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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,
May was the third wetter-than-normal month across parts of the Pacific Northwest, following a dry winter in the region. The month was drier than normal in the southern Plains to the Great Lakes eastward to the Appalachians and Northeast.
The May precipitation pattern at the primary stations in Alaska was mixed and mostly drier than average for Hawaii. In Puerto Rico, the precipitation signal was mixed during May, based on National Weather Service radar estimates of precipitation. May streamflow averaged near normal for Puerto Rico and dry for the Hawaiian Islands.
Long-term moisture deficits persisted in many areas. Six-month dryness was evident for parts of the Northwest, South and Southeast. The Pacific Northwest has experienced a dry winter followed by a wet spring, though 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 though long-term deficits remain across parts of the Southwest and West, and much of the central to northern Plains. These are reflected in the end of May U.S. Drought Monitor map. 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.
Parts of the central U.S. have been persistently dry for the spring, with states from Texas to Pennsylvania showing dryness at the 3-month timescale. Both Michigan and Illinois ranked 3rd driest on record for the spring.
Some regional highlights:
Reservoir levels in the West reflected the abundance (Arizona) or lack (many other states) of precipitation, or preventative actions taken by water managers in anticipation of summer shortfalls (Washington). 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 26% 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 May.
Over the last three decades, November-May precipitation in Oregon's Division 7 has experienced large swings from dry (1975-77) to wet (1982-84) to dry (1990-92) to wet (1996-98) and back to dry (2000-03) (see graph to left). When the annual values are smoothed with a 5-year weighted average (blue line in graph below), these multi-year oscillations appear to have greater amplitude than those in the preceding 80 years of instrumental record.
The graph to the right also shows a millennial-length tree-ring record (1000-1996) that corresponds well to the variability in November-May precipitation (red line; 5-year weighted average of annual values). This record is the average of three tree-ring chronologies from central Oregon. The growth of the western juniper trees at these sites is influenced mainly by the moisture in the soil at the start of the growing season, delivered by storms the previous winter and spring. The correlation between annual values of the 3-chronology average and of November-May precipitation is 0.734, indicating a high degree of shared variance.
The tree-ring record, as a proxy for precipitation, can put the variability of the last 30 years into a much longer perspective. While there is no exact analog for recent conditions, the tree-ring record shows periods in which there were similarly large oscillations, such as in the early 1200s, early 1400s, late 1500s, and mid-1700s. There are also long periods in which the amplitude of variability was generally lower, most notably 1800-1900. Also, there are a number of dry anomalies that match or exceed the extremes in the instrumental records, the most severe around 1071, 1352, and 1580. Another severe dry event in the mid-1600s has two closely separate peaks (1644, 1652). More recent significant dry events occurred around 1737, 1794, and 1844, the last one being less severe but more sustained. Knapp et al. (2004), examining the occurrence of severe, sustained droughts in the interior Pacific Northwest using a broader network of western juniper tree-ring chronologies, found widespread evidence for these last three dry events. Based on analyses of modern droughts, they ascribed the occurrence of these paleodroughts to persistent Pacific blocking highs which deflect winter and spring storms from the region.
Resources:
Divisional climate data, including precipitation for Oregon Division 7 as shown above, can be obtained from NCDC.
The three tree-ring chronologies used as a proxy record for Oregon Division 7 precipitation are from sites named Horse Ridge Recollection, Table Rock-Arrow Gap, and Frederick Butte Recollection. These data, contributed by Dave Meko and others from the University of Arizona's Laboratory of Tree-Ring Research, are available from the World Data Center for Paleoclimatology, International Tree-Ring Data Bank.
The data for the three chronologies can be found by entering their names into the Tree-Ring Search Engine.
References:
For questions on technical or scientific content of this report, please contact:
Richard Heim:For general climate monitoring questions, please contact:
CMB.Contact@noaa.govFor climate data orders, please contact the National Climatic Data Center's Climate Services and Monitoring Division:
NCDC.Orders@noaa.gov
http://lwf.ncdc.noaa.gov/sotc/index.php
