Drought - December 2007
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|Contents Of This Report:||
|A series of weather
systems moved across parts of the western and southeastern drought
areas during December, bringing much needed short-term relief. Dry
conditions prevailed from southern Texas
into the Tennessee Valley, over parts of Florida,
and across other parts of the West.
The December precipitation pattern over Alaska
was mixed, but generally dry conditions dominated over the last
3 months. December was wetter than normal across most of
Rico and the Hawaiian
In the Southeast, three cold frontal low pressure systems in the last days of the month dumped abundant rainfall along a path from the Louisiana delta and southern Alabama, across central Georgia, to the Carolinas and southeast Virginia. The rain was not enough to end the long-term drought, where deficits of 15 to 30 inches had accumulated during 2007 in the core dry areas. But it did reduce the size of the exceptional drought area and contracted other drought categories on the U.S. Drought Monitor (USDM) map. According to USDM statistics, the percent area of the Southeast experiencing exceptional (D4) drought (the worst drought category) shrank from 31.5% on December 4, 2007 to 22.0% by January 1, 2008.
For the western U.S., a broad area of above-normal precipitation fell from the Southwest to the central Rockies, with smaller areas in the Pacific Northwest and northern Rockies. Some improvement to drought conditions occurred in the Southwest during late November and early December, but the rain and snow were not enough to make up for the long-term precipitation deficits. About 48% of the western U.S. (Rockies westward) fell in the moderate to extreme drought category, and about 32% fell in the severe to extreme category (as defined by the Palmer Drought Index) by the end of this month. According to USDM statistics, the percent area of the West experiencing moderate to exceptional drought hovered around 54% throughout the month. For the contiguous U.S. as a whole, all USDM drought categories (D1-D4) shrank from 58.6% on December 4 to 54.6% by January 1.
Long-term dryness also persisted in the western Great Lakes and northern and southern High Plains.
|Southern Texas (climate
division 9) has experienced extremely dry conditions during the
last four months, with 2007 having the third
driest October-December in the 113-year record. This follows
very wet conditions earlier in the year, which made 2007 the
year on record in spite of the recent dryness.
The Southeast has experienced persistently dry conditions during 2007. The core drought area stretched from the southern Appalachians to eastern Mississippi. This core area experienced the 39th driest December in the 113-year record, but the driest May-December through January-December (see table below).
The record dry year in 2007 caused the Palmer Drought Index for this area to rapidly reach extreme drought conditions. Even though 2007 shattered previous annual precipitation records, the cumulative precipitation deficit over 2 consecutive years was barely a record, with the droughts of 1925 and the 1950s reaching similar levels of severity.
The persistent dryness has depleted soil moisture, ravaged pastures, and dried up streams, wells, and springs. Voluntary and mandatory water restrictions continued in many communities in California and in the Southeast. Soil moisture was most severely affected in the Southeast, western Great Lakes, northern High Plains, and parts of the Far West. Streamflow (both modeled and observed) averaged below-normal in the western Great Lakes, parts of the West and New England, and especially in the Southeast. As of January 1, reservoirs in most western states averaged below normal.
A more detailed drought discussion can be found below.
According to the Southeast Regional Climate Center, drought continued to dominate the climate of much of the Southeast region, a condition which has persisted for at least 5 months in many areas, creating an ever-increasing number of water supply problems. By December 11, 36% of the area was suffering an exceptional drought. Thanks to two significant rainfall events in the second half of the month, this was down to 22% by year's end. Only the Florida panhandle, the Atlantic Coast, and the extreme northwestern portions of Virginia were free of some degree of drought, and 74% of the area had at least a moderate drought. The rain events produced amounts between 4 and 7 inches across most of the region but were particularly intense in the coastal areas of South Carolina and Georgia. In Savannah, Georgia, 7.30 inches of rainfall was received in a 24-hour period spanning December 20-21, contributing to a monthly total of 9.44 inches, the wettest December on record. This station, after a November which was the 6th driest of any month since the beginning of the 20th century, was well on its way towards the driest year on record until almost the very end of the year.
With temperatures running some 4-8° F higher than normal throughout December, and with low relative humidities in many areas, the threat of forest fires was added to the drought concerns.
For the first time in the agency's history, the South Florida Water Management District (SFWMD) on December 13 declared an extreme District-wide water shortage, directly affecting more than five million South Florida residents and thousands of farms and businesses. At its monthly meeting, the District's nine-member Governing Board adopted a groundbreaking water shortage order, Modified Phase III, instituting a one-day-a-week watering schedule for residential landscape irrigation to conserve regional water supplies, beginning January 15, 2008. The water level in Lake Okeechobee, the source of water for the Florida Everglades and the primary back-up water supply for five million South Floridians, was at its lowest elevation ever recorded for the month of December.
As noted by the Southern Regional Climate Center, during the month of December, precipitation totals were generally above normal in the north and below normal in the south. The only the exceptions were in central and eastern Tennessee, where monthly values were below normal, and in southern Louisiana, where values were above normal. In southwestern Texas, conditions were quite dry with most stations reporting less than one inch of precipitation for the entire month. For the majority of these stations, this equates to 50% or lower of normal precipitation. Similar percentages of normal precipitation were also observed in south central Tennessee and northeastern Mississippi. In Mississippi, precipitation totals were generally at most only one to two inches below normal, except in the northeast and east central portions of the state, where four inch monthly deficits were observed. In Tennessee, lower than normal precipitation in the central and eastern portions of the state allowed drought conditions to persist. As was the case last month, the severe drought conditions were concentrated in the eastern half of the state. As of the end of December, 53.8% of the state was in severe drought or worse, of which 46.8% was classified as extreme drought, and 19.9% as exceptional drought. Small areas of moderate drought were present in south central Texas and northwestern Oklahoma.
As explained by the Midwest Regional Climate Center, December was a wet month across all but a few isolated areas of the Midwest. Precipitation was heaviest in Minnesota, Iowa, and Wisconsin, as well as along the Ohio River. The only areas with slightly below normal precipitation were portions of southern Lower Michigan and extreme southeastern Kentucky. At the end of the month, the U.S Drought Monitor continued to depict extreme drought in extreme southeastern Kentucky, and small areas of moderate drought in Minnesota, Wisconsin, Michigan, Missouri, and Illinois.
As noted by the High Plains Regional Climate Center, December precipitation was above normal for much of the High Plains region, while the western Dakotas remained dry. There was little to no change in the U.S. Drought Monitor for the region, other than an expansion of the abnormally dry to moderate drought (D0-D1) categories which took place across portions of western Kansas and eastern Colorado.
As summarized by the Western Regional Climate Center, precipitation varied greatly across the West with the Intermountain and Southwest well above normal and most of Montana extremely dry. The Pacific coast region was mostly normal to slightly above. By the end of the month, mountain snowpack was near or above normal for most of the West except for the Sierra Nevada of California which languished at 53% of normal. In Alaska, most of the primary stations were drier than normal for December with snowfall also below normal. For the year, 2007 was generally above normal in temperature and, except for parts of the Intermountain region, below normal in precipitation. The Los Angeles Airport ended 2007 with their 4th driest year (4.89 inches), based on records dating back to 1945.
The NOAA Alaska-Pacific River Forecast Center reported that National Weather Service, USDA Natural Resource Conservation Service, and Environment Yukon snow course and telemetered snow sites indicated that accumulated snow water equivalents throughout interior Alaska are generally less than half of normal for December. Snow accumulations in the upper Chugach mountains of the Kenai Peninsula were near normal, while accumulations in the Canadian Yukon River basin were slightly above normal, and snow water equivalents in the southeast Alaska panhandle were up to 150% of normal.
As noted by the Northeast Regional Climate Center, December was wet in the Northeast. Dry conditions eased; according to the US Drought Monitor, only the extreme southern tip of West Virginia and Maryland's Eastern Shore were still experiencing severe drought. To the north, abnormally dry conditions were limited to Rhode Island and eastern Connecticut. For the year, the Northeast received 103% of normal precipitation in 2007. Coastal areas and the southern part of the region saw the driest annual conditions. Maryland received only 83% of normal precipitation; Delaware, 86% and Connecticut, 89%. The three wettest states in 2007 were Maine, New Hampshire and Vermont, each one tallying 111% of their normal yearly precipitation total.
|A detailed review of
drought and moisture conditions is available for all contiguous U.S.
states, the nine standard regions, and
the nation (contiguous U.S.):