Sunday, April 30, 2017

Above-Average Tornado Activity Doesn't Necessarily Mean Much for Climatological Peak of Tornado Season, History Shows

Jonathan Belles
Published: April 27,2017

With a few days left in April, the first four months of 2017 are off to a quick start for tornado activity in the United States.
Tornadoes occur when the right conditions line up in the atmosphere. Meteorologists usually look for certain patterns that can bring severe weather – including tornadoes – and these patterns often come and go over periods of weeks or months.
Therefore, this early-year activity doesn't conclusively say anything about the number of tornadoes we'll see in May or the rest of the year, NOAA says.
The number of weather systems that will feed on favorable ingredients beyond a week into the future is hard to forecast, but one thing is for sure: there will be tornadoes in May. How many? Stay tuned; there have been more than usual so far this year.
(MORE: Tornado Central | Your 7-Day Severe Weather Outlook)

Very Busy Start for Tornadoes

Preliminary tornado reports through Tuesday remain above the 10-year average from 2005 to 2015 with nearly 500. According to NOAA, 2017 is already more than halfway to the seasonal total of 2016 which was a below-average tornado year with 971 total.
Preliminary tornado reports Jan. 1-April 23, 2017.
(Storm Prediction Center/NOAA)
Usually, during the winter months, the warmth and needed moisture are capped to the Gulf Coast states. This year, favorable winds aloft and at the surface brought the unseasonably favorable conditions northward – sometimes as far as the Midwest.
(MORE: U.S. Warm Records Crushing Cold Records by Over 5-to-1 Ratio)
The above-average temperatures that started 2017 likely contributed to the higher-than-average tornado counts in the first four months of the year.
The number of reports is usually filtered and condensed by the Storm Prediction Center (SPC) to get a more accurate count. Often, multiple people will report a single tornado, and these reports are all added to the list of local storm reports (LSR's). The actual number of tornadoes is usually less than the tornado reports.
Tornado count and percentiles from 1954 to April 25, 2017. Data is preliminary for 2017.
(Storm Prediction Center)
Although tornado counts are still being filtered and verified, the number of actual tornadoes is approaching 500 through Tuesday, which is near a record for most tornadoes to-date since 1954. This number of actual tornadoes has flirted with or surpassed record levels to date since the beginning of March.
2017 would be the first time since records began in 1950 with the first four months topping 100 tornadoes in the U.S., according to Steve Bowen, director of impact forecasting at the reinsurance company Aon Benfield.
It is important to note tornado counts have been, at least in part, artificially growing since 1950 due to the rise of storm spotters and social media – especially in the last couple decades.

May is the Peak of Tornado Activity

Although there is no defined tornado season in the U.S. since tornadoes occur year-round, tornado activity peaks in the late spring.
(MORE: Tornado Risk by Month)
"Tornado season" occurs whenever the best recipe for severe weather ingredients comes together in a given location. For communities farther north, tornado season is shorter and occurs when temperatures are generally warmer. To the south, this season can be year-round.
May is the peak month for tornado activity with an average of 276 twisters during the 20-year period spanning 1996-2015. June is second with 217 tornadoes, on average.

The 1991-2015 average number of May tornadoes in the U.S. is 269, according to
During May, activity shifts from what is typically marked as Dixie Alley in the South to the Plains' Tornado Alley. This is typically where low-pressure systems are greeted with an abundance of moisture from the Gulf of Mexico and plenty of heat to kick off storms.
According to the SPC, May owns five of the top 10 most damaging tornadoes in terms of inflation-adjusted damage:
(MORE: The Most Tornado-Prone Locations in the U.S.)
According to Dr. Greg Forbes, severe weather expert at The Weather Channel, four of the top 10 tornado outbreaks have also occurred in May:
These outbreaks are ranked using 11 parameters combined into a "Forbes Impact Index."
MORE: Plains Severe Flooding, Tornadoes, May 24 - June 3, 2016

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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