Outbreaks of severe thunderstorms and tornadoes have ramped up in recent weeks. There is one big reason why, but a lot has to happen before a tornado can form.
Warming Gulf Waters
Early in the spring, we commented that colder-than-average weather in the South this winter greatly chilled the Gulf of Mexico. This below-average trend continued through April, despite well above-average air temperatures since March. It takes water much longer to warm (and cool) compared to the air.
A recent satellite image of surface Gulf of Mexico water temperatures. Orange and red areas indicate temperatures in the 80s. Light blue and yellow areas indicate temperatures in the 70s. Image courtesy of Rutgers University.
Water temperatures continued to average well below normal through April, but this deficit now has to be made up. Most waters in the Gulf have now warmed to seasonal levels and in some cases are now trending above average.
High Dew Point Temperatures
Warm Gulf of Mexico waters are a major ingredient for violent thunderstorms and tornadoes. The warmer the water, the higher the dew point can become in the air passing over the shallow sea.
The dew point is the temperature to which the air must be cooled to become saturated. The higher the dew point, the easier it is for the atmosphere to become saturated. A dew point temperature of around 70 degrees seems to be a magic number, and we had dew points around that mark on Monday over the southern Plains.
In order to start the saturation process, there has to be substantial heating of the ground by the sun. The rays of the sun are intense during May. With each passing day, 90-degree temperatures become more common.
Temperatures surged into the 90s over part of the area where the thunderstorms formed on Monday.
Rising columns of heat formed and cooled over the southern Plains Monday, allowing the towering clouds to form.
In many cases, simply hot, humid days are not enough to bring out the worst thunderstorms. While, yes pop-up, garden variety thunderstorms are quite common during the midsummer, to get those nasty, damaging thunderstorms you need an extra boost.
The approach of a cold front, a sea breeze boundary or even a wedge of dry air can be enough to give the rising hot air the extra boost to get high enough in the atmosphere to take advantage of wind shear.
We had a dry wedge of air advancing eastward into the hot, humid air on Monday. That phenomena is known by meteorologists as a "dry line."
It is a known fact that winds increase with height. However, this increase can be much stronger in certain situations and is known as wind shear.
Wind shear causes the air to rotate in the vertical near the ground. As this rotation reaches the rising column of air around a thunderstorm, it causes a spin in the horizontal, in turn often causing the entire thunderstorm to rotate.
The more heating, moisture and wind shear available, the faster the rising, cooling and spinning of the air.
A Tornado is Born
It is in these established rotating thunderstorms that where most tornadoes are born. While the thunderstorm itself becomes a large, slowly rotating vortex, a tornado is a much smaller, yet much faster, spin-off vortex from the parent thunderstorm.
The Setup Tuesday Night/Wednesday on the Plains
While the conditions are not quite a favorable for violent thunderstorms and tornadoes over the Plains with this second outbreak, more than a handful of twisters could still be triggered.
The air is starting off cooler and drier, and the wind shear is not quite as intense. However, the Gulf of Mexico has not gotten any cooler and the sun is a little higher in the sky than the day before.
Story by AccuWeather.com Expert Senior Meteorologist Alex Sosnowski
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