News•September 10, 2012

Mixture of Grim News, Relief in Drought Forecast for Fall

The record drought in the U.S. is forecast to ease in some regions during the fall, but a majority of the drought-stricken regions from the West to the High Plains may be left high and dry by prevailing weather patterns through the end of November, according to the federal government’s latest seasonal drought outlook.

The outlook, issued by the National Oceanic and Atmospheric Administration (NOAA) on September 6, is based on a variety of weather and climate data, including computer model simulations and studies of how El Niño events — such as the one currently developing in the tropical Pacific — tend to affect the weather across the country during the fall.

NOAA's Seasional Drought Outlook, issued on Sept. 6. Click on image to see a larger version. Credit: NOAA/CPC.

As Climate Central recently reported, the drought has proved to be quite stubborn in the face of increased rainfall, including downpours from the remnants of Hurricane Isaac, which struck the Gulf Coast during the last week of August and then spread tropical moisture into the Ohio Valley, Mid-Atlantic, and Southeast.

Through September 4, about 63 percent of the land area of the Lower 48 states was suffering under some level of drought, and the area under “exceptional drought” — the most severe level — constituted 6.14 percent of the nation’s land area.

The drought was most likely triggered by natural climate variability, but it was aggravated by extreme heat during the summer, which may have had some ties to manmade global warming.

The majority of the drought-stricken area is not expected to see much improvement through November, the outlook shows. The NOAA map is colored brown, indicating persistent drought conditions, from Texas northward to the Central Plains, and then stretching west across the central and northern Rockies, all the way to the West Coast.

“As you move west in the Plains it’s a dry time of the year, so it becomes harder to overcome deficits that have accumulated,” said Rich Tinker, a drought forecaster with NOAA's Climate Prediction Center in Maryland. Farther west in the Rocky Mountains, Tinker said, significant drought alleviation may have to wait until the winter.

The drought outlook calls for the most improvement to take place in the East, where wetter fall weather will help ease the dryness from this past spring and summer. Some relief is also expected in the Southeast, including parts of Georgia, which has been suffering from drought conditions since August 2010.

The outlook also shows improving drought conditions in the northern High Plains and Ohio Valley, where cooler temperatures and increasing chances of snow during the fall may lessen drought impacts, even if precipitation deficits are not completely erased. For these areas, as well as the East, the transition to fall should lead to improving drought conditions, since cooler temperatures will tend to reduce the evaporation of soil moisture, unlike during spring and summer, when temperatures soared to record levels, worsening the drought.

Map showing where rainfall was above average, average, or below average during August. Click on the image for a larger version. Credit: NOAA.

During a typical fall, the Ohio Valley and the East also tend to receive more generalized and moderate rain storms compared to the localized thunderstorms that occur during the summer.

Tinker said the areas that are predicted to get some relief would not necessarily see a complete end to the drought, as precipitation and soil moisture deficits remain quite deep in many areas. “Improvement doesn’t mean alleviation,” Tinker said.

Relief is also called for in the Southwest, where above-average rainfall associated with the summer monsoon has already helped ease the long-lasting and severe drought there. Drought conditions may also ease in the Central Rockies, where the fall outlook shows enhanced chances for above-average precipitation. But the outlook states that “no relief is in sight” for other drought-affected parts of the Rockies westward.

Beyond the Lower 48 states, drought conditions are also forecast to engulf all of the Hawaiian Islands throughout the fall.