from the NWS:
A study published recently in
npj Climate and Atmospheric Science, by Vittorio A. Gensini of Northern Illinois University and Harold E. Brooks of the
National Severe Storms Laboratory, looked into the possibility that tornado frequencies are changing across the United States. Their findings include a decrease in the traditional "Tornado Alley" of the Great Plains and an increase in the Southeast's "
Dixie Alley". This study generated a fair amount of buzz, so we thought we'd briefly list out a few of the more important points mentioned in the study:
- After removing non-meteorological factors, the annual frequency of U.S. tornadoes through the most reliable portions of the historical record has remained relatively constant.
- Detecting spatial shifts in tornado frequency is challenging. Tornadoes are short-lived and affect very small geographical areas. Also, tornado reporting procedures have varied dramatically over the years and from one region to another.
- The study used the Significant Tornado Parameter (STP) to account for tornado frequency. STP is designed to highlight the existence of atmospheric ingredients favoring large storms capable of producing EF2-EF5 tornadoes, like what is typically seen in the Great Plains.
- It should be noted that even if the atmosphere is supportive of tornadoes with the right winds, moisture, and instability, thunderstorms won't form without a strong enough trigger to spark them (like a cold front). The STP does not account for whether or not a trigger is present.
- However, STP values correspond with tornado reports closely enough such that STP is a suitable index to use. STP is especially useful in January, February, March, May, and December.
- The period analyzed in this study was from 1979 to 2017.
- A significant upward trend in tornado frequency was found in portions of the Southeast, Midwest, and Northeast.
- Both tornado reports and tornado environments indicate an increasing trend in portions of Mississippi, Alabama, Arkansas, Missouri, Illinois, Indiana, Tennessee, and Kentucky.
- It should be kept in mind that STP is designed to evaluate environments favorable for large storms typical of the Great Plains, and not necessarily small spin-ups that we see in regions farther east.
- Unfortunately, increases in tornado frequency in the American South juxtapose with a population that is especially vulnerable to tornadoes. The Southeast already represents a maximum in the occurrence of tornado casualties.
Keep in mind that this study was not associated with nor is it necessarily endorsed by NOAA or the National Weather Service. This information is presented here simply because of public interest.
It should be noted that this is just one study. Nevertheless, it is important for residents of the Ohio Valley and southeast United States to always be prepared for severe weather, including tornadoes. Right now, when the weather is quiet, is an excellent time to put together a
tornado safety plan for your family. Have a survival kit and your plan in place well before severe weather strikes!
I would argue that one really needs to start in 1990 or perhaps 1995 for the U.S. tornado
database to best determine trends. Why?
Technological advances, an increase in scientific knowledge and understanding, social
changes, and modifications of standards and practices have affected the overall net
increase in tornado count in the U.S. over time.
1) There is a significant spike in annual tornado count starting in 1953. This was the year
the Severe Local Storms (SELS) unit, forerunner of SPC, was established for around the
clock severe storm monitoring. Tornado reporting was encouraged by the U.S. Weather
Bureau for documentation and watch/warning verification purposes, hence the increase in
count.
2) Starting in 1990, you see another spike to above 1000 in tornado count. Part of the
reason for this is that this year marked the first installation of Doppler radars
(WSR-88Ds) and around this time, storm chasing was beginning to ramp up in
popularity. Both of these factors resulted in more tornadoes being reported, verified,
and documented.
3) During the 1990s, the National Weather Service (NWS) underwent the Modernization
and Associated Restructuring (MAR). Investigation of tornado reports and post-damage
storm surveys at each new Weather Forecast Office (WFO) county warning area
increased, leading to a more representative count of tornadoes per year.
4) Storm chasing really took off after the release of the movie 'Twister' in 1996 and has
continued to grow ever since. This continued to sustain and increase higher annual
tornado counts.
5) By the middle part of the first decade of the new millennium, digital cameras were
rapidly replacing traditional slide and print film. This eliminated the long processing
times of film, allowing for not only more tornadoes to be verified and documented, but
also in a more timely manner.
6) By the end of the first decade of the new millennium, the expansion of social media such
as Facebook and Twitter allowed instant reporting of tornadoes in real time. Wireless
Internet had been firmly established for the now ubiquitous smartphone/mobile device
and combined with built-in digital still picture and video cameras, this made tornado
reporting and verification even faster and more efficient.
7) At the start of the 2010s, all WSR-88Ds were upgraded with dual-polarization
capabilities. This allowed the detection of the tornadic debris signature (TDS) within
storms. This further assisted in the detection and documentation of tornadoes,
especially those in remote areas and embedded in microbursts/downbursts.
8) By the mid-2010s, specialized satellites had been launched with spatial resolution
as high as 5 meters. Open areas with little in the way of damage indicators could
be examined after a storm suspected of producing a tornado occurred. Narrow
swaths in crop fields could now be detected indicating brief tornadoes that
otherwise would have been missed due to remoteness, darkness, or heavy
precipitation.
9) Also by the mid-2010s, drone use became widespread and this assisted in more aerial
surveys of suspected tornado paths, and also allowed surveys in areas not easily
reachable from the ground
The increases in U.S. annual tornado counts have been affected markedly from changes in
technology, population, and public awareness, as three examples. Detection and
documentation of tornadoes continues to improve with time. For example, we are finding
tornadoes in the Rockies and Great Basin are more common than once thought as
population continues to grow and people visit more remote areas with the mobile
capabilities to take pictures/videos of the tornadoes. It is also has become apparent that
tornadoes with squall lines are more common than once thought, due to very detailed and
thorough surveys done by the NWS. Currently, it is estimated about 18% of all U.S.
tornadoes occur with squall lines.
