What is the Dryline and How Does it Contribute to Severe Weather? (Credit: University of Wisconsin-Madison)

DISCUSSION: ​As we start to head into the spring months, a common meteorological boundary will be frequently mentioned. A boundary can be one of the most important ingredients when forecasting severe weather. One of these boundaries is known as the dryline. The dryline is a boundary that separates moisture. It’s important to note that it is not a front since a front is a boundary which has a strong density gradient such as the difference in temperatures across the boundary. It is essentially a boundary which has moist air to the east and dry air to the west and is a very common feature. The dryline can occur on a nightly basis during the summertime in the High Plains, although it is mostly referenced during springtime. The dryline is more notable when low pressure systems move into the plains. These systems can cause a contrast between moist and dry air. Dry air comes from the Rocky Mountains, while humid air is brought up from the Gulf of Mexico, thus a sharp gradient. The southerly winds help to supply moisture from the Gulf of Mexico ahead of the dryline. In the diagram below, the dryline is depicted as the scalloped line running down from eastern Oklahoma through eastern Texas. Out ahead of this line in Texas the dewpoints are in the mid 50s to around 60 degrees Fahrenheit. Behind the line, dewpoint temperatures are in 30s and 40s. It is not uncommon to have dewpoints in the 70s ahead of the dryline and dewpoints in the 20s behind it. Along this line is where the strongest storms would be expected to form.  This is because of the convergence of the differing moisture boundaries and wind shift. Usually, there is also a southeast flow along and ahead of the boundary while there is a westward flow behind it. This further contributes to the convergence. 

​Image Courtesy: University of Wisconsin-Madison 

As the strong winds come into contact with each other, it has nowhere to go so it is forced upward. The stronger the convergence, the greater the updraft, and hence the stronger the storms which develop. The strongest convergence occurs when the winds are more direct to one another, such as a westerly wind and an easterly wind. 

​Image Courtesy: University of Illinois 

The dryline is forced to the east during the daytime hours as the shallow layer of moist air near the boundary gets mixed out by the daytime heating.The turbulent mixing of the dry air above and the moist air below reduces the dewpoint temperature which helps to mix the dryline eastward. During the evening hours the dryline retreats back to the west, but the storms which formed are able to continue eastward as momentum carries them.This propagation of the dryline back to the west occurs because the diurnal heating has been lost, thus turbulent mixing also ends. Radiative cooling is more effective in dry air which enable the dryline to retreat back to the west as the temperature is cooled back to its dewpoint temperature. The animation below clearly depicts where the dryline is. The yellow and orange colors indicate dry air while the blues and greens indicate moist air. 

Image Courtesy: University of Wisconsin-Madison

A large majority of violent tornadoes in the Midwest are due to the dryline. An example is the El Reno tornado of May 31st, 2013 which resulted in eight deaths. With spring right around the corner, we can expect the dryline to become more active once again.

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©2019 Meteorologist Corey Clay