As the leaves fall across Kentucky, they are falling on dry ground. Following widespread flooding across a large portion of Kentucky in early May, the late summer and early fall have seen the emergence of drought.
Precipitation totals have been running well below normal from western Kentucky through the Ohio River Valley. Dry conditions have spread eastward through the Bluegrass and are now starting to become evident in the mountains of eastern Kentucky. (A list of precipitation statistics from selected Kentucky Mesonet sites has been provided by the Kentucky Climate Center at WKU.)
The drought developed during the late summer months of 2010 and was caused primarily by a broad ridge of high pressure that was centered over the Gulf Coast states. Surface winds were directed from the Gulf, which brought high humidity and hot temperatures to Kentucky throughout the summer.
“Typically, the combination of hot temperatures and high humidity is enough to spark drenching thunderstorms throughout the summer,” said Greg Goodrich, assistant professor of geography and coordinator of WKU’s meteorology program. “In the summer of 2010, subsidence from the persistent ridge of high pressure along the Gulf Coast suppressed thunderstorm activity and allowed the drought to develop.”
A lack of landfalling tropical systems also added to the rainfall deficit.
“Droughts are unique. Each one has its own signature as reflected in how widespread it is, when during the year it develops, and how it intensifies,” said Stuart Foster, state climatologist and director of the Kentucky Mesonet and Kentucky Climate Center at WKU. “This drought has a couple of important characteristics. First, it has intensified very rapidly through the late summer and into fall. Second, while impacts on agricultural and increased fire risk are evident, impacts on municipal water supplies have thus far been almost nonexistent.”
Above normal precipitation during spring and into the summer means that reservoirs in the upper reaches of some of our major river basins have been full, helping to supplement water availability in drought-affected areas.
Analysis of previous late summer droughts suggests that the dryness is likely to continue. “More often than not, dry weather in late summer continues into the fall,” Foster said. “Yet, if the weather pattern does change, we can go from very dry to very wet.”
Looking back more than a century, when conditions were as dry as they have been this year in August and September, the months of October and November were more often than not below normal for precipitation, particularly in the western half of the state. The late summer drought of 1953 was particularly intense, and lasted into December before more normal precipitation returned. More recently, 1998, 1999 and 2008 were dry through August and September with precipitation remaining below normal during October and November. Meanwhile in 1983, a dry period in the late summer was followed by an unusually wet period in October and November.
Rarely does a late summer drought extend through the following winter in Kentucky. “A hot summer with below average rainfall is common when La Niña develops in the tropical Pacific, as it did this year,” Goodrich said.
The previous La Niña summer of 2007 also featured drought with record heat in Kentucky, most notably in August. Fortunately, most La Niña winters experience above average rainfall, although La Niña autumns are mostly dry. Based on the historical record and the outlook for La Niña, there is hope for relief through the winter months.
Contact: Stuart Foster, (270) 745-5983; Greg Goodrich, (270) 745-5986.