HOUSTON - "Excellent! Joe, we have major action! The cap is breaking and towers are going up 30 miles up the dryline!" That was a quote from the 1996 blockbuster hit, "Twister" as the gang of meteorologists set off to wrangle the atmospheric bull: the tornado.
Scroll to the 13:10 second mark:
The same phenomenon that the movie touched on occurred Tuesday night. A string of violent and deadly tornadoes moved east over the Texas Panhandle and into western Oklahoma destroying countless houses and property and fatally injuring at least one person in an obliterated mobile home park.
In all, some 27 tornadoes were reported on Tuesday evening with countless reports of hail and wind damage.
The nemesis: the dryline.
Drylines are fairly intuitive; they separate the dry desert air of the west from the warm, moist air flowing north out of the Gulf of Mexico. It's simply the line between the two air masses.
To the west of the dryline, the air is often of desert origins and of higher elevation. Even though the warm, moist air out east of the dryline is more buoyant, dry air can still overrun it creating instability. It also cultivates a microburst threat as gulf air evaporates into the dry air aloft. Since evaporation is a cooling process, it can cool the environment in the cloud and create a dense, cold pocket of air that can literally fall out the bottom of the cloud.
Check out how quickly storms can blow up in the radar imagery taken yesterday just hours apart:
Notice in the first image how quiet the Panhandle is in the early afternoon hours. This is likely due to a cap, a mid-level layer of warm air that keeps moisture from rising. However, as the day time wears on and the ground continues to warm, the cap is broken because the surface temperature is equal to or warmer than the air above it allowing storms to erupt along the dryline.
Still can't see the dryline? Check out this image:
The dew points, an indication of how much moisture is in the air, are exceptionally low out in west Texas. Places like Midland reporting only a 16° dewpoint temperature which means the air temperature would have to drop to 16° for the air to become completely saturated! Wow!
Compare that with a place like Dallas that is almost at a 70° dewpoint. That's moist! When a gradient that extreme exists over a short distance, trouble often brews in the form of severe thunderstorms.
Drylines rarely advance as far east as Houston because we are located on the coast where a maritime environment exists almost perpetually. Therefore the violent, long-lived tornado threat is limited. In fact the last strong tornado to hit this region was an F4 tornado that hit Channelview in November 1992.
Next time you see a yellow humped line on a weather map in west Texas, you can almost guarantee, if conditions are right, that severe thunderstorms are sure to follow.
And now you know!
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