drought Monitor released Thursday showed 29 percent of the country in drought, and 12 percent of the country in exceptional drought, the largest extent on record (though records only go back to 2000).

The drought conditions covering large sections of the U.S. have evolved very quickly. A year ago, there was essentially no extreme or exceptional drought over the U.S. The New York Times has an excellent illustration showing the evolution of “extreme to exceptional” drought over time

Of all states, drought conditions continued to be most extensive and intense in Texas, with 72 percent of the state in exceptional drought. But drought conditions have rapidly expanded in Oklahoma, where exceptional drought now covers 40 percent of the state, a marked 10 percent increase from last week.

In the south central U.S., the drought and heat go hand in hand. At weather.com’s blog, meteorologist Stu Ostro, explained the self-reinforcing cycle:

In the southern states, particularly the southern Plains, afternoon high temperatures have consistently been particularly extreme, assisted by how dry the soil is. Rather than some of the sun’s energy going into evaporating soil moisture, it gets efficiently converted into quickly-rising temperatures each day.
And in turn, the soil dries out even more, worsening the drought.

Ostro also demonstrated that while the drought in 2011 across the U.S. is nowhere near as extensive as it was in the Dust Bowl years of 1934 and 1936, what’s unique about 2011 is the “extraordinary dichotomy of extreme wet/dry that exists in such close juxtaposition to each other” (CWG’s Andrew Freedman has also written about this).

In other words, just to the north and northwest of where it’s been extraordinarily dry and soils are parched, it’s been extraordinary wet and soils are saturated. So when the huge heat ridge builds over the central U.S. over the weekend, intersecting the northern regions with high soil moisture, humidity levels will go through the roof in places like Minneapolis and Des Moines, even if actual air temperatures are slightly dampened.

When you add into the equation the abundance of “sweaty corn” around these same areas, humidity levels are going to be incredible. (Corn increases humidity by releasing large quantities of water into the atmosphere through evapotranspiration.)


Monday maximum heat index forecast (NOAA)

For all of these reasons, a huge area of the Plains and upper Midwest is under an excessive heat watch. Heat indices are likely to range from 105-115 degrees, with dewpoints near 80.

The National Weather Service is sounding the alarm about these conditions.

“This heat is dangerous on many levels,” said Jack Hayes, director of the National Weather Service. “Temperatures and humidity levels are high, the heat will be prolonged, and very warm temperatures overnight won’t provide any respite. All of these factors make this an unhealthy situation, especially those in the upper Midwest who are not use to such heat.”


Friday maximum heat index forecast (NOAA)
As I discussed yesterday, the heat and humidity is likely to gradually migrate eastward, with the Southeast through the mid-Atlantic dealing with excessive heat and humidity by mid-to-late week. The projected maximum heat indices from Charleston, South Carolina to Baltimore, Maryland (including Washington, D.C.) range from 105-110 degrees.

By  |  03:40 PM ET, 07/15/2011

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