Tornado Drought

This weekend saw the largest tornado outbreak of the year, with 37 tornadoes reported on Saturday--primarily across Oklahoma, Kansas, Missouri, and surrounding states, and 17 more tornadoes across the northern Midwest on Sunday. However, no deaths or major damage was reported with any of these tornadoes, continuing the trend of this unusually kind 2005 tornado season.

Only five people have died since January 1, and none in April, May, or June, usually the worst months for tornado activity. The record fewest number is 15, set in 1986. So, 2005 could challenge this record, with the busiest part of tornado season already behind us (however, note that 2002 also was looking this way, with only 11 tornado deaths by June, but 37 fatalities occurred in November of that year). For comparison, tornadoes kill an average of 51 people a year, with 19 of these deaths in May, the deadliest month. For the continental Unites States, May had a preliminary tornado count of 129--the thirteenth lowest on record.

And for the first time ever, Oklahoma went the entire month of May without a tornado. Oklahoma's average is 21 tornadoes in May. The previous low was two tornadoes, in 1988. The record is 90, set in 1999 (the May 3 Oklahoma City tornado of 1999 marked the last F5 tornado to affect the U.S., and remains the fourth most expensive tornado ever--over $1 billion in damage--and the tornado with the highest winds ever measured--318 mph).

Why so few tornadoes this year? A very cold upper-level low pressure system over the Great Lakes and Northeast has provided unseasonable cool weather from the northern Plains to the mid-Atlantic states and prevented much middle-level moisture from moving northward from the Gulf of Mexico. This moisture is the key fuel needed to energize tornadic thunderstorms.

What's the outlook for the remainder of tornado season? The next 10 days of June look to be pretty normal, as far as tornado activity goes. The cold upper-level low pressure over the Northeast is gone now, and has been replaced with a typical jet stream pattern of frequent low presure systems passing across the country that favor the usual amount of tornado activity. Two separate weather systems embedded in this jet stream pattern could trigger tornadoes today over New York and surrounding states, as well as North Dakota and Montana.