It all began to unfold yesterday morning when the Storm Prediction Center (SPC) in Norman, Oklahoma (which itself was struck by a violent, wedge-like EF3 tornado – the city, not the SPC) had issued a rare 'high risk' warning of violent, large and long tracked tornadoes. Well, that is exactly what happened once they began to develop.
So, what causes a tornado outbreak to occur? To those less versed in tornadic activity and the ingredients that need to come together to cause such an event to take place, here's some helpful information.
First, while there is no single agreed upon definition, generally more than six tornadoes in a day in the same region is defined as an outbreak. These outbreaks typically occur in the Spring months from March through June, and they usually take place in the 'Tornado Alley' section of the Great Plains. May is the most active month, averaging more than 300 tornadoes during the latest 3-year period, which is nearly 25 percent of all tornadoes observed during any given full calendar year.
There are generally three atmospheric criteria that need to come together perfectly (or not so perfectly, depending on how you look at it) for a major tornado outbreak to commence.
- A lifting mechanism, such as an upper level trough, surface cold front, outflow boundary, dry line (or a combination of these), which acts as a focal point for storm initiation.
- Pronounced wind shear, which is a change of wind speed and direction with height. Fast low-level winds also boost the risk.
- A deep layer of high moisture content.
Well, yesterday all these ingredients came together at the right time (or not so right time, maybe?)across Kansas and Oklahoma, and the atmosphere became extremely ripe for supercell thunderstorms to form. Supercell thunderstorms are storms with rotating updrafts that can produce extremely large hail, damaging straight-line winds, and violent tornadoes. The result: long-lived twisters with winds up to 150 miles per hour, spouting hail the size of baseballs, toppling stores, flattening schools, ripping up towns and leaving thousands without power!
More severe weather is possible through Wednesday, including parts of Oklahoma and Kansas.