Tuesday, March 28, 2017

April's Tornadic, Dangerous Reputation

Jonathan Belles, Jon Erdman
Published: March 28,2017

While tornadoes can be spawned any time of year the volatile mix of climatological ingredients exist, April is a particularly dangerous month for tornadoes in parts of the U.S.
In the period between 1991-2015, April comes in only behind May and June for average U.S. tornado counts. That average is admittedly skewed by April 2011, in which 758 U.S. tornadoes set a record for any month.
(MORE: Tornado Central)
A swath from the central and southern Plains east into the Tennessee Valley is most susceptible to swarms of tornadoes in the first full month of spring. However, the tornado threat also rises markedly in April much farther north into the Mississippi and Lower Ohio Valleys.
Typical April tornado risk areas. Areas typically with the highest risk of tornadoes in April are contoured white.
Tornadoes by month in the Birmingham, Ala. metro from 1950-2015. (Source: Dr. Greg Forbes)
Tornadoes by month in the Dallas metro area from 1950-2013. (Source: Dr. Greg Forbes)
Dating to 1950, here is a list of metro areas who have seen a peak in tornadoes during the month of April, according to severe weather expert Dr. Greg Forbes (Facebook | Twitter):
  • Atlanta
  • Birmingham, Huntsville and Tuscaloosa, Ala. 
  • Cincinnati
  • Dallas
  • Jackson, Miss. 
  • Little Rock
  • Memphis
In addition, while not necessarily a peak month, many more metro areas have experienced a significant rise in tornado counts in April, compared to March:
  • Chicago
  • Houston
  • Kansas City
  • Lubbock
  • Nashville
  • St. Louis
  • Wichita
(MORE: The Future of Tornado Warnings: More Precise, More Lead Time, Fewer False Alarms)
In fact, the three worst tornado outbreaks in U.S. history, according to Forbes, all occurred in April:
  1. April 3-4, 1974 "Superoutbreak"
  2. April 26-28, 2011 "Superoutbreak"
  3. April 11-12, 1965 "Palm Sunday Outbreak"
Four of the top 10 worst single tornadoes in U.S. history also have taken place in April, Forbes says:
Of the 59 tornadoes strong enough to be rated F/EF5 from 1950-2016, just over one-third of those – 20 tornadoes – occurred in April. These include four EF5 tornadoes in the April 2011 Superoutbreak and seven F5 tornadoes in the April 1974 Superoutbreak. To show the occasional northward reach of April's tornado threat, one F5 tornado scoured through southwest Lower Michigan, including the city of Grand Rapids, on April 3, 1956.
Incidentally, April is not just a dangerously tornadic month in the U.S. What is officially considered the world's deadliest single tornado killed an estimated 1,300 in Bangladesh on April 30, 1989.
(INTERACTIVE: Check Your 7-Day Severe Weather Forecast)

Why April is Notoriously Tornadic

To answer this, we have to consider what's happening both near the surface and at jet-stream level.

Avg. April Highs

Average April jet-stream level winds, based on 1981-2010 climatology. Dark red shading indicates typical location of strongest jet-stream level winds in April.
Heading through spring, the sun is increasingly higher in the sky, so there is more daylight available to warm the Earth's surface.
Ahead of frontal systems swinging through the Plains and South, temperatures will warm into the 70s, 80s, or perhaps 90s in April than in March or certainly February and January.
Coincident with warming temperatures are increases in both the magnitude and depth of moist air ahead of the aforementioned frontal systems. By April, surface dew points in the 60s, or even 70s, penetrate farther north, supplying buoyancy for severe thunderstorms to grow.
(VORTEX-SE Investigates: Tornadoes in the Southeast May Be Influenced by Mountainous Terrain)
Overtopping this increasingly warm and humid pre-frontal air mass is a still-powerful jet stream.
An average April jet stream is strongest over the southern U.S., over that warm and humid air.
A common outbreak scenario involves a pronounced southward dip in the jet stream, or upper-level trough, that plows east into the Plains or South with winds aloft spreading apart, forcing strong upward vertical motion in the atmosphere.
The jet stream also provides deep wind shear, or changing wind speed and direction with height, supportive of rotating supercell thunderstorms.
If wind shear is particularly strong in the first few thousand feet near the surface, these supercells would more likely produce tornadoes.
Check our Tornado Central page for the latest on the current severe weather threat.  

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