Published: 2008-05-21 13:32:03
Updated: 2008-05-21 13:32:03
Posted May 21, 2008 1:32 p.m. EDT
MIKE MOSS SAYS: Jimmy, The tornado alley area of the eastern plains and southern midwest is often subject to relatively warm, most air in the lower atmosphere flowing north from the Gulf of Mexico in advance of cold fronts and deepening upper level troughs pushing in from the northwest (the upper toughs and surface lows sometimes in the process of strengthening along and behind the fronts due to the influence of strong air flow across the Rocky Mountains). Lift due to the front, to upper level divergence in advance of the upper trough, and sometimes to prefrontal convergence at low levels can combine to produce bands of showers and thunderstorms that form in an environment characterized not only by great instabilty, but also by strong vertical wind shear, often with both large amounts of speed shear over a great depth and a notable directional turning near the surface. These conditions are favorable for the formation of supercell thunderstorms, and when the combination of factors is maximized, for the formation of strong to violent tornadoes.
The topography of North America plays into making this combination of ingredients most common across the "tornado alley" region during the spring and early summer and again to a lesser degree in the fall. On our side of the Appalachians, it is not unusual to have less unstable air trapped in place during this time frame or to have the most intense lift associated with the lows (surface and aloft) that form to our west bypass us as they track across the midwest or northeastern U.S., leaving tornadoes on average fewer and weaker. That being said, it certainly is not impossible for the pattern to occasionally favor our part of the country with the proper combination, and we have a history that includes tornadoes of F3 and occasional F4 intensity to show for it, we just are not climatologically favored as much.
You asked more specifically about the summertime, when we tend to become most unstable and to have the highest dew points (and therefore plenty of low level moisture). That much is tru, and it is also truwe that in the summertime uneven heating in the mountains of our state often serves an an initiator of thunderstorms (along with seabreeze fronts due to differential heating between our land surface and the ocean to the east over the coastal plain). However, by the time we get well into summer, a couple of important factors begin to play a smaller role. By then, cool airmasses have retreated well to our north, along with the most significant frontal boundaries and the strong upper level disturbances, traveling surface waves of low pressure, and associated strong winds and wind shear that go along with those features. So, in many cases our summer thunderstorms occur in an environment that is quite unstable due to low level heat and humidity, but they often are lacking in deep moisture and they often are lacking in the larger scale dynamics that help to organize ordinary since or multi-cell thunderstorms into the supercells that are more significant tornado threats. For that reason, many of our summer thunderstorms are of the "routine" variety and most of those that reach severe limits do so based on hail size or the potential for brief but intense downburst winds driven by precipitation drag and/or evaporative cooling. That doesn't mean a tornado is impossible in the summer here, but they are less likely than in the spring or fall, and they are much less likely than in "tornado alley" during those seasons, despite our nearby mountains and the Gulf Stream to our east.