Cool Weather Contributing to Historically Low Tornado Count

May 9, 2013 · 0 comments

The lack of tornadoes in the United States from May of 2012 to April of this year has been “remarkable,” according to Harold Brooks of NOAA’s Severe Storms Laboratory. In that time, there have been an estimated 197 tornadoes rated EF1 or stronger (exact totals are available through January; estimates for February through April won’t be confirmed until July). That is 50 fewer tornadoes than the previous low for a 12-month period (not including overlapping periods, such as April 2012-March 2013), established from June 1991 through May 1992. (Reliable data go back to 1954.)

The even better news is that there have been only seven fatalities from tornadoes in the last year, which according to Tom Grazulis of the Tornado Project is the second-fewest on record, and his reliable records date back to 1875. The only yearlong period (again, not including overlapping periods) with fewer fatalities was September 1899-August 1900, with 5.

According to AccuWeather’s Alex Sosnowski, the dearth of twisters may be attributed to a pattern of dry, cold air east of the Rockies, which has affected the formation of thunderstorms in Tornado Alley. According to Sosnowski,

The wedge of cool air forces the base of the clouds from the thunderstorms to be higher off the ground.

This setup limits not only the number of tornadoes but also damaging wind gusts, since most of the action is occurring several thousand feet above the ground. The pattern can still produce a number of storms with hail.

Additionally, the jet stream has been farther south than normal, stifling the movement of warm, moist air from the Gulf of Mexico to the southern United States. If this large southward dip in the jet stream continues through the spring and into the beginning of summer, severe storms will be inhibited even as seasonal temperatures finally begin to arrive.