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Tornadoes On Vacation?
by Judy Graphenteen

In the United States, tornado season, which typically peaks from March through July, has officially ended. This year, there was an active early tornado period during the months of January through mid-April, with a particularly dramatic day of tornadoes on April 14, 2012 (over 80 tornadoes across the central United States). But after mid-April, the tornado count drastically dropped, and this 2012 tornado season became extremely quiet. July 2012 recorded a preliminary report of only about 24 tornadoes, which is unusually low. Overall, the 2012 tornado season was the complete opposite of the 2011 season.
 It appears that La Niña months bring a heighten risk for tornadoes across the United States. The jet stream typically produces stronger winds, which adds more shear to thunderstorms. This shear is one of the many vital ingredients needed for tornadoes to form. Early 2012 brought upon a weak La Niña, however, spring 2012 saw a fading La Niña that eventually became neutral. High pressure has simply dominated the weather pattern across the central United States. High pressure has provided a lot of sinking air that has produced extensive drought and massive heat waves across the central and eastern United States. Tornadoes typically occur along the northern portions of the United States in the summer months, when the jet stream is further north along the US/Canadian border. However, the jet stream was further north, and severe weather and tornadoes became a bigger issue for Canada in parts of Saskatchewan.
 The warm and dry conditions this past spring along with the blazing heat and drought this summer gave us a lack of tornadoes. Unusual warmth during the late winter and early spring, including warm Gulf of Mexico waters, likely contributed to a high number of early-season tornadoes. A strong jet stream left over from the La Nina pattern also played a role. A persistent southerly flow frequently drew warm, moist air from the Gulf, and combined with the strong jet stream; both necessary ingredients for violent thunderstorms and tornadoes. The ground was much warmer than usual over the central part of the nation. Typically in the early spring, a cold ground enhances a temperature inversion that really limits thunderstorm formation. More recently, a diminishing La Nina pattern has allowed the corridor of tornado-producing thunderstorms to shift farther west, when compared to last year.   During the summer, while necessary surface heating is present, the jet stream is typically rather weak. In the fall, while the jet stream strengthens, heat needed to generate violent storms is decreasing. Exceptions to this rule include some tropical storms and hurricanes during the late summer and early fall. Pockets of gulf waters are still unusually warm. In order for severe tornado outbreaks, everything needs to be perfect in the atmosphere. Despite quiet weather, people should not let their guard down.  
 --  Judy Graphenteen  




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