Weather Explainers

Why Drylines Are Important in Severe Weather and Tornado Forecasting

At a Glance

The dryline is a spring and summer feature that is critical in severe weather forecasting.
Drylines most often occur in the Plains states and can trigger thunderstorms that produce tornadoes.

You've probably heard of a cold or warm front, but there is an equally important surface boundary you may not be familiar with. It's called a dryline.

Drylines typically set up north to south through the Great Plains – in states including Nebraska, Kansas, Oklahoma and Texas – during spring and summer. They separate warm and moist air from the Gulf of Mexico to the east from hot and dry air originating from the Southwest on the west.

They are associated with large-scale low-pressure systems, which also have an attendant cold and warm front.

(MORE: Tornado Central)

A dryline (yellow) over west Texas and west Oklahoma in combination with the cold (blue) and warm (red) fronts associated with a low-pressure system. Ahead of the dryline are supercell thunderstorms that developed on May 16, 2017 which spawned tornadoes.

Volatile weather conditions can exist – namely the development of supercells that can unleash large hail, damaging winds and tornadoes – ahead of the dryline boundary.

Where that dryline sets up on a severe weather day dictates what locations are in the danger zone east of the boundary. Forecasters are able to analyze the location of a dryline in real time based on surface observations, and that allows them to determine where severe storms may begin to erupt. Sometimes a severe thunderstorm or tornado watch can be issued before the storms even form, or when they are in their infancy.

Conversely, locations well to the west of the dryline are typically in clear with no severe weather worries.

(MORE: What is a Supercell?)

Storms develop along a dryline because the dry air behind the boundary is less dense than the moisture-rich air ahead of it. The rising air that results from this collision triggers the formation of thunderstorms on the immediate eastern side of the dryline.

If the air aloft in the atmosphere above the dryline is too warm, however, the atmosphere can be "capped", resulting in no thunderstorm development.

The denser dry air associated with the dryline forces the moist air to rise resulting in the formation of possible thunderstorms. (NOAA)

When drylines develop, they typically advance east through the Plains during the daytime, then retreat west at night. They can occasionally reach the Mississippi River region when a strong storm system charges east, NOAA says. Sometimes a dryline is overtaken by the advancing cold front located to its west.

Where the dryline passes through, there's a sharp drop in humidity, skies clear and sometimes temperatures increase. Winds typically shift from a moist south or southeast direction to a dry southwest or west direction.

(INTERACTIVE: See How a Tornado Forms)

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 aren't exclusive to the United States.