Thunderstorms and the Dry Line

Clayton, New Mexico — way east of the Rocky Mountains, out on the Great Plains. A humid June afternoon. We had finished up in court and were sitting around in the judge’s office. The phone rang. The judge handed it to me. My friend, the airport weather forecaster, spoke abruptly: “Margaret, if you’re gonna go, you’d better git!” So after a quick weather briefing — scattered thunderstorms — I preflighted my Navion and took off on the 160-mile flight west to Taos.

I flew regularly from Taos to Clayton for a day’s work. Taos, at 7,095 feet above sea level, is bordered on the east by the Sangre de Cristo Mountains, which rise to 13,161 feet at Wheeler Peak. I usually flew over a lower part of the range before settling down to 7,500 feet along the treeless prairies east of the mountains. Flying to Clayton meant descending to 4,965 feet a few miles from the Texas/Oklahoma border. On the plains, the only airport along the route was in Springer, New Mexico, about 85 miles from Clayton.

So when I took off from Clayton that afternoon, I was flying over terrain that was very well known to me. I was familiar with the occasional working windmills that gave me surface winds, with the various abandoned ranch houses, with the little volcanic hills that speckled the plains. I was familiar, even, with where to look for the white rumps of grazing antelope.

And now in the air, a line of huge cumulus buildups loomed right and left of course, but there was plenty of open sky between them. That’s what “scattered” meant to me. If I needed to turn away, I had a straight shot 90 miles south to Tucumcari. And to the northwest appeared an avenue of clear air beyond the mesas, 85 miles up to Trinidad, Colorado. These were safe airport havens. Flying toward a wide space ahead, I could see far distant to the western plains at the base of the mountains beyond the line of clouds.

Leveling off at 8,500 feet, I checked the panel: 21 inches manifold pressure, 2,150 rpm, fuel pressure 13 psi, oil pressure and temperature normal, EGT and CHT where I wanted them, ammeter showing a slight charge. Everything was good.

When I looked up, those cumulus clouds towering on both sides surprised me. They seemed to be meshing together high above me. I was too close to view the tops. Columns of rain had formed and were falling some miles off to the south and north. So I dropped down closer to the ground, where I could still see farther west, tightened the strap on the mountaineering helmet that I always wore while mountain flying and persevered.

Slogging along, I studied the turbulent, massed clouds closely. Above to my right, a small funnel cloud drooped out of the cumulus, twisting down in a spidery, gray thread then retreating. I’d never seen a funnel cloud before. My escape route to the southeast toward Tucumcari was still open, so I motored on, peering at the writhing patterns in the cumulus ahead. Next time I looked back, the escape route behind me was blocked.

All too gradually I realized that I was approaching the outer edge of a huge circle of cloud, a great, dark vault that rose overhead. Thunderstorms and virga — columns of precipitation that don’t reach the ground — flanked the sides of my route. The airplane engine droned steadily on, and I flew under the eastern rim of the dome. The air was nice and clear under there — inviting, opening a course westward beyond the far rim out on to the sun-dappled plains. The needle on the rate of climb indicator twitched and rose to 500 fpm. Intending to stay level, I trimmed nose down. The air was perfectly smooth.

The vertical speed indicator continued to rise: 1,000 feet per minute. Now I was really going up. I glanced at the height of the arch overhead — evil-looking lemon gray, slightly roiled by wind. Now the far border of the dome in front of me was lower than my flight path.

It was too late to turn around. The air was still smooth as stainless steel, but the airplane was ascending steadily and peaceably at 1,000 fpm up into the center of the canopy of cloud. Suddenly, I saw that my shiny Navion, and I with it, was being siphoned up into the maw of a thunderstorm.

Quickly, I reduced power to idle and pointed the nose down. Soon I was at VNE, 195 mph, with the nose slanted down and the throttle all the way back, but I was nonetheless rocketing up toward the height of the dome. I didn’t lower gear or flaps to slow down because gear-down speed is 100 mph. That tough Navion airplane structure, designed like its ancestor the P-51, could withstand exceeding VNE.

I just concentrated on the dive and watched the approach of the black rim of cloud on the far side of the dome as I plunged forward. But I wasn’t losing any altitude. I was knifing down the updraft, edging into the thunderstorm on its side.

Below, I could now see sunlight on the ground and blowing dust whirled up by the storm. Finally, I reached the edge of the updraft and started actually descending, hurtling toward safety and the Earth.

And without warning, the airplane was snatched from its smooth dive, torn like a leaf from a branch, flogged this way and that, tumbling left to right. It was uncontrollable. I gripped the seat pan with both hands, braced myself between the rudder pedals and seat back, and scrunched my body into its smallest possible shape but was still thrown violently against the side of the cockpit and the roof. My hard hat whacked against the Plexiglas window and the aluminum windshield pillar. In between lurches, I leveled the wings with rudder as gently as I could, and when I could finally grasp the throttle, I added power. Refuge lay in that sliver of air close to Earth. The air smoothed out. Perhaps 200 feet above the grasslands, my small silver airplane bumped along over the whorls of dust and then in bright sunshine finally relaxed into its normal rhythm.

Picking the microphone up off the floor, I radioed Albuquerque Center and asked if they were painting a thunderstorm east of Springer.

“Yes,” they replied.

“Tops?” I asked.

“Tops are at 48,000.”

“Thanks and good day,” I said.

There, ahead in the late afternoon sunlight, were the two mesas I used as guidelines for my route across the mountains. I climbed up to 10,500 feet and skimmed across the ranges toward home.

So that was a dry line thunderstorm.

The Great Plains dry line is a surface boundary, aligned north/south, between two different air masses: dry air, heated as it rides east downhill from the high terrain of the Rocky Mountains, rumbling into moist, cooler air trending up to the west from Kansas, Oklahoma and Texas. The atmospheric pressures, wind directions and humidities of the air masses are different. Where they meet, powerful convection is forced by mixing, and the result is a line of huge powerful thunderstorms. Storm chasers revel in the dry line as it is the source of supercell thunderstorms.

The dry line is not called a front because the general weather doesn’t change essentially on either side of the line. The dry line retreats toward the mountains at the end of the day, and later the next morning, it flows east again toward lower terrain as the atmosphere heats up and the warm air slides down along the plains. So the dry line sloshes, as it were, back and forth across the eastern plains of Colorado, New Mexico and west Texas. It is most prevalent in late spring and early summer. Because no terrain feature exists to interrupt the flows, thunderstorms continue building along the dry line, fed by the collision of the differing air masses.

You can find the dry line forecast on aviationweather.gov under the heading "Forecasts," then "Prog Charts," then "Surface," which will bring up five panels reaching out 48 hours. The dry line, if one exists and is forecast, is depicted on the prog charts as an orange line with orange scallops on the eastern side. If you are flying across the Great Plains in spring or early summer, you should always check the progs for the dry line.

Take, for example, the prog chart for June 20, 2013. It showed an extensive dry line plotted all the way from the plains of Wyoming south across eastern Colorado and eastern New Mexico down to southwest Texas. Assuming thunderstorms formed all along this line, flight routes to Casper, Cheyenne, Denver, Colorado Springs, Albuquerque and El Paso might be hazardous or blocked. In addition, this particular dry line showed a prominent bulge over eastern Colorado. Research indicates that higher altitude westerly winds initiate the bulge. To put a very complicated process simply: The wind shear involved in a dry line bulge may cause its burgeoning storms to lean over, twist and hatch tornadoes.

If your weather briefing mentions scattered thunderstorms in the forecast for the Great Plains, you should check for a dry line. “Scattered thunderstorms,” according to the National Weather Service Glossary, means an “area coverage of convective weather affecting 30 percent to 50 percent of a forecast zone.” So it is tempting to take off toward scattered thunderstorms because you imagine your route might be available over half the terrain. A dry line implies much more hazardous circumstances than mere scattered or isolated thunderstorms. If on the way you are talking with Flight Watch and observe a bulwark of building cumulus, ask if there’s a dry line and if severe thunderstorms are forecast. If that’s the case, retreat.

Dry line thunderstorms metastasize unbelievably fast. We all know about not flying through the beautiful blue of a sucker hole. Usually, we think of sucker holes as something rather small that might wrap you fast in gauzy, bumpy clouds. In the case of storms coalescing along the dry line, a nice, miles-wide sucker hole will grab you quickly with lethal updrafts, downdrafts and extreme turbulence. Sometimes, if you are flying west in a moist air mass toward a dry line, you will not be able to see the buildups because of poor visibility in the damp air mass.

An early sign of a severe thunderstorm is virga, curtains of rain or snow sinking toward, but not hitting, the ground. Not many pilots are acquainted with virga. Virga mark skies that are very hazardous. The descending columns of precipitation are cold and therefore heavy. Virga displaces air, so right next to the downdraft is an updraft. The shear between downdraft and accompanying updraft really tosses you around. Rainy virga is usually gray, and snow, white. In snow virga, you have no forward visibility at all. Often, lightning flashes along the shafts of virga.

In thunderstorm weather, if airports are few and far between, you may have no other option but to fly under virga. Sometimes, the least rough air is a few hundred feet above the ground, but to fly low, you must be familiar with the terrain and watch out for antennas and towers that may have sprouted in uninhabited places.

When you’re flying in turbulence near virga, don’t flail with aileron against a wing drop. Level the wings gently with rudder, or if you’re thrown into a steep bank, use coordinated controls to regain level flight. You steer with your feet, not the yoke, remember? And when you are approaching an airport threatened by thunderstorms — and you have no other place to land — keep one radio tuned to the AWOS, and each cycle, listen for wind and pressure changes. Depending on the circumstances, you may need to select a different runway at the last minute.

So what are the major signs of dry line thunderstorms to look out for? During the preflight, check the dry line forecast and search for high dew points in airports east of the dry line. In the air, be on alert for nasty-looking virga streaming below cumulus clouds that are bulking up in all vectors ahead. Lightning and funnel clouds should also serve as warnings that it’s time to turn around, land in a hospitable Great Plains town and look forward to taking off in benign skies early the next morning.

Click next to get a look at some extra dry line warning signs.

Dry Line Warning Signs

Look Ahead for the Dry Line: If you are flying across the Great Plains in spring or early summer, you should always check the progs for the dry line. The dry line, if one exists and is forecast, is depicted on the prog charts as an orange line with orange scallops on the eastern side.

The Danger of Virga: An early sign of a severe thunderstorm is virga, curtains of rain or snow sinking toward, but not hitting, the ground. Virga mark skies that are very hazardous.

Read more from Margaret W. Lamb in her feature "Deadly Downdrafts: Understanding the Risks" here.

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