Why Drylines Are Important in Severe Weather and Tornado Forecasting

Drylines may not be as well known as cold fronts and warm fronts, but they are important for understanding severe weather across a particular location in the U.S.

The Definition

Drylines typically set up across the Central U.S. during spring and summer. These boundaries, often draped over states like Nebraska, Kansas, Oklahoma and Texas, separate warm and moist Gulf air to the east from hot and dry air originating from the Southwest on the west. Hence the name dryline.

Drylines are associated with large-scale low-pressure systems, and you often see these systems with a cold front, warm front and a dryline, as shown below.

Powerful thunderstorms instigated along the dryline can cause large hail, damaging winds and tornadoes. The storms form on the east side of this boundary, along the warm, moist side. I'll break down more of the why later in this article.

Because the storms form on the east side of the boundary, the location of the dryline on a severe weather day determines which locations are in the danger zone.

Forecasters can see the location of the dryline based on surface observations, and from there, they can determine where the storms will start to pop. The stronger the line, the more confident they will be to issue severe thunderstorm and/or tornado watches east of the boundary.

For those living on the western side of the boundary, you would be in luck. Locations west of the dryline are typically clear with no severe weather worries.

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The Science Behind Drylines

Storms develop along a dryline because the dry air to the west of, or behind, the boundary is more dense than the moisture-rich air to the east of, or ahead of it. Air that is less dense gets forced up at these boundaries. The rising air that results from this collision triggers the formation of thunderstorms on the immediate eastern side of the dryline.

However, its not always going to cause severe storms. If the air aloft in the atmosphere above the dryline is too warm, the atmosphere can be "capped" or too stable. This will result in no thunderstorm development.

When drylines develop, they typically advance east through the Plains during the daytime, then retreat west at night. However, these drylines rarely advance as far east as the Mississippi River before retreating. Sometimes a dryline is overtaken by the advancing cold front located to its west, which can cause storms to occur.

So how does it feel on the ground when a dryline passes through?

Usually, there's a sharp drop in humidity, skies clear and sometimes temperatures increase. Moist winds typically shift from a south or southeast direction to a dry southwest or west direction.

In a severe weather setup where thunderstorms ignite along a dryline, there can also be other thunderstorms that develop in association with a warm or stationary front that extends from the parent low-pressure system.

Drylines, although not as well known, are a predictable weather pattern that should still be taken seriously, especially if you live to the east of one of these boundaries after it forms.

Rob Shackelford is a meteorologist and climate scientist at weather.com. He received his undergraduate and master’s degrees from the University of Georgia studying meteorology and experimenting with alternative hurricane forecasting tools.