Weather Break: Tornadoes in January

This is a transcript of the Weather Break radio show for Wednesday, January 16, 2008. This episode was written by Dr. Jon Schrage.

With all the coverage of politics and elections in the news last week, you could be forgiven if you missed hearing about the amazing series of storms that moved across the United States. A series of areas of low pressure--known as cyclones--moved ashore on the West Coast, bringing heavy rains at low altitudes and up to 11 feet of snow in the mountains. Out ahead of these storms, the counterclockwise rotation of the winds around the cyclones pulled very warm air from the Gulf of Mexico northward all along the Eastern Seaboard. On just one day last week, over 80 cities in the eastern US set record high temperatures for the day, with people showing up for the New Hampshire primaries in short-sleeved shirts.

Of course, whenever you have two very different air masses meeting like they did last week in the central United States, a meteorologists starts thinking about severe weather. The boundary between two air masses is known as a front, and the front separating cold, Canadian air in the west from tropical air in the east was unusually strong for the month of January. A strong jet stream aloft above the front provided the necessary wind shear to produce tornadic thunderstorms in five states--Oklahoma, Arkansas, Missouri, Illinois, and, of all places, Wisconsin. The Storms Prediction Center in Oklahoma is saying that there reports from 42 tornadoes on Monday, January 7.

Now, January isn't exactly the peak of tornado season. Tornadoes certainly can happen in January, especially during years of El Nino conditions in the Pacific. However, these tornadoes generally occur about as far south as they can in the US, in the southern states of Mississippi, Alabama, Georgia, for example. Farther north, tornadoes are much more rare in the winter. The news media were reporting that northern Illinois hadn't had a confirmed tornado almost sixty years. The Milwaukee Journal Sentinel reported that there hadn't been a January tornado in Wisconsin since 1844.

Why are tornadoes so rare in northern states in the winter? There are a couple of factors, actually.

Firstly, a strong thunderstorm generally requires warm, moist air at the surface. Air like that is relatively easy to lift into the updraft of a supercell thunderstorm or a squall line. In the winter, we all know that warm, moist air just isn't that common in states like Wisconsin and Illinois, due to the likely snowcover on the ground, the low sun angles during the day, and the short length of day.

Secondly, as mentioned earlier, most tornadic thunderstorms happen along a frontal boundary that separates different air masses. In the spring and summer, tornadoes are particularly common along a type of front called a dryline, which separates hot, dry air coming from places like TX, New Mexico, and northern Mexico from moist air coming from the Gulf of Mexico. Drylines are not likely to form in winter, however, because the usual source regions for hot, dry air are not particulary warm. Without strong drylines to foster the growth of tornadic thunderstorms, only the less-favorable cold fronts and warm fronts are available for tornadogenesis, and these fronts mainly occur farther south during the winter--not surprisingly, in the southern states of Mississippi, Alabama and Georgia.

Last week, the unusually strong cyclones coming ashore along the west coast set up conditions across the central US that somewhat recreated the pattern we would see in the spring, with warm, moist air surging northward and colliding with cooler air along a strong frontal boundary. All the pieces were in place for an extremely rare, January outbreak of tornadoes.