NOAA's National Centers for Environmental Information
U.S. Drought Highlights:
- On the national scale:
- Severe to extreme drought affected about 12 percent of the contiguous United States as of the end of April 2006.
- Moderate to extreme drought affected about 31 percent of the contiguous U.S.
- Dryness affected many of the same areas this month that have experienced dry conditions during the last several months. April was much drier than normal along the south Atlantic and Gulf coasts and central High Plains, and also across the northwestern Great Lakes. About 3 percent of the contiguous U.S. was very dry (i.e., precipitation in the bottom 10th percentile of the historical record).
- Above-normal precipitation continued to bring improvement to parts of the northern Rockies and central and northern Plains. During the month, beneficial rains brought some relief to the drought area in eastern Oklahoma and western Arkansas and parts of central and northern Texas, but it was not enough to compensate for the significant deficits of the last 12 months.
- Dryness intensified and spread along the Atlantic and Gulf coasts. For the last three months drought had been concentrated in the middle Atlantic states and in southern Louisiana, but the areas of dryness were spreading throughout the Southeast. Long-term moisture deficits persisted across most of the Mississippi Valley, southern to eastern Texas, and the Southwest.
- Very warm temperatures across most of the eastern two-thirds of the country worsened conditions in many drought-stricken areas.
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 12 percent of the contiguous United States as of the end of April 2006, about the same as last month
- about 31 percent of the contiguous U.S. fell in the moderate to extreme drought categories (based on the Palmer Drought Index) at the end of April
- 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 14 percent of the contiguous U.S. fell in the severely to extremely wet categories at the end of April
- 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
Dryness affected many of the same areas this month that have experienced dry conditions during the last several months. April was much drier than normal along the south Atlantic and Gulf coasts and central High Plains, and also across the northwestern Great Lakes. About 3 percent of the contiguous U.S. was very dry (i.e., precipitation in the bottom 10th percentile of the historical record).
Above-normal precipitation continued to bring improvement to parts of the northern Rockies and central and northern Plains. During the month, beneficial rains brought some relief to the drought area in eastern Oklahoma and western Arkansas and parts of central and northern Texas, but it was not enough to compensate for the significant deficits of the last 12 months.
Dryness intensified and spread along the Atlantic and Gulf coasts. For the last three months drought had been concentrated in the middle Atlantic states and in southern Louisiana, but the areas of dryness were spreading throughout the Southeast. Long-term moisture deficits persisted across most of the Mississippi Valley, southern to eastern Texas, and the Southwest.
The April precipitation pattern at the primary stations in Alaska was near to above normal over most of the state. Across Hawaii, the precipitation pattern was dry most everywhere. In Puerto Rico, the month was predominantly wet, based on National Weather Service radar estimates of precipitation.
Some regional highlights:
- Several states had the tenth driest, or drier, multi-month seasons (February-April, November-April, May-April).
- Numerous wildfires burned across Florida, and were especially widespread during the latter part of April into early May.
- End-of-month and month-averaged soil moisture conditions, based on model computations (CPC-1, CPC-2, MRCC), were drier than normal across a broad swath from the Southwest and central Plains to the Atlantic coast. The models also indicated dry soil moisture conditions in parts of Alaska, and improved soil moisture conditions near the surface and at depth across the Midwest.
- April streamflows were below seasonal norms across much of the Gulf Coast and East, and parts of the central Plains and the Southwest, as computed by models and based on USGS observations.
- Drought conditions in the Southwest continued during April. About 23 percent of the western U.S. (Rockies westward) fell in the moderate to extreme drought category (as defined by the Palmer Drought Index) as of the end of April. Aggregated reservoir levels in the West (provided by the USDA) reflected the long-term precipitation deficits in most states.
Paleoclimatic Perspectives
April 2006, Pre-Instrumental Perspective, Central New Mexico
New Mexico has been experiencing very dry conditions for much of the last seven years. The Central Highlands section of the state (climate division 6) has been especially hard hit. New Mexico Division 6 had the eighth driest "year" (May-April) in the 1895-2006 historical record during 2005-2006 and ranked second driest for several seasons (see table above). The cumulative 60-month precipitation graph is shown to the right. While the last 5 to 7 years generally have been dry, the aggregate deficits of the 1950s were much drier.
April 2006 brought little relief to this winter's severe drought in the Southwest. New Mexico division 6 received only 1.08 inches of precipitation (21% of the 1950-2000 average) from November 2005-April 2006, which virtually ties 1904 (1.07 inches) as the driest November-April period since records began in 1895.
The graph below (annual values in light blue, 5-year weighted average in dark blue) shows the winter (November- April) precipitation, 1895-96 to 2005-06, for New Mexico Division 6. The graph also shows a 989-year tree-ring reconstruction (1000-1988; annual values in light red, 5-year smoothed values in dark red) of November-April precipitation for central New Mexico (Division 6). This reconstruction was generated by Ni et al. (2002) from long, moisture-sensitive tree-ring records from the Southwest. The correlation between the annual values of the tree-ring reconstruction and November-April precipitation is 0.649, indicating a moderate degree of shared variance. The reconstruction places the winter precipitation variability of the last century in central New Mexico into a much longer perspective.
Paleoclimatic tree-ring reconstruction and observed
precipitation for New Mexico Division 6
for the total period 1000-2006 - 200 KB Image
The tree-ring reconstruction shows multi-year periods of persistent winter drought (orange bars on the graph) that appear to be more severe than any in the past century, such as events in the 1080s, 1660s, and 1780s. Conversely, the period 1326-1362 (blue bar) was persistently wet, and longer than any such period of the last century, with only one winter during this 37-year interval reconstructed as being drier than the long-term mean. Because of the moderate uncertainty around the reconstructed values, it is difficult to determine how often extreme one-year winter droughts similar to 1904 and 2006 have occurred. The reconstruction does show 10 years with less than 2 inches of winter precipitation, or about one per century, though these years are not evenly distributed over time.
In addition to its severity, the current winter drought in New Mexico is notable for being preceded by two wet winters; 2004 and 2005 had 128% and 183% of average precipitation, respectively, during November-April. Swetnam and Betancourt (1998) analyzed fire occurrence in the Southwest from 1700-1900 using a network of fire-scarred trees and tree-ring reconstructions of summer Palmer Drought Severity Index (PDSI). Their work shows that the largest fire years (in terms of spatial extent) tended to occur during severe drought years that were preceded by 1-3 wet years, which encouraged the growth of fuels. Hopefully, in 2006, sufficient moisture will come to the Southwest in time to avoid repeating this pattern.
Resources:
- Divisional climate data, including precipitation for New Mexico Division 6 as shown above, can be obtained from NCDC.
- The reconstruction of winter precipitation for New Mexico Division 6 can be obtained from the NOAA-NCDC Paleoclimatology Branch.
References:
- Ni, F., Cavazos, T., Hughes, M. K., Comrie, A. C. and Funkhouser, G. 2002. "Cool-season precipitation in the southwestern USA since AD 1000: Comparison of linear and nonlinear techniques for reconstruction." International Journal of Climatology 22: 1645-1662.
- Swetnam, T. W., and Betancourt, J. L. 1998. "Mesoscale disturbance and ecological response to decadal climatic variability in the American Southwest." Journal of Climate 11: 3128-3147.