Can dry air assist severe thunderstorm outbreaks?

For this explainer topic, let’s focus on a meteorology term called the “dry line”. Meteorologists forecasting in the southern Great Plains, particularly western Texas, are very interested in this atmospheric feature! They typically are a south-to-north oriented air mass boundary between very dry air spilling out of the Front Range of the southern Rockies that eventually mixes with humid Gulf of Mexico air masses. The physical geography in this part of the world allows dry lines to be fairly common, even in the absence of large-scale low pressure systems.

Spatial contrasts in available surface water vapor content (measured by dewpoint temperature and commonly denoted as “Td”) may be dramatic when comparing weather observations on either side of the observed dry line frontal feature. A Td difference greater than 50 degrees is not out of the question! For example, dewpoints in the 60s and 70s east of an advancing dry line can quickly plummet into the 30s or lower west of the dry line. So, what’s all the fuss about? Well, dry air is denser than humid air. This means a dry line moving into a humid air mass acts as a lifting mechanism for moisture. Add into the equation a passing vigorous low pressure circulation entering the Great Plains region and it can be recipe for a severe thunderstorm outbreak.

Under such scenarios, using satellite and Doppler radar it's possible to see thunderstorm initiation first flare up along and just ahead of the dry line on the humid air mass side and then propagate with prevailing storm steering winds aloft (typically moving in a westerly to easterly direction). When environmental conditions are conducive (i.e., vertical wind shear, ample low-level moisture, and instability caused by large vertical contrasts in temperatures – cold over hot) then rotating supercell storms become possible with threats of large hail, damaging winds, and even tornadoes. Passing large-scale weather disturbances combined with a dry line’s ability to lift moisture is an ideal setup for creating supercell outbreaks!

Of note, the Desert Southwest region of North America in general is a major contributor to dry air masses carried into the southern Great Plains by the prevailing Westerlies aloft (Polar Jet/storm track). Additionally, air flowing down from higher elevations on the leeward side of the Rockies tends to warm and dry an air mass further upon reaching the plains through a process called adiabatic heating. This long distance geographical relationship between the Desert Southwest and Great Plains supercell potential assisted by dry lines intrusions helps highlight how it's important to look beyond the "local" area when weather forecasting!

-JWM

Photo Credit: Jonny William Malloy