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Submitted by George Elliott on Sun, 04/28/2013 - 8:06am.
It’s that time of year again. Tornado season is upon us. I’ll talk more about the nature and formation of tornadoes in another paper. In this paper, since it’s the beginning of tornado season across the country, I want to talk about the climatology of these intense storms, as opposed to the dynamical workings.
As warm and moist air from the Gulf of Mexico begins to spread north more frequently during the warmer months (in addition to spreading farther inland, as well), violent thunderstorms are more likely. During January and February, when cold and generally stable air masses cover much of the country, very few thunderstorms reach the intensity necessary to produce tornadoes. For tornadoes to develop, very strong thunderstorms are a must, and thunderstorms need heat and moisture to work with in order to form. Obviously, January and February are not known for frequent visits of warm and moist air across the U.S. That said, the very few tornadoes that occur during January and February usually occur along the Gulf Coast or in Florida. In general, very few tornadoes occur in the west, but a few have occurred in California in January and February when major winter storms slam into the west coast.
It is not until March that tornado season really begins to accelerate. March is the month in which genuine springlike winds begin blowing across the country, particularly across the south. Cold and dry air masses still invade the U.S. on a frequent basis, of course, and these mixing air masses make for prime conditions to develop thunderstorms strong enough to produce tornadoes. The most frequent area for this to occur in March is in all the Gulf Coastal states. This does not preclude tornado weather elsewhere, obviously, but the most frequent occurrence of tornado weather during March is in the states of Louisiana, Mississippi, Alabama, and Georgia.
As the days get longer, and heating from the sun becomes more direct, warm, humid, and unstable air masses begin to more frequently spread across the country, and penetrate farther north. That said, the clashing of air masses, i.e., warm, moist, and unstable air, with colder, drier, more stable air, happens with greater frequency and farther north during the months of April and May. As a matter of fact, maximum tornado occurrence in April and May ranges anywhere from the southern states across the central and southern plains states. Prime locations include Texas, Oklahoma, Kansas, the southern states, and into the Tennessee Valley.
As the days continue to warm, and colder air masses continue to retreat northbound during the latter part of May into June, tornado frequency fades across the south and the plains. At this time, maximum tornado frequency spreads to the upper Midwest, Great Lakes, and Mid-Atlantic states.
As summer becomes entrenched across the U.S., tornado frequency is greatest across the northern tier of the U.S. and New England, although the number of tornadoes are much fewer than during spring.
There are some notes to make regarding this seemingly smooth transition of the tornado season. Firstly, if warm and humid air masses make it far enough north, combined with the proper dynamics, tornadoes can occur anywhere during any month. This paper is dealing with the “normal” progression of tornado season. Also, during the autumn months, when colder air masses are moving farther south, and the warm and moist air is in retreat, tornado occurrence does not retreat in the same way as it commenced during the spring and early summer. Tornado frequency is much less in the autumn, although there is a “mini-pickup” in the south during November.
Not too many tornadoes occur in the west. This is due to topography, and the fact that the western U.S. (anywhere from the high plains westward), is not in a favorable location with respect to clashing air masses. Very few tornadoes occur in New England as well. New England is so far north, the required airmass battles to form tornadoes do not occur too frequently.
Lastly, hurricanes can spawn tornadoes upon landfall. With that in mind, summer and autumn tornadoes occur in areas prone to landfalling hurricanes. Also, Florida frequently is subject to waterspouts (smaller, generally weaker funnels over open waters) that move ashore, thus increasing the statistical occurrence of tornadoes over Florida.
That’s about it regarding the climatology of tornadoes. Remember, though, tornadoes have hit every state, at all times of the day, during all months of the year. We’re just talking averages here.
By: George Elliott