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Climate of 2006 - July
U.S. Drought Watch

National Climatic Data Center, 14 August 2006

Global Analysis / Global Hazards / United States / Extremes
Use these links to access detailed analyses of Global and U.S. data.

Contents Of This Report:

National Drought Overview
Detailed Drought Discussion
State/Regional/National Moisture Status
Pre-Instrumental Perspective
Drought Indicators
Additional Contacts
Questions

National Overview

Based on the Palmer Drought Index, severe to extreme drought affected about 28 percent of the contiguous United States as of the end of July 2006, an increase of about 1 percent compared to last month. By contrast, about 8 percent of the contiguous U.S. fell in the severely to extremely wet categories.
About 51 percent of the contiguous U.S. fell in the moderate to extreme drought categories (based on the Palmer Drought Index) at the end of July.
On a broad scale, the previous two decades (1980s and 1990s) were characterized by unusual wetness with short periods of extensive droughts, whereas the 1930s and 1950s were characterized by prolonged periods of extensive droughts with little wetness (moderate to extreme drought, severe to extreme drought).

A file containing the national monthly percent area severely dry and wet from 1900 to present is available for the severe to extreme and moderate to extreme categories.
Historical temperature, precipitation, and Palmer drought data from 1895 to present for climate divisions, states, and regions in the contiguous U.S. are available at the Climate Division: Temperature-Precipitation-Drought Data page in files having names that start with "drd964x" and ending with "txt" (without the quotes).

Detailed Drought Discussion

By the end of July, the area of drought covered the Plains from the Rockies eastward to the Midwest and the South from Arizona to the Atlantic Ocean (August 1 Drought Monitor). In the drought areas, soil moisture was low, streamflow was especially low in the northern and central Plains and the Southeast, and vegetative health was poor.

Coupled with very dry conditions were high temperatures and evaporation in the drought stricken areas.

Drought and high temperatures impacted many sectors of the economy. Crops were highly stressed or dying, livestock was dying or prematurely sold because of a lack of feed and water, water restrictions were common in many areas, and wildlife in search of food and water was reported in urban areas. Disaster conditions have been declared by the governors of several states. Dry conditions also led to dozens of fires in the western half of the country. Drought impacts have been collected and summarized by county at the National Drought Mitigation Center's Drought Impact Reporter.

Cracked soil in Iowa (photo courtesy of Dave Reinig, Little Sioux, Iowa)

Severely stressed Iowa corn field (photo courtesy of Dave Reinig, Little Sioux, Iowa)

The July precipitation pattern at the primary stations in Alaska was below normal in most of the southern part of the state. In Hawaii, the precipitation pattern was dry throughout the State. In Puerto Rico, the month was predominantly dry in the central and western interior areas, based on National Weather Service radar estimates of precipitation.

Map showing July State Precipitation Ranks

Some regional highlights:

Several states experienced the tenth percentile of precipitation, or drier, multi-month seasons (May-July, February-July, August-July).
Month-averaged and end-of-month soil moisture conditions, based on model computations (CPC-1, CPC-2, MRCC), were drier than normal across the Plains and South. At depth, soil moisture west of the Mississippi River continued much below normal.
Low streamflows, as computed by models and based on USGS observations, continued in the Southeast westward and northward into the Plains.

Western U.S. percentage area wet and dry, 1996-2006

Drought conditions in the Southwest continued during July. About 30 percent of the western U.S. (Rockies westward) fell in the moderate to extreme drought category, and about 14 percent fell in the severe to extreme category (as defined by the Palmer Drought Index) as of the end of the month. The percent of average basin precipitation at the end of the month reflected the very dry conditions in Arizona and New Mexico.

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.):

STATES:

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

REGIONS:

Northwest	West North Central	East North Central
Northeast	Central	Southeast
South	Southwest	West

NATIONAL:

Contiguous U.S.

Pre-Instrumental Perspective

July 2006 Paleoclimatic Analysis for Northeast Wyoming

The long-term drought that has persisted in Wyoming since late 1999 has hit the northeastern corner of the state particularly hard. In Wyoming Division 6 (Belle Fourche Drainage), precipitation since August 2001 has been only 80% of the 60-month normal, and the NWS/NCEP objective drought indicator blends for late July 2006 showed Division 6 to be in the worst drought category. This year began with near-normal precipitation from January-April 2006, but then June and July brought only 35% of normal rainfall.

Wyoming Division 6 August-July 60-month precipitation, 1895-2006

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Precipitation Ranks for
Wyoming Division 6, 2005-2006
Period	Rank
Jul	6^th driest
Jun-Jul	2^nd driest
May-Jul	6^th driest
Apr-Jul	14^th driest
Mar-Jul	13^th driest
Feb-Jul	11^th driest
Jan-Jul	11^th driest
Dec-Jul	11^th driest
Nov-Jul	10^th driest
Oct-Jul	15^th driest
Sep-Jul	11^th driest
Aug-Jul	20^th driest

1996-2006 Wyoming Division 6 monthly precipitation anomaly

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Wyoming Division 6 Palmer Hydrological Drought Index, 1900-2006

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The graph below (annual values in light blue, 5-year weighted average in dark blue) shows the annual (August-July) precipitation, 1896-2006, for Wyoming Division 6. The value for 2006 (13.22") is only the 20th lowest since 1896, but it marks the seventh year in a row with below-normal precipitation. The most persistent drought, however, was the Dust Bowl, with 12 years of below-normal precipitation (1929-1940), including the lowest 60-month total precipitation (1934-1938; indicated with yellow arrow) on record. The second-lowest 60-month total is the current drought, 2002-2006 (orange arrow).

Paleoclimatic tree-ring reconstruction and observed precipitation for Wyoming Division 6 for the total period 1603-2006

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larger image (200 KB)

The graph to the left also shows a 388-year tree-ring record (1603-1990; annual values in light red; 5-year smoothed values in dark red) that corresponds well to the variability in August-July precipitation. This record is the average of three tree-ring chronologies (Douglas-fir and ponderosa pine) from Wyoming, South Dakota, and North Dakota. The correlation between the annual values of the tree-ring record and August-July precipitation is 0.671, indicating a high degree of shared variance. The tree-ring record captures the multi-year variability of the observed precipitation record particularly well.

The tree-ring record, as a proxy for precipitation, can put the persistent droughts of the last century in northeastern Wyoming into a much longer perspective. There are four 5-year periods in the tree-ring record (red arrows) with average ring-width anomalies similar to that for 1934-1938, suggesting that those periods were droughts of similar severity to the worst part of the Dust Bowl, and by extension, similar to the current drought. The most severe of these paleodroughts, judging from the ring-width anomalies, was from 1756-1760. But no paleodrought appears to match the overall persistence of the Dust Bowl years. The current drought, like the Dust Bowl, appears to be among the worst multi-year droughts of the past 400 years.

Resources:

Divisional climate data, including precipitation for Wyoming Division 6 as shown above, can be obtained from NCDC.
Tree-ring data, including the chronologies for Whiskey Mountain (Wyoming), Burning Coal Vein (North Dakota), and Buckhorn Mountain (South Dakota), can be obtained from the International Tree-Ring Data Bank (ITRDB), World Data Center for Paleoclimatology.

Reference:

Sieg, C. H., Meko, D., DeGaetano, A. T., and Ni, W. 1996. "Dendroclimatic potential in the Northern Great Plains." Pages 295-302 in J. S. Dean, D. M. Meko, and T. W. Swetnam, Eds., Tree Rings, Environment and Humanity. Tucson: Radiocarbon.

Drought Indicators

The following indicators illustrate the drought conditions this month:

Palmer Drought Indices,
Standardized Precipitation Index,
long-term (36 to 60 month) 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
- hydrologic year precipitation,
satellite-based observations of vegetative health,
National Weather Service model calculations of
National Weather Service model calculations of soil moisture using the Leaky Bucket Model,
Midwest Regional Climate Center model calculations of soil moisture,
streamflow maps maintained by the USGS.

Additional Contacts:

For all climate questions other than questions concerning this report, please contact the National Climatic Data Center's Climate Services Division:

Climate Services Division
NOAA/National Climatic Data Center
151 Patton Avenue
Asheville, NC 28801-5001
fax: 828-271-4876
phone: 828-271-4800
email: ncdc.info@noaa.gov
For further information on the historical climate perspective presented in this report, contact:

Richard Heim
NOAA/National Climatic Data Center
151 Patton Avenue
Asheville, NC 28801-5001
fax: 828-271-4328
email: Richard.Heim@noaa.gov

-or-

Ned Guttman
NOAA/National Climatic Data Center
151 Patton Avenue
Asheville, NC 28801-5001
fax: 828-271-4328
email: Ned.Guttman@noaa.gov

-or-

Jay Lawrimore
NOAA/National Climatic Data Center
151 Patton Avenue
Asheville, NC 28801-5001
fax: 828-271-4328
email: Jay.Lawrimore@noaa.gov