Residents in the valleys of eastern Kentucky recently experienced a rare weather event as the daily range of low and high temperatures exceeded 50 degrees.
The Kentucky Mesonet station near Paintsville Lake in Johnson County recorded a morning low temperate of 38.7 degrees on April 2. As the sun rose and a light breeze stirred, the temperature began to climb to 90.4 degrees by mid-afternoon. The diurnal temperature range, found by subtracting the day’s low temperature from the high, was 51.7 degrees.
The event was not limited to one site. Kentucky Mesonet stations in valley locations near Booneville in Owsley County and Jackson in Breathitt County also recorded a diurnal range that exceeded 50 degrees. A diurnal temperature range near 30 degrees is more typical of this time of year.
“Based on Kentucky’s recorded climate history, this was an extremely rare event,” said Dr. Stuart Foster, state climatologist and director of the Kentucky Climate Center at WKU. “Interestingly, this event marked the 100th anniversary of a similar event that occurred in late March of 1910. In that year, morning frost in many Kentucky valleys gave way to temperatures in the mid to upper 80s in the afternoon at some locations around the state. When we first identified this historical event, we thought that the data were incorrect. Now we understand that was not the case.”
In both instances, a similar set of conditions came together. First, a high pressure system settled over the eastern United States with warm, dry air. Second, the event occurred in the early spring before leaves came out on the trees. Later in the spring, trees with leaves contribute to evapotranspiration that adds water vapor to the atmosphere. This condition (water vapor in the atmosphere) moderates daily maximum and minimum air temperatures. However, with evapotranspiration still low in the early spring, solar radiation has greater heating power during the daytime hours leading to rapid rise in near-surface air temperature.
Finally, the phenomenon is limited to valleys. In the evening hours, cooler air settles in the valleys as the winds become calm. Moreover, dry and clear nights allow rapid radiational cooling. As a result, valley bottoms become much cooler than surrounding ridges and hilltops.
Meteorologists at the Jackson office of the National Weather Service have termed this phenomenon the ridge/valley split. Collaborative research between the National Weather Service and WKU is investigating the conditions that contribute to the occurrence of a ridge/valley split.
“The ability to identify the conditions that lead to a ridge/valley split in advance greatly aids our efforts to forecast daily temperatures in eastern Kentucky,” said Gary Votaw, Science and Operations Officer at the National Weather Service Forecast Office in Jackson. “Under the right conditions, valley temperatures can dip more than 20 degrees below ridgetop temperatures. At other times, early morning temperatures are actually warmer in the valleys. The Kentucky Mesonet is helping us become more aware of the spatial variability for this phenomenon and also how to anticipate its occurrence.”
While meteorologists have been aware of this phenomenon, the Kentucky Mesonet now provides a valuable source of data that will help to improve daily temperature forecasts in eastern Kentucky and elsewhere among the hills and valleys of the commonwealth.
About the Kentucky Mesonet: The Mesonet has 49 stations operating across Kentucky. The stations collect real-time weather and climate data on temperature, precipitation, humidity, solar radiation, wind speed and direction. Data is packaged into observations and transmitted to the Kentucky Climate Center at WKU every five minutes, 24 hours per day, throughout the year and is available online at www.kymesonet.org.
Stations are located in Adair, Allen, Barren, Boone, Breathitt, Breckinridge, Bullitt, Caldwell, Calloway, Campbell, Carroll, Casey, Christian, Clark, Clinton, Crittenden, Cumberland, Fayette, Franklin, Fulton, Graves, Grayson, Hardin, Harrison, Henderson, Hopkins, Jackson, Johnson, Knott, Knox, Lewis, Lincoln, Logan, Madison, Marshall, Mason, McLean, Mercer, Metcalfe, Morgan, Nicholas, Ohio, Owen, Owsley, Rowan, Taylor, Trigg, Union and Warren counties.
Site license agreements have been reached in Lawrence, McCreary, Marion, Muhlenberg and Pike counties. Mesonet officials are actively pursuing sites in about 20 other counties, including Bath, Bell, Carter, Clay, Harlan, Hart, Pendleton, Powell, Pulaski, Rockcastle and Todd.
State Climatologist Stuart Foster is director of the Kentucky Mesonet and the Kentucky Climate Center. Dr. Rezaul Mahmood, associate professor of Geography and Geology, is associate director of the Kentucky Mesonet and the Kentucky Climate Center. The Kentucky Mesonet staff includes meteorologists and staff with expertise in instrumentation, information technology, quality assurance, and education outreach. The Kentucky Mesonet also provides opportunities for WKU student employees and interns to work side-by-side with professional staff.
Initial funding for the project was secured by U.S. Sen. Mitch McConnell through a $2.9 million federal appropriation for the Kentucky Climate Center, part of WKU’s Applied Research and Technology Program in the Ogden College of Science and Engineering.
Contact: Stuart Foster, (270) 745-5983.