NOAA's National Centers for Environmental Information
U.S. Drought Highlights:
- On the national scale, severe to extreme drought affected about 4 percent of the contiguous United States as of the end of June 2005.
- Moderate to extreme drought affected about 13 percent of the contiguous U.S.
- The month was drier than normal in a broad swath from the Southwest and southern Plains to the Great Lakes, then eastward to the mid-Atlantic and Northeast states. Parts of this swath have been drier than normal for the last four months (March, April, May, June).
- For the Pacific Northwest as a whole, June was the fourth near to wetter-than-normal month following a dry winter, although parts of the region were drier than normal this month.
- Long-term moisture deficits (last 24 to 36 to 60 months) persisted across parts of the West into the northern High Plains and central Plains.
Please Note: The data presented in this drought report are preliminary. Ranks, anomalies, and percent areas may change as more complete data are received and processed.
National Overview
On the national scale,
- severe to extreme drought affected about 4 percent of the contiguous United States as of the end of June 2005, a decrease of about 1 percent compared to last month
- about 13 percent of the contiguous U.S. fell in the moderate to extreme drought categories (based on the Palmer Drought Index) at the end of June
- 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
- about 27 percent of the contiguous U.S. fell in the severely to extremely wet categories at the end of June
- a file containing the national monthly percent area severely dry and wet from 1900 to present is available
- 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).
Regional Overview
June was drier than normal in a broad swath from the Southwest and southern Plains to the Great Lakes, then eastward to the mid-Atlantic and Northeast states. Parts of this swath have been drier than normal for the last four months (March, April, May, June). For the Pacific Northwest as a whole, June was the fourth near to wetter-than-normal month following a dry winter, although parts of the region were drier than normal this month.
The June precipitation pattern at the primary stations in Alaska was mixed but mostly drier than average along the coast and wetter than average at the interior stations. Across Hawaii, most of the stations were drier than average. In Puerto Rico, the precipitation signal was mixed, based on National Weather Service radar estimates of precipitation. June streamflow averaged near normal for Puerto Rico and the Hawaiian Islands.
Long-term moisture deficits persisted in many areas. Six-month dryness was evident for parts of the Northwest, northern Rockies, southern Plains, Mississippi to Ohio valleys, Great Lakes, and mid-Atlantic states. The Pacific Northwest has experienced a dry winter followed by a wet spring, although spring excesses did not significantly ease six to seven years of precipitation deficits. The southwestern U.S. has been very wet during the winter and spring, although long-term deficits remained across parts of the Southwest and West, and much of the central to northern Plains. This is reflected in the end of June to beginning of July U.S. Drought Monitor maps. The Southwest has recovered at the 12 to 24 month timescales, but still shows dryness in some parts at the 36 to 60 month timescales.
The southwestern U.S. has been very wet during the winter and spring, although long-term deficits remained across parts of the Southwest and West, and much of the central to northern Plains. This is reflected in the end of June to beginning of July U.S. Drought Monitor maps. The Southwest has recovered at the 12 to 24 month timescales, but still shows dryness in some parts at the 36 to 60 month timescales.
Some regional highlights:
- Three states had the tenth driest, or drier, June in the 111-year record.
- Four states had the tenth driest, or drier, April-June on record.
- Illinois had the second driest March-June on record. By the end of June, the Farm Service Agency (FSA) executive directors of 47 counties in Illinois had filed drought reports, which is the first step to initiate a government disaster declaration (Illinois Farm Bureau, July 1, 2005).
- As noted by the National Weather Service, by the end of the month the state of Texas had 78 counties with burn bans, or approximately one-third of all counties statewide.
- End-of-month and month-averaged soil moisture conditions were drier than normal across parts of the southern Plains, Pacific Northwest, and mid-Atlantic states, and much of the Great Lakes to Mississippi and Ohio valleys, based on model computations (CPC-1, CPC-2, MRCC). The models also indicated dry soil moisture conditions in eastern Alaska and near the surface and at depth across the Great Lakes to Ohio Valley.
- According to end-of-June USDA observations, more than 50 percent of the topsoil moisture was rated short to very short (dry to very dry) in several states from the southern Plains to the Mississippi Valley, and eastward to the mid-Atlantic coast. This is drier than the 5-year and 10-year averages across much of the area.
- Streamflow levels were below seasonal norms across parts of the Pacific Northwest, northern Rockies, southern Plains, Great Lakes, Mississippi and Ohio valleys, and mid-Atlantic states, both as computed by models and based on USGS observations.
- Several large wildfires were burning by the end of the month in central Alaska and the U.S. Southwest. The Alaska fires were burning in areas that experienced severe dryness last summer and are on the edge of the area with current dry soil moisture conditions. The unusually heavy precipitation during late fall to spring in the Southwest resulted in rapid and extensive undergrowth which provides abundant fuel for fires.
- The precipitation pattern for the West during late autumn to winter (November-February) consisted of unusually wet conditions in the Southwest and very dry conditions in the Northwest. The spring to early summer months (March-June) have seen near to wetter than average conditions across much of the West. In spite of the recent precipitation, the hydrologic year to date (October-June) precipitation pattern still consists of a sharp north-south gradient in moisture receipt.
- Reservoir levels in the West reflected the long-term precipitation deficits in many states. The percent area of the western U.S. (Rockies westward) experiencing moderate to extreme drought (as defined by the Palmer Drought Index) decreased from about 67% in July 2004 to under 10% by October. Intensification of drought in the Pacific Northwest (Nov., Dec., Jan., Feb., Mar., Apr.) resulted in an expansion of the western drought area to about 28% by the end of February. Above-normal precipitation from storms during the last several months in the Pacific Northwest brought the western area coverage down to near 11% by the end of June.
Paleoclimatic Perspectives
June 2005 Drought Pre-Instrumental Perspective
Bighorn Basin, Wyoming and Montana
Despite near-average precipitation over the past 12 months, Wyoming and Montana have been mired in long-term drought since mid-1999. The Bighorn Basin in north-central Wyoming and extreme south-central Montana is representative of conditions in both states, with precipitation in the corresponding climate division (Wyoming Division 4) since July 2000 totaling 84% of the 60-month normal (based on 1950-2000) (also see graph to left).
The graph to the right (annual values in light blue, 5-year weighted average in dark blue) shows the average annual (July-June) precipitation, 1896-2005, for Wyoming Division 4. Several severe multi-year drought events can be seen in this record: around 1900; the mid-1930s Dust Bowl; the 1950s; and the last six years (2000-2005), all of which have been below the long-term average.
Also shown on this graph is a 739-year tree-ring reconstruction (1260-1998) of Bighorn Basin annual precipitation (annual values in light red; 5-year smoothed values in dark red), developed by Gray et al. (2004). This reconstruction is based on four long tree-ring chronologies (one Douglas-fir, three limber pine) from the Bighorn Basin, and was calibrated on an instrumental precipitation record (1907-1996) averaged from five long-term weather stations in the Bighorn Basin, four of which are within Wyoming Division 4: Buffalo Bill Dam, WY; Lovell, WY; Powell Field Station, WY; Worland, WY; and Bridger, MT. The reconstruction was calibrated on a 13-month "annual" period (June-June), but it correlates well with the Wyoming Division 4 annual (July-June) precipitation. Over their common period (1896-1996) the correlation is 0.602, indicating a high degree of shared variance. The precipitation units shown are standardized for comparison; negative values indicate below-average precipitation, and positive values indicate above-average precipitation.
The tree-ring reconstruction can put the precipitation variability of the last century in north-central Wyoming into a much longer perspective. First, the most severe single-year droughts of the 20th century (e.g., 1934, 1956) were probably matched or exceeded on numerous occasions in the previous 650 years. The reconstruction also shows several extended dry periods much longer than those in the instrumental record, including events in the late 1200s and the late 1500s. The latter event has been found, from other tree-ring reconstructions, to have extended through much of North America (Stahle et al. 2000). These events, were they to recur, would likely pose a greater challenge to human activities and ecosystems in the Bighorn Basin than even the severe droughts of the 1930s, 1950s, and 2000-2005.
Resources:
- Divisional climate data, including precipitation for Wyoming Division 4 as shown above, can be obtained from NCDC.
- The data for the Bighorn Basin precipitation reconstruction are available from the World Data Center for Paleoclimatology.
References:
- Gray, S. T., Fastie, C. L., Jackson, S. T., and J. L. Betancourt, 2004. "Tree-ring-based reconstruction of precipitation in the Bighorn Basin, Wyoming, since 1260 A.D." Journal of Climate 17 (19): 3855-3865.
- Stahle, D. W., Cook, E. R., Cleaveland, M. K., Therrell, M. D., Meko, D. M., Grissino-Mayer, H. D., Watson, E., Luckman, B. H., 2000. "Tree-ring data document 16th century megadrought over North America." EOS Transactions, American Geophysical Union 81 (12): 121-125.