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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.
On the national scale,
Dryness was observed in May over much of the country. The Southwest, western Texas, western High Plains and the south Atlantic states have experienced very dry conditions for the last several months. The most severe conditions in May were in the Desert Southwest to the northern Plains, and the Big Bend region of Texas. About 11 percent of the contiguous U.S. was very dry (i.e., precipitation in the bottom 10th percentile of the historical record).
Coupled with the very dry conditions were high temperatures in the Southwest and Plains. The combination of high temperatures and very little precipitation led to extreme drought as defined by the Palmer Z Index.
Dryness persisted in the mid-Atlantic States and southern Appalachians, central Florida and along the Louisiana coast. Evaporation in these areas was high, and streamflow was low.
The May precipitation pattern at the primary stations in Alaska was below normal throughout most of the state. However, the southeastern coastal region was wetter than normal. Across Hawaii, the precipitation pattern was wet in the Southeast and progressively drier towards the Northwest. In Puerto Rico, the month was predominantly dry in the east and wet along the northwest coast, based on National Weather Service radar estimates of precipitation.
The wheat harvest in Oklahoma is expected to be the worst in 50 years due to drought, wildfires, high winds, hail storms, insects and frost damage (AP, 5/22). On May 9, the USDA authorized a federal drought disaster declaration in all but one of the counties in Arizona.
Some regional highlights:
May 2006, Pre-Instrumental Perspective, Southern Texas
Near-average rainfall occurred in far southern Texas in May, but it wasn't nearly enough to relieve severe drought conditions exacerbated by rainfall far below average during the previous five months. Since November 2005, precipitation in Texas Division 9 has been only 48% of average (based on 1950-2000), and 59% of average since June 2005. Even after the May 2006 rains, most of Division 9 (Southern Texas) was classified as being in "exceptional drought" (D4) on the late May to early June U.S. Drought Monitor.
The graph below (annual values in light blue, 5-year weighted average in dark blue) shows the winter-spring (November-May) precipitation, 1896-2006, for Texas Division 9. The value for 2006 (4.98") is the 2nd lowest since 1896, after 1971 (3.16"). The most striking multi-year anomaly in this record is the 1950s drought (indicated with orange bar), which had seven consecutive years (1950-1956) with below-average winter-spring precipitation.
The graph to the left also shows a 341-year tree-ring record (1652-1992; annual values in light red; 5-year smoothed values in dark red) that corresponds well to the variability in November-May precipitation. This record is the average of three tree-ring chronologies (Douglas-fir, post oak, and Montezuma bald cypress) from Texas and northern Mexico. The correlation between the annual values of the tree-ring record and November-May precipitation is 0.686, 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 winter precipitation variability of the last century in southern Texas into a much longer perspective. The record shows a number of individual years in the approximately 250 years prior to 1895 that may have had very low winter-spring precipitation similar to the lowest values in the past century. Among multi-year droughts, the 1950s appears unusually persistent and severe even in the context of the entire tree-ring record. Prior to 1950, the ring-width index was below average for seven years in a row only once (1711-1717; red bar), and the total cumulative ring-width anomaly for that period was not as great as for 1950-1956, suggesting it was not quite as severe as the 1950s drought.
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