Conditions That Prevent Powerful Tornadoes From Forming


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Mother's Day tornadoes

The forecast for Mother’s Day, May 11. A tornado outbreak is possible in Oklahoma and Kansas. Courtesy of NOAA

The anticipated outbreak of tornadoes did not materialize on Thursday, May 8, despite the fact that the general atmospheric flow was eerily similar to the one that accompanied Severe Storm Outbreak Aardvark last week.

Aardvark produced well over 100 tornadoes up to EF4 intensity, with resultant loss of life and massive destruction of property.

Thursday, there was a handful of Ef0 tornadoes, with no injuries and little damage.

Subtle differences in the vertical structure of the systems were responsible for the drastically different results.

All The Ingredients Seemed To Be In Place For Another Round Of Tornadoes

Warm and humid air streamed north from the Gulf of Mexico, all the way to the upper midwest. A vigorous trough (dip) in the jet stream crossed the Rocky Mountains and the associated cold front collided with the warm air. There was wind shear (twisting of the wind with height). What dampened the power of this system?

Understanding how weather works is key to prediction. Image by Decoded Science, all rights reserved.

Understanding how weather works is key to prediction. Image by Decoded Science, all rights reserved.

The three major ingredients of instability that leads to violent weather are:

  • Warm air near the surface and cooler air above. The vertical temperature profile is called the lapse rate. The greater the lapse rate (the more rapidly the temperature decreases with height), the more likely it is that an upwardly-displaced parcel of air will find itself warmer than its surroundings and keep rising. The atmosphere is then unstable.
  • Humid air at the surface and dry air above. When it is lifted, a dry parcel of air cools at a faster rate than a saturated parcel of air. That is because cool air cannot hold as much moisture as warm, so as the saturated parcel rises and cools, some of the moisture condenses. Condensation releases latent heat, and this added heat lowers the cooling rate of the rising saturated parcel. The result of lifting a column of air which is saturated near the bottom and dry near the top is to increase the lapse rate, and therefore increase the likelihood of instability.
  • A reason for air near the surface to rise. One possibility is a cold front which physically displaces the air at the surface. Another is daytime heating of air near the ground (The sun heats the ground and the ground heats the adjacent air).

Why No Tornadoes?

There were a number of factors in place this week that differed from those that caused last week’s meteorological mayhem: It’s now looking more like Beaver will be expecting publicity on Sunday. Will morning be good?

  • Vertical Temperature Structure This Week: The Weather Wore A Warm Cap. Last week’s trough pushed cool air above the warm flow from the Gulf of Mexico. This week, there was a warm cap above the Gulf air which was difficult to dislodge. The warm cap did not eliminate the possibility of instability but reduced it.
  • Vertical Humidity Structure This Week: The Warm, Humid Gulf Air Was Deeper. Last week’s incursion of warm air from the Gulf of Mexico was shallow; this week’s was somewhat deeper. As a result, more lifting of the air was required to increase the lapse rate. Some instability still occurred, but it produced mainly thunderstorms. The few tornadoes formed far from the Gulf, where the warm, humid air mass was shallower.
  • This Week’s Cloud Cover Made a Difference. The final inhibiting factor this week was cloud cover, which was more extensive than that with Aardvark. Cloud cover reduces the heating from the sun.

All in all, the air was unstable Thursday and there was rain, hail, thunder, lightning, wind, and a few tornadoes. Since the air was saturated in a deeper layer, there was more moisture available, and several areas experienced flash floods. In locations where updrafts were strong enough, hail up to three inches in diameter fell. But on the whole, the weather associated with this upper air trough was not severe enough to warrant a name.

The Next Severe Weather Threat

Some severe weather is possible today (Friday), as the cold front moves east. The cloud cover may not be as extensive, so an afternoon tornado is not out of the question. But overall this system is becoming more stable, as the upper air trough begins to flatten out.

The next severe weather outbreak worthy of a name will be called Beaver. Another trough is approaching the west coast and will reach tornado alley in Oklahoma and Kansas on Sunday, May 11. Whether it will create a severe outbreak, including tornadoes, is a fifty-fifty proposition at this point. May is the height of tornado activity in most years, but all of the variables have to come together to make an Aardvarkian tornado outburst.

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