NOAA's National Centers for Environmental Information
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U.S. Drought |
National Drought Overview
| As the year began, moderate to extreme drought* covered about one-third of the contiguous United States including much of the West and parts of the central Plains and upper Mississippi Valley. Precipitation during the subsequent 8 months resulted in drought areas east of the Rockies contracting or disappearing, and lack of precipitation caused new drought areas to develop. During this same period, long-term drought conditions over much of the West remained generally steady. As a result, the national moderate-to-extreme drought area basically fluctuated between 22 to 32 percent. Generally wetter conditions near the end of the year brought improvement to much of the West, causing a decrease in the national drought area to about 5 percent by the end of October. The percent area of the nation in severe to extreme drought followed a similar pattern throughout the year. |
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*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 100 years (the period of instrumental record) 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. |
Regional Drought Overview
| The 2004 national
drought affected the western U.S. for much of the year, but other
parts of the country were affected at different times. In fact,
many parts of the country experienced short-term (i.e., monthly) weather at both extremes
of the moisture spectrum during 2004. The driest month of the
year was March, both on a
national scale as well as for
many regions of the country.
The first three months of the year were persistently dry for the Northeast. January-March 2004 ranked as the 3rd driest such 3-month period in the 110-year record for the region. |
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For much of the Southeast, this year started out dry after very wet conditions during most of 2003. The 7th driest spring (March-May) and 11th driest January-May resulted in expanding drought conditions in the region. However, a wet summer and copious rainfall from several tropical systems in September ended the threat of drought for most of the Southeast. |
| The western U.S. was affected by drought for much of 2004. A series of winter storms during the start of the 2003-2004 cold season brought a promising mountain snowpack to a large part of the West by the end of February 2004. |
| But an unusually dry and warm March caused extensive early melting of the pack and set the stage for worsening drought conditions during the summer. The West region had the 2nd driest March-September in the 110-year record, the Northwest region had the 8th driest March-April on record, Montana ranked 3rd driest for February-April, and Wyoming ranked 10th driest for March-June. Streamflow averaged well below normal and soil moisture, pastures and rangeland dried out during June-August. Even by the end of the year, reservoirs across much of the West remained depleted. In March and again by mid-summer (July), fully two-thirds of the West (Rockies westward) was classified in moderate to extreme drought according to the Palmer Index. |
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| A shift in the weather pattern brought above-normal precipitation to parts of the West, especially the Southwest region, beginning in September and continuing into October, November, and December. The percent area of the western U.S. experiencing moderate to extreme drought fell to near 10 percent by October. |
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| In retrospect, the percent area rose above 20 percent in November 1999 and stayed above this level persistently for the last 5 years. At its peak (August 2002), this drought affected 87 percent of the West (Rockies westward), making it the second most extensive and one of the longest droughts the region has experienced in the last 105 years. In spite of the abundant rain and snow, both short- and long-term drought persisted across parts of the West (especially the northern sections). |
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| Drought was especially severe in Alaska this year. After a wet spring, the summer months (June-August) were very dry, with many large wildfires breaking out across the east central interior regions. By the end of October, over 6.6 million acres were blackened across the state. This made the 2004 fire season the worst on record for Alaska. The dry conditions were accompanied by unusually hot summer temperatures. |
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| More detailed information on drought during 2004 can be found on NCDC's monthly climate summary pages. |
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 branchs during 2004:
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