DRYLINE MAGICby Tim Marshall
(published in Weatherwise Magazine April-May 1992)
Every spring veteran storm chasers head to west Texas to catch a weather show they call "dryline magic." The dryline is a boundary that separates hot, dry air to the west from warm, moist air to the east. During the spring it lurks on the western high plains. The dryline moves east during the day, acting like a rotary plow, churning up the warm, moist air ahead of it. If there is enough moisture and instability in the warm air, severe storms can form, storms that often produce high winds, large hail, and tornadoes.
A dryline storm is more isolated, more visible, and more violent than any other type of storm. Storm chasers love them. Once they've seen one, most forget all about the fast-moving squall lines associated with cold fronts and the low-visibility storms along warm fronts.
I've driven through many dry-lines. One morning, while driving east from Lubbock, Texas, in hot, dry air, it seemed as if I suddenly hit a waterfall! The parched air in the car immediately became saturated, and the windows all fogged up. Looking north and south along the dryline, I could see the cross sections of the two distinct air masses. To the west, the sky was sharp and clear, and distant cumulus clouds were crisp and white. In contrast, to the east, the air mass was hazy and cloudy, and I could make out only a few rows of yellow fuzzy cumulus before visibility faded into obscurity.
Later that day, on my return westward, I suddenly plowed through a wall of dust denoting the leading edge of the dryline. The muggy, oppressive air in my car was quickly evacuated, and it felt like I had driven into a blast furnace. My lips were chapped and my teeth gritty.
Topography plays an important role in dryline development and movement. The elevation from western Nebraska south through western Texas averages about 3,000 feet above sea level. Air descending the eastern slopes of the Rockies warms and dries out as it sinks, creating a hot, dry, cloud-free zone that gives birth to the dryline. As the parched air moves eastward toward lower elevations, it encounters more and more moisture and has more and more air to mix. This slows it down, and by mid-afternoon the dryline usually stalls.
By early evening, the dryline is in full retreat back to the high plains, pushed westward by low-level winds on the moist side of the boundary. Sometimes the dryline repeats this cycle for an entire week, producing severe storms day after day.
Dryline storms tend to be severe for several reasons. Upward forcing along the dryline tends to occur at specific points associated with waves or bulges along the boundary. Surface moisture convergence is enhanced at these points, and it is moisture that fuels storms. As a result, dryline storms tend to be more isolated and more severe than other storms, since they don't have to compete with neighboring storms for moisture.
The dryline is identified on the weather map by a sharp gradient in dew point temperatures, usually around 50°F. Sometimes, 300-60° differences may occur between adjacent weather stations. In strong dryline situations, I have even seen negative dewpoints in the dry air, with relative humidities in the single digits!
In order to determine the probable movement of the dryline, I check the morning weather balloon soundings to see how deep the low-level moisture is east of the dryline. A moisture layer only a few thousand feet thick will tend to mix quickly, allowing the dryline to advance eastward rapidly. I have seen some drylines reach speeds of 50 m.p.h, when there is little low-level moisture in their paths. On the other hand, a moisture layer around 5,000 feet thick will tend to slow down the mixing process and could bring the dryline to a halt.
Forecasting dryline storm development is tricky and can leave storm chasers sunning themselves, rather than stalking a monster storm. The dryline will not produce a severe storm by itself. It usually has to wait for an upper-air disturbance to lift the warm, moist surface air to its east through a stable layer (temperature inversion) aloft. I study the morning soundings to determine the strength of this inversion. The thicker the inversion, the more difficult it is for a storm to form. Only the right combination of surface and upper-air conditions can lead to explosive storm development.
Once a storm does develop along the dryline, there is no guarantee it will become severe, since this will depend on other factors such as the amount of vertical wind shear present. The turning of the wind from southeast at the surface to westerly aloft (directional shear) is needed to cause storms to rotate. In addition, an increase of wind velocity with height (speed shear) will knock over weak storms so only the strongest storms survive. Dryline storms eventually move off the boundary into deeper low-level moisture where they can feed and grow.
One of the key features I look for in forecasting dryline storms is a dry-line bulge. This forms when strong surface winds in the dry air accelerate a portion of the dryline eastward ahead of the rest of the boundary. (Such winds can cause intense dust storms that obscure the sun.) Sometimes a surface low-pressure center forms along the dryline bulge, enhancing moisture convergence and increasing the chances for storm development.
A dryline-frontal intersection is also an area of enhanced moisture--and surface wind--convergence. The front can be cold or warm, or a thunderstorm outflow boundary. Storms that form at such an intersection are literally located in a narrow canyon that channels moisture and wind into the storm. Stationary or slow-moving cold fronts create a "point" for storm development at the intersection. In contrast, a rapidly moving cold front merging with a dryline frequently creates a "line" of storms or a squall line.
Dryline Storm Chasing
When chasing dryline storms, I try to position myself in the moist air, about 20 to 40 miles ahead of the dryline. If the dryline is too close, I may find myself battling blowing dust and strong gusty winds that can slow me down. On the other hand, if the dryline is too far to the west, I have to plan for extra driving time to get through the intervening drizzle, low clouds, and fog.
As I drive, I monitor weather conditions at selected cities by listening to radio stations that report dew-point temperatures. I also carry a wet-bulb thermometer to measure the amount of moisture in the air.
As the afternoon develops, I look for a boiling, foaming line of cumulus to the west of the moist-air stratocumulus cloud cover. This marks the mixing mechanism of the advancing dryline. I particularly look for dusters of cumulus towers that seem to grow and die repeatedly in the same area, possibly indicating the presence of a bulge, a frontal intersection, or a low-pressure center. Such agitated areas of convection are good severe-storm generators.
Once a storm does develop, I look around carefully to make sure there are no others nearby that could interfere with the development of my storm. I then judge the distance and movement of the storm and plan my chase route accordingly.
Many times dryline storm chasing is a waiting game. If the temperature inversion aloft is too strong, storms will not develop, no matter how unstable the surface air east of the dryline. On the other hand, I have learned never to turn my back on the dryline as long as there is daylight left; storms can explode and become severe in a mere 30 minutes--even on a retreating dry-line.