August 2007 PHDI

U.S. Drought Summary

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The data presented in this drought report are preliminary. Ranks, anomalies, and percent areas may change as more complete data are received and processed.
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Top of Page National Drought Overview

Dry conditions predominated during much of the year across large parts of the Southeast, West, and Upper Great Lakes. March, May, August, and November were especially dry over large areas. Increased evaporation from anomalously warm temperatures combined with the lack of precipitation to exacerbate the drought conditions, especially during the summer months. A new core dry area developed over the mid-Atlantic states during the summer and fall. Unusually wet conditions in the southern and central Plains and into the Lower Great Lakes by late summer and early fall kept the three main drought areas (West, Southeast, Upper Great Lakes) separated. The January 1, 2008 U.S. Drought Monitor depicts conditions at the end of 2007.

Several short-lived dry episodes occurred in other regions throughout the year, notably in the Ohio Valley and Northeast in May and the northern Plains in July. The percent area* of the contiguous U.S. experiencing moderate to extreme drought grew steadily from 16 percent in January to a peak of about 42 percent by August, declined during the next 2 months, expanded again in November, and declined again in December.
Percent Area of the Contiguous U.S. in Moderate-Extreme Drought, Jan 1996-present
Percent Area of the Contiguous U.S. in Moderate-Extreme Drought, Jan 1900-present

*This drought statistic is based on the Palmer Drought Index, a widely used measure of drought. The Palmer Drought Index uses numerical values derived from weather and climate data to classify moisture conditions throughout the contiguous United States and includes drought categories on a scale from mild to moderate, severe and extreme.

The most extensive national drought coverage during the past 110 years (the period of widespread reliable instrumental records) occurred in July 1934 when 80 percent of the contiguous U.S. was in moderate to extreme drought. Although the current drought and others of the 20th century have been widespread and of lengthy duration, tree ring records indicate that the severity of these droughts was likely surpassed by other droughts including that of the 1570s and 1580s over much of the western U.S. and northern Mexico.

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Top of Page Regional Drought Overview

Impacts from this year's drought were felt especially hard by the agricultural and hydrological communities. Low streams, reservoirs, and stock ponds and depleted soil moisture ravaged pastures, rangeland, and cropland as the growing season progressed. Very dry soil moisture conditions stretched from coast to coast at the beginning of August, but improved in the Great Lakes and northern Plains by month's end. Agricultural conditions were especially harsh in the West and Great Lakes to Southeast at the end of July when many states had more than half of their pasture and rangeland in poor to very poor condition. Unusually hot temperatures in the West during July exacerbated the drought conditions. The heat spread into the Southeast during August. By the end of September, pasture and rangeland conditions improved in the Great Lakes region after beneficial rains but worsened in the Southeast. August 5, 2007 Percent Area of State USDA Topsoil Moisture Dry to Very Dry
August 2007 Standardized Temperature Index

The governors of several states implemented water conservation measures or declared drought disasters or states of emergency. These include Alabama, Florida, and Georgia in the Southeast, and California and Oregon in the West, during May. In July, the governors of Maryland and Tennessee made similar declarations for part or all of their states. In September and October the list expanded to include Connecticut, Delaware, and North Carolina.

Parts of the Southeast have been dry since December 2006. This dryness has been associated with a strong subtropical Bermuda High which stretched across the North Atlantic into the Southeast, deflecting tropical storm systems away from the region for much of the spring and summer. A strengthening La Niña during the last half of the year compounded the dryness. Regionwide, every month in 2007, with the exception of October and December, averaged drier than normal. This persistent lack of precipitation has resulted in a record dry March-May and December-September for the region. Southeast Region precipitation departures, January 1998-present

3-month SPI, March-May 2007 9-month SPI, January-September 2007

Several southeastern states had record or near-record dryness during 2007. Especially dry periods include: February-April, February-May, March-May, January-June, and March-August. The drought depleted soil moisture, dessicated vegetation, shrank rivers, and dried up wells. Many communities implemented water restrictions as municipal water supplies dwindled. Wildfires plagued Florida during March and April, and Lake Okeechobee's level fell to 8.94 feet by the end of May, breaking the record low of 8.97 feet set back in May 2001. Alabama Power, the state's largest utility, operated some of its coal plants at significantly reduced levels by summer to avoid raising water temperatures in the Coosa, Black Warrior and Mobile rivers. The Tennessee Valley Authority shut down Brown's Ferry Number 2 nuclear power plant in August due to inadequate streamflow needed to cool the reactor. January-December 2007 statewide precipitation ranks
Tennessee Statewide Precipitation, February-August, 1895-2007

Mississippi Statewide Precipitation, January-June, 1895-2007 Georgia Statewide Precipitation, March-May, 1895-2007

In the western U.S., the 2006-2007 wet season had a couple wet months (November 2006 and February 2007), but otherwise conditions were dry. By April 1, the normal snowpack season peak, mountain snowpacks were significantly below normal across most of the region. The percent area of the West (Rocky Mountains to West Coast) experiencing moderate to extreme drought (as defined by the Palmer Drought Index) expanded to 73% by August before decreasing again. Large wildfires became widespread in the northern areas by mid-summer and in southern California during the fall. According to a University of Nebraska report, the October 2007 level in Lake Mead (in the Southwest U.S.) fell to 1111.1 feet, which is 15 feet lower than the same time last year, the lowest October level since 1964, and the fourth lowest October level in 70 years of record.

Percent Area Western U.S. experiencing moderate-extreme drought and wet spell conditions, January 1996-present 9-month SPI, September 2006-May 2007

In the Lake Superior drainage area (Upper Michigan, northern Wisconsin, and northeast Minnesota), May-August was persistently dry and most of the previous 12 months were very dry (May-August, September-August). The late spring to early fall period is normally the wettest season for this region. Unusually dry conditions during this season for the last several years resulted in a record low level for Lake Superior, which impacted shipping and recreational activities. The average Lake Superior water level at the end of August 2007 was 600.4 feet above mean sea level, a foot lower than a year earlier and two inches lower than the previous record of 600.5 feet in 1926. In September, the level dipped 1.6 inches beneath the previous low for that month reached in 1926, according to NOAA's Great Lakes Environmental Research Laboratory. The U.S. Army Corps of Engineers, which uses a different measuring technique, calculated the September level at 4 inches below the record. Lake Superior region precipitation, April-August, 1895-2007

6-month SPI, June-November 2007 During the summer and fall the drought expanded into the mid-Atlantic region. July, August, September, and November were especially dry months for this region.

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Top of Page Pre-instrumental Drought Perspective

Tree ring records provide a useful paleoclimatic index that extends our historical perspective of droughts centuries beyond the approximately 100-year instrumental record. Several paleoclimatic studies have shown that droughts as severe or worse, both in magnitude and duration, than the major 20th century droughts have occurred in the U.S. during the last thousand years. The following paleodrought reports have been prepared by the NOAA/NCDC Paleoclimatology and Climate Monitoring branches during 2007:

Citing This Report

NOAA National Centers for Environmental Information, Monthly Drought Report for Annual 2007, published online January 2008, retrieved on July 24, 2024 from