Issued 8 August 2011
Contents Of This Report:
Map showing Palmer Z Index

National Drought Overview


Detailed Drought Discussion


July 2011 was a warm and dry month (fourth warmest and 20th driest, based on data back to 1895) when weather conditions are averaged across the country. But this reflected regional extremes (in both monthly precipitation and temperature) which resulted from persistent weekly regional patterns of precipitation (weeks 1, 2, 3, 4, 5) and temperature (weeks 1, 2, 3, 4, 5) anomalies throughout the month. Hot and dry weather expanded drought in the South, Midwest, and Northeast, and intensified the record drought in the Southern Plains. While earlier droughts are unsurpassed in terms of duration, the current drought in parts of the South is more intense when measured by the Palmer Hydrological Drought Index. USDM statistics rated 61 percent of the Southeast in moderate to exceptional drought at the end of July, while in the Southern Plains 84 percent was rated in moderate to exceptional drought and 47 percent in exceptional drought. The drought was accompanied by dessicated soils, failed crops, and hot temperatures, with more than 2750 daily high temperature records and 6100 warm daily low temperature records tied or broken. These hot temperatures intensified evapotranspiration. "Exceptional" drought (D4) expanded during July, resulting in the largest national footprint of D4 in the 12-year history of the USDM. About 32 percent of the contiguous U.S. was experiencing moderate to exceptional (D1-D4) drought by August 2nd, about four percent more than at the end of June.

U.S. Drought Monitor map from August 2, 2011
The U.S. Drought Monitor drought map valid August 2, 2011.

By the end of the month, the core drought areas in the U.S. included:

Palmer Drought Index

Palmer Z Index map Palmer Hydrological Drought Index map

The Palmer drought indices measure the balance between moisture demand (evapotranspiration driven by temperature) and moisture supply (precipitation). The Palmer Z Index depicts moisture conditions for the current month, while the Palmer Hydrological Drought Index (PHDI) and Palmer Drought Severity Index (PDSI) depict the current month's cumulative moisture conditions integrated over the last several months. As seen on the July 2011 Palmer Z Index map, areas of low precipitation and widespread hot temperatures resulted in short-term drought across much of the Southern to Central Plains, East Coast, Ohio Valley, and Great Lakes this month. Wet conditions are evident on the Z Index map over a large area from the West to the Northern Plains, and parts of the Central Gulf Coast. Compared with the June 2011 PHDI map, the July 2011 PHDI map indicates that drought conditions intensified in the Southern to Central Plains and Southeast; moist conditions intensified across parts of the West; and moist conditions lessened in parts of the Northeast and Ohio Valley. The July 2011 PHDI map also reflects the long-term nature of the drought conditions. The Z Index and PHDI maps in combination show that monsoon showers (Southwest) and scattered thunderstorms (Central Gulf Coast) brought limited relief to some drought areas, and the wet Northeast to Ohio Valley dried out this month, but for the rest of the country — it rained where it was already wet and was drier than normal over the existing drought areas.

Standardized Precipitation Index

1-month Standardized Precipitation Index 2-month Standardized Precipitation Index 6-month Standardized Precipitation Index

The Standardized Precipitation Index (SPI) measures moisture supply. The SPI maps here show the spatial extent of anomalously wet and dry areas at time scales ranging from 1 month to 24 months. Dryness is evident across much of the Southern Plains and Southeast at the 1 month to 12 month time scales (with some indication even at 24 months), and in parts of the Ohio Valley to Northeast at 1 to 2 months. The Central Gulf Coast is wet at 1 and 2 months, but dryness reasserts itself from 3 to 12 months. Similarly, monsoon showers brought near normal conditions to much of the Southwest at 1 month, but dry conditions become evident beginning at 2 and 3 months. A large area of exceptional dryness is visible in the Southern Plains at 2 months, but it is most evident at 6 to 9 months. Wet conditions are evident in the West at 1 and 2 months, especially along the Pacific Coast. Widespread wetness dominates across the West to Northern Plains beginning at 3 months, and from the Ohio Valley to Northeast beginning at 6 months. This illustrates the persistence of the dry and wet areas.

9-month Standardized Precipitation Index 12-month Standardized Precipitation Index 24-month Standardized Precipitation Index

Agricultural and Hydrological Indices and Impacts

Leaky Bucket model soil moisture percentiles
Leaky Bucket model soil moisture percentiles
USGS monthly streamflow percentiles
USGS monthly streamflow percentiles

Drought conditions were reflected in numerous agricultural, hydrological, and other meteorological indicators, both observed and modeled. Across the drought area from the Southwest to portions of the Northeast, streamflows were low, some groundwater well stations were at or near record low levels for this time of year, soil moisture was depleted, water restrictions were implemented in many communities, and pastures, rangeland, crops, and natural vegetation were ravaged. Parts of the Southern Plains and Far West had no days with rain at all in July. A large area of dry soils was indicated for Mexico connected to the Southern Plains dryness. The excessive heat across the southern and eastern U.S. increased evaporation and placed additional stress on vegetation across much of the country. This summary is based on the following observed and modeled indicators:


USGS groundwater map
USGS groundwater map.


VegDRI (Vegetation Drought Response Index) map
VegDRI (Vegetation Drought Response Index) map.


Map of maximum consecutive dry days
Map of maximum consecutive dry days.

Regional Discussion

July had a mixed precipitation pattern across Alaska. Many interior and southern coastal stations were drier than normal, but surrounding stations were wetter than normal. Below-normal precipitation was widespread in the southern and central portions of the state for the last 2 to 3 months, and at longer time scales (6, 12, 24, 36 months, and water-year-to-date [October-July]). July monthly streamflow was below normal in the southern coastal basins. The August 2nd USDM map had an eighth of the state in the abnormally dry category.

July was wetter than normal over Puerto Rico. There were a few areas with below-normal rainfall at longer time scales (2, 3, and 6 months, year to date, and water year to date [October-July]), but the August 2nd USDM map had no drought or abnormally dry areas on the island.

The rainfall pattern over the Hawaiian Islands was mixed during July, with most stations in the drought areas of the Big Island receiving below-normal rainfall. Drought conditions expanded during July, increasing from 16 percent of the state in moderate to severe drought at the end of June to 21 percent at the end of July. For the southern islands, long-term rainfall deficits remained at most time scales (last 2, 3, 6, 12, 24, 36 months, and water-year-to-date [October-July]).

State precipitation ranks, July 2011 State precipitation ranks, January-July 2011

On a statewide basis, July 2011 was drier than normal for 23 states, mostly stretching from the Southern Plains to the Northeast. Hardest hit were Texas and Oklahoma which ranked second and ninth driest, respectively. For the Southern Plains states, especially Texas, the dryness extends back to October 2010, with several "seasons" ranking as the driest on record. Record dryness for several seasons is also noted for Louisiana, New Mexico, and Oklahoma.

Texas statewide precipitation, January-July, 1895-2011 New Mexico statewide precipitation, January-July, 1895-2011

On a more localized basis, record dryness has occurred for at least one climate division in the Southwest, Southern Plains, Southeast, or Mid-Atlantic Coast at every time scale from the one-month period, July 2011, through the 12-month period, August 2010-July 2011. Record warm temperatures have occurred for at least one climate division during most of these last 12 "seasons", especially in the Southern Plains during the spring and summer:

Precipitation ranks:
Temperature ranks:

The combination of intense dryness and high evapotranspiration due to extremely warm temperatures has resulted in a rapid increase in the percent area of the Southern Plains experiencing drought and a rapid intensification of the PHDI. Two-thirds of the South region was rated in extreme to exceptional drought (USDM categories), while virtually all of Texas was under moderate to exceptional drought, nine-tenths under extreme to exceptional drought, and three-fourths in exceptional drought. The rapid increase of drought in Texas has resulted in record dry PHDI values for several climate divisions, rivalling (divisions 4 and 6) and in some cases exceeding (divisions 1, 2, and 5) the intensity of the earlier drought of record. The PHDI in climate divisions in New Mexico (divisions 6 and 7) and Louisiana (division 1) has rivalled or eclipsed the drought of record in those areas, and even southeast Georgia (division 9) has PHDI values as extreme as the worst drought of the last 70 years. In all of these cases, even though the intensity of the current drought is record or near-record, the duration of earlier droughts exceeds the duration of the current drought.

Texas climate division 2 PHDI, January 1900-July 2011 New Mexico climate division 7 PHDI, January 1900-July 2011 Louisiana climate division 1 PHDI, January 1900-July 2011

Percent area of the Western U.S. in moderate to extreme drought, based on the Palmer Drought Index
Percent area of the Western U.S. in moderate to extreme drought, based on the Palmer Drought Index.

The precipitation pattern over the West in July was mixed. July was drier than normal in parts of the Pacific Northwest, Northern Rockies, and Intermountain Basin, but this did little to change the overall pattern for the water year to date (October-present) — extremely dry conditions have been persistent from Arizona and New Mexico into eastern Colorado, but abundant precipitation has fallen across the rest of the West. This is evident in both the low elevation station precipitation as well as the high elevation (SNOTEL) station precipitation, modeled soil moisture, and PHDI. An analysis of early data by the USDA indicated that reservoir levels were, on average, below normal in New Mexico, where drought was an issue, to near or above normal in the other western states. According to the USDM, 19 percent of the West was experiencing moderate to exceptional drought at the end of July, about the same as June, while the Palmer Drought Index statistic was about 14 percent, also the same as last month. When the statistics for the Arizona-New Mexico-Colorado drought area (the Southwest drought area) are aggregated, the percent area in moderate to exceptional (USDM categories) drought has fluctuated between 60 and 70 percent for the last five months. The percent area in the exceptional and extreme to exceptional categories steadily increased from March to June then leveled off with monsoon showers in July, holding steady at about 37 percent in extreme to exceptional drought at the end of this month.


State/Regional/National Moisture Status
A detailed review of drought and moisture conditions is available for all contiguous U.S. states, the nine standard regions, and the nation (contiguous U.S.):

alabama arizona arkansas california colorado connecticut
delaware florida georgia idaho illinois indiana
iowa kansas kentucky louisiana maine maryland
massachusetts michigan minnesota mississippi missouri montana
nebraska nevada new hampshire new jersey new mexico new york
north carolina north dakota ohio oklahoma oregon pennsylvania
rhode island south carolina south dakota tennessee texas utah
vermont virginia washington west virginia wisconsin wyoming

northeast u. s. east north central u. s. central u. s.
southeast u. s. west north central u. s. south u. s.
southwest u. s. northwest u. s. west u. s.

Contiguous United States


Drought Indicators
The following indicators illustrate the drought conditions this month:


Contacts & Questions
For additional, or more localized, drought information, please visit:

Citing This Report

NOAA National Centers for Environmental Information, Monthly Drought Report for July 2011, published online August 2011, retrieved on July 19, 2024 from