January 12, 2006
Cold Decembers often are followed by warm Januarys
WEST LAFAYETTE, Ind. It's no fluke that January temperatures so far have been mild enough that this month can be considered a thaw, according to climatologists based at Purdue University.
Early December temperatures indicated that it could be the sixth-coldest Indiana December in 110 years of recordkeeping, said Indiana State Climatologist Dev Niyogi, a Purdue assistant professor of agronomy and earth and atmospheric sciences. Warming during the last 10 days of the month, however, eliminated this possibility and led into a January thaw, which is something that happens as often as 65 percent of the time in the state following a subnormal December.
"A January thaw exists when maximum temperatures are at least 6 to 10 degrees above the local normal maximum temperature for at least seven consecutive days," Niyogi said. "Of course, the normal maximum temperature will vary somewhat from north to south across Indiana, so we used 40 degrees or higher as our basis for estimating a thaw."
Niyogi and Kenneth Scheeringa, a Purdue professional meteorology assistant and associate state climatologist, studied the years 1981 through 2005 to determine the frequency of January thaws. They found that such mid-winter temperature patterns occur most often in far southwestern Indiana at a rate of about 30 percent. In central Indiana, the rate is about 15 percent, while in north central Indiana it's about 8 percent.
A January thaw occurring following a December with below-normal mean temperatures is even more significant, Niyogi said.
"In the 25 years studied, a below-normal mean temperature month of December led into a warmer-than-normal January about 53 percent of the time in central and southern Indiana," he said. "In northern Indiana, the odds climbed to as high as 65 percent."
In addition, January thaws tend to occur more in the western part of the state because warm air masses approach Indiana from the southwest in advance of fast-moving cold polar fronts, Niyogi said. The warmer air enters southwestern Indiana first and often lingers there before the cold air sweeps into the entire state. Also, snow cover tends to be greater in eastern Indiana, slowing the warm-up process.
The mild end of December and beginning of January has offset the especially cold late fall and early winter, the climatologists said. The winter mean temperature has returned to normal.
The rest of the winter in Indiana will continue to have swings from warmer-than-normal to colder-than-normal temperatures. The temperature will depend on regional wind patterns since no large-scale climate system, such as El Nino, is dominating weather patterns this season, the climatologists said.
Writer: Susan A. Steeves, (765) 496-7481, firstname.lastname@example.org
Sources: Dev Niyogi, (765) 494-6574, email@example.com
Kenneth Scheeringa, (765) 494-8105, firstname.lastname@example.org
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