Tornado drought: only 3 U.S. tornadoes during May's first week

With just three tornadoes during the period May 1 - 7, 2013 has had the third-fewest U.S. tornadoes during the first week of May since record keeping began in 1950. The only year with fewer tornadoes during the first week of May were 1970 (zero) and 1952 (two.) During the ten year period 2003 - 2012, the U.S. averaged 73 tornadoes during the first week of May, with a high of 239 during May 1 - 7, 2003. The three May 2013 tornadoes have all been week EF-0 twisters, and none have hit the Midwest's tornado alley:

May 2: An EF-0 tornado with 70-mph winds hit Boca Raton, Florida, tossing lawn furniture around, downing a few trees and fences, and breaking one window. The tornado stayed on the ground for just four tenths of a mile and was only 50 yards wide; nobody was hurt.

May 4: An EF-0 tornado damaged barns and outbuildings near Elkton, Florida.

May 6: An EF-0 tornado with 50 mph winds passed through Bakersfield, California. The 25-yard wide tornado lasted two minutes and stayed on the ground for 1/4 mile. This tornado was not related to a supercell thunderstorm like most tornadoes are, but instead was a "landspout" tornado, caused by the interaction of the inflow into a thunderstorm with a ground-based swirling area of winds.


Video 1. One of May 2013's scant total of three tornadoes during the month's first week: an EF-0 tornado with 50 mph winds passes through Bakersfield, California at 7:10 pm PDT on May 6, 2013. The 25-yard wide tornado lasted two minutes and stayed on the ground for 1/4 mile.

Here are the record low years for the May 1-7 period, as I tabulated using 1950 - 2012 data from the Tornado History Project:

1970 - 0
1952 - 2
2013 - 3
1951 - 3
2011 - 4
1966 - 5
1957 - 7
1963 - 8
1962 - 8


Figure 1. Running 12-month total of (E)F1+ tornadoes starting in January 1954 and ending in April 2013. The record high and record low have both occurred during the past three years. EF-1 and stronger tornadoes are better to use when studying long-term trends in tornadoes, since weaker EF-0 tornadoes were significantly undercounted in the first few decades of the historical record. Image credit: Dr. Harold Brooks, U.S. Severe Weather Blog

Unusually cold spring weather to credit for the 2013 tornado drought
The low May tornado numbers come on the heels of an usually quiet March and April for tornadoes. The cause? Unusually cold weather in the Midwest, including last week's remarkable May snowstorm. Thanks to the cold spring in the Midwest during 2013, and the 2012 Midwest drought, the 197 EF-1 and stronger tornadoes that occurred during May 2012 - April 2012 was an all-time minimum for any twelve-month period since at least 1954, wrote tornado researcher Harold Brooks at the U.S. Severe Weather Blog (previous minimum: 247 tornadoes from June 1991-May 1992.) The death toll of just seven was also a record low for any twelve-month period since 1950. This is all the more remarkable since this record minimum in tornado numbers occurred less than two years after the record maximum: 1050 EF-1 and stronger tornadoes from June 2010 - May 2011. The extraordinary contrast underscores the crazy fluctuations we've seen in Northern Hemisphere jet stream patterns during the past three years. Call it "Weather Whiplash" of the tornado variety. A blog post by meteorologist Patrick Marsh of NOAA's Storm Prediction Center argues that the record 12-month tornado maximum of 1050 EF-1 and stronger tornadoes from June 2010 - May 2011 was a 1-in-62,500 year event. The record 12-month minimum of 197 EF-1 and stronger tornadoes that occurred from May 2012 - April 2013 was a 1-in-3000 to 1-in-4000 year event. In Marsh's words: "Anyway you look at it, the recent tornado "surplus" and the current tornado "drought" is extremely rare. The fact that we had both of them in the span of a few years is even more so!"


Figure 2. A reminder that May tornado seasons that start out quietly don't always end that way: the May, 2011 tornado season had only four tornadoes during the first week of May. However, the EF-5 tornado that hit Joplin, Missouri on May 22, 2011 cost $2.8 billion, making it the most expensive tornado in world history.

The forecast: quieter than usual during the coming week
The weather will warm up over tornado alley the remainder of the week, but tornado activity should will stay well below average--NOAA's Storm Prediction Center is forecasting a "Slight Risk" of severe weather over isolated portions of the country Wednesday through Friday, with damaging winds, not tornadoes, expected to be the primary threat. But things can change in a hurry in the Midwest. The first week of May in 2011 only had four tornadoes, yet that month ended up as one of the most devastating tornado months in history: 326 tornadoes, 178 deaths, and well over 1,000 injuries. The EF-5 tornado that hit Joplin, Missouri on May 22, 2011 cost $2.8 billion, making it the most expensive tornado in world history.

Jeff Masters