©2009 by WindLogics Inc.åÊ Reprinted by permission.

Tornadoes are one of the most intriguing weather phenomena of the Plains and Midwest.åÊ These severe storms can occur just about anywhere in the U.S., but they tend to be concentrated in the region from Texas northward through South Dakota and Iowa ‰ÛÒ a region known as “tornado alley.”åÊ This is the region where the two necessary ingredients for severe storms, abundant low-level moisture and vertical wind shear (changes in speed and direction with height), are most frequently found.

Every severe weather season is unique, but 2009 has been especially unusual.åÊ April, May and June are typically the most active months for severe storms, but this year these storms went into hiding during the climatological peak of the tornado season. In tornado alley, the number of tornadoes was approximately 75% below average from mid-May to mid-June.åÊ Many areas of the Northern Plains reported only 10-20% of the normal number of tornadoes during the peak of the season.

Unusual Weather Pattern Diverts the Jet Stream

To understand why this occurred, we must look westward over the Pacific Ocean where an unusual and highly persistent weather pattern called a “Rex Block” developed in early May. This blocking pattern diverted the main axis of the jet stream up into extreme northwest Canada before it dipped southward again into the Northern Plains of the United States.

Normal spring weather systems follow the jet stream from southwest to northeast across the Plains, but in this case the weather systems moved from northwest to southeast pulling cool and dry Canadian air through the central United States.åÊ This created a situation in which the two main severe weather ingredients rarely came together.åÊ The Rex Block broke down in mid-June, allowing a return of more typical severe weather patterns across the Plains, only to become reestablished again in mid-July.

Related Impacts on Wind Energy Production

These weather patterns explain more than just the tornado report anomalies. They also explain some atypical wind energy production across the central United States.

Over portions of the Central and Southern Plains, the position and orientation of the jet stream during May negatively impacted the wind resource as the number of weather systems normally moving through the region was greatly reduced.åÊ Because of the northwest orientation of the jet stream, low-level jet activity (where higher-speed winds sometimes reach down to hub heights) was also below average.åÊ This weather pattern also negatively impacted the July wind resource in the Southern Plains as it prevented the western portion of the Bermuda High from developing – -anåÊ important feature for driving the Plains’ summer wind resource.

But over the Northern Plains and Upper Midwest, this weather pattern positively impacted the wind resource;åÊ the close proximity to the jet stream and stronger than average jet strength ushered more frequent and stronger weather systems through the area than would be typically seen during the late spring and summer months.

These weather events clearly show the value of widespread geographic dispersion of wind plants and also highlight the need to carefully understand the long-term wind resource and variability.åÊ While unusual, the events of the past season are part of normal weather variability.åÊ Wind is a relatively stable fuel source when viewed over longer time periods, but seasonal variability is a natural weather attribute that can best be mitigated through geography, diversity and a long-term view.

Thanks to Dr. Cathy Finley, Senior Atmospheric Scientist, for this analysis.