Contents Of This Report:
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.
By the end of the month, the core drought areas in the U.S. included:
- a large area of moderate (D1) to exceptional (D4) drought across the Southwest, Southern and Central Plains, and into the Lower Mississippi Valley;
- areas of moderate to exceptional drought in the Southeast;
- areas of moderate to severe (D2) drought along the Mid-Atlantic Coast and into the Northeast;
- parts of Hawaii, where moderate to severe drought persisted; and
- areas of moderate drought developing in the Ohio Valley.
Palmer Drought Index
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
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.
Agricultural and Hydrological Indices and Impacts
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 (U.S. Geological Service) observed streamflow;
- NOAA Climate Prediction Center (CPC) modeled runoff anomalies and percentiles;
- VIC (University of Washington Variable Infiltration Capacity macroscale hydrologic model) 1-, 2-, 3-, and 6-month runoff percentiles;
- NLDAS (North American Land Data Assimilation System) modeled streamflow anomalies and percentiles;
- NLDAS model runoff anomalies and percentiles;
- USGS groundwater observations (real-time network, climate response network, total active network);
- USDA (U.S. Department of Agriculture) observed soil moisture conditions and comparison to 5-year average and 10-year average;
- the Palmer Crop Moisture Index (CMI), which steadily worsened and expanded throughout the month (weeks 1, 2, 3, 4, 5);
- CPC modeled soil moisture anomalies and percentiles for end of July;
- CPC's Leaky Bucket model soil moisture percentiles;
- NLDAS modeled soil moisture percentiles for the top soil layer and total soil layer;
- VIC modeled soil moisture percentiles;
- USDA observed pasture and rangeland conditions;
- Vegetation Drought Response Index (VegDRI);
- satellite-based Vegetation Health Index (VHI);
- the USGS agro-hydrologic model (Soil Water Index, Water Requirement Satisfaction Index);
- rainfall observations (plotted by the USGS and NOAA National Weather Service [NWS]);
- percent of normal precipitation and precipitation percentiles (NWS, High Plains Regional Climate Center station observations, Leaky Bucket model);
- USGS number of days with precipitation and maximum number of consecutive dry days;
- temperature departures from normal and percentiles;
- number of record warm daily low temperatures, record daily high temperatures, record monthly high temperatures, and all-time record high temperatures set in July 2011 (from NCDC's daily records analysis).
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]).
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.
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:
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.
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.):
|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|
- Palmer Drought Indices
- Standardized Precipitation Index
- long-term (36 to 60 month) percent of normal precipitation maps
- airport station percent of normal precipitation maps
- statewide precipitation rank maps
- Cooperative station percent of normal precipitation maps
- percent of average maps for the SNOTEL stations in the western mountains provided by the Western Regional Climate Center
- National Weather Service model calculations of soil moisture, runoff, and evaporation
- National Weather Service model calculations of soil moisture using the Leaky Bucket Model
- topsoil moisture conditions observed by the USDA and mapped by the Climate Prediction Center
- pasture and range land conditions observed by the USDA and mapped by the Climate Prediction Center
- streamflow maps maintained by the USGS
Contacts & Questions