The supercell setup

Home damage from Tuesday courtesy Gary H on Click2pins

If you were affected by Tuesday’s tornado outbreak, you probably don’t care much if this weather set-up compared to the Channelview tornadoes back on Nov. 21, 1992. The comparison, however, is remarkable and we can always learn from the past when it comes to just how weather situations come together.

We sometimes think too casually that, when cold, dry air meets warm, moist air, we get storms and tornadoes. That would be where a cold front forms and warm air is forced up into the sky very quickly where it cools to create clouds, rain and storms. But that cold/warm collision doesn’t always cause strong storms. That is just the beginning.

We additionally look for the presence of a low pressure system near us. That is important because when air pressure gets lower, the winds circulating around the center get stronger. The lower the pressure, the stronger the winds. When that low is right over you, so are the high winds. Think of that like the eye of a hurricane -- the closer you are to the eye, the stronger the wind (and to be clear, the eye of a hurricane IS where the lowest pressures are). So if that low is nearby, watch out, that is the center of energy!

Finally, the mid and upper level winds, the jet streams, must be strong and moving right across the supercell. That’s the most critical part. In our situation, southeast winds are coming from the Gulf, northwest winds are coming from the approaching low, and now the jet streams are supplying strong southwest and west winds. All those winds from different directions, called wind shear, begin the rotation that forms a tornado. In addition, the jet stream winds literally tilt the storm and that tilt separates the air going up and down keeping the storm unstable. That instability allows it to continue for miles and miles before finally collapsing.

The bottom line is that all those ingredients don’t come together very often to form a supercell thunderstorm and, even when they do, only 30% of supercells manage to form tornadoes. Of course, when that happens, watch out.

Looking back at the Channelview tornadoes we can clearly see the cold front and the low in almost the exact same place as where we found Tuesday’s front and low. I have them side-by-side below:

courtesy National Weather Service archives

And then there is the jet stream wind which was amazingly strong last Tuesday. Here is the wind around 18,000 feet up running 70 to 80 mph:

This is the 500mb chart showing winds from the southwest at 70-80mph. Courtesy

Even stronger winds were found at 34,000 feet where commercial airplanes fly at around 150 mph. Notice the dark red area of strongest winds!

Jet stream winds right over our area Tuesday around 150mph. Courtesy

I don’t have a comparison of jet stream winds from 1992, but I can assure you they were there and they were just as strong. For what it’s worth, you might recall the tornado outbreak from Halloween 2015. Look at those similar map features:

Same synoptic set up for a tornado outbreak! Courtesy National Weather Service

So while cold fronts and low pressure systems often move across the area, the upper level winds are key to big tornado outbreaks. Fortunately, all those parameters at the same time are rare for our area. But experience makes for better forecasting the next time.

Have a great weekend and watch the rain on Sunday, especially on feeder roads and low-lying areas!


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About the Authors:

KPRC 2's chief meteorologist with four decades of experience forecasting Houston's weather.