The Texas Dry Line

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Every Spring an interesting phenomenon occurs in Texas. The west to east Jet Stream across the U.S. is still strong and a lee side (east of the Rockies) area of Low pressure develops near the Texas panhandle. Into this area of low pressure rich moisture laden air is drawn in from off the Gulf of Mexico and carried northwestward toward the low. At the same time very warm and dry air is drawn toward the low from the southwest off the desert of the west and Mexico. The dry air and the moist air usually collide somewhere over west Texas. The boundary is easy to find by studying dewpoints across the state. This is the famous west Texas dry line.

The dry line behaves somewhat like a front. The convergence of wind from the southeast and southwest can initiate upward motion and if upper level conditions are right the dry line can trigger monster thunderstorms. The dry line is an inefficent thunderstorm trigger partly because the hot dry desert air runs out on top of the humid air and effectively suppresses thunderstorm development. That condition is what meteorologists call a capped atmosphere. However its that same cap that can make any storms that do form extra powerful. Like a lid on a pressure cooker the pressure to form a thunderstorm (daytime heating, convergence, upper level forcing) can build up to the point where the cap breaks and the updrafts through that break can become extra strong. That’s when the dryline initiates supercell thunderstorms. A supercell is a very large rotating thunderstorm that can produce huge hail and even tornadoes.

Fortunately for South Texas the dryline usually resides in west Texas. That’s the area called “Tornado Alley”. Interestingly enough the dryline usually triggers the violent storms late in the afternoon or evening far to our west. Any storms that fire off there will move slowly eastward and usually arrive here late at night when the atmosphere is cooler here and therefore less energetic. That’s why we are not in “Tornado Alley”. Nonetheless tornadic supercells occasionally do make into our area so we are not totally immune to the power of the Texas dryline.

During summer as the jet stream weakens the easterly trade winds begin to blow across our entire state. The dryline disappears. Humidity is carried far west into the mountains and desert areas of the southwest. There the monsoon season begins. In our area the trigger for thunderstorms becomes the sea breeze boundary on the coastal plain. Thunderstorms form on the sea breeze during the early afternoon and get carried inland toward San Antonio and arrive here during the late afternoon and evening hours. Without a dryline in summer the atmosphere is usually uncapped and much less likely to form huge hail and tornadoes.

So if you ever noticed that most of our springtime thunderstorms arrive at night, now you know why.