What are “dry thunderstorms”?

Hold on! Doesn’t a thunderstorm need lots of moisture? Well, an individual storm really only needs enough moisture. Essentially, a bit of low or midlevel moisture combined with even modest atmospheric instability can be sufficient for lifting that moisture up into the atmosphere to cool and condense into tall clouds.

 

Although the lack of deep moisture in any dry thunderstorm regime greatly diminishes a flash flooding threat, it’s important to not let your guard down with this seemingly mundane thunderstorm type. There are unique overlapping weather hazards to consider with an atmosphere conducive for dry thunderstorm initiation. 

Let’s explore a bit why the development of dry thunderstorms are an important weather forecast consideration. To help explain, I want to highlight the southwestern United States where a very hot air mass beginning to mix with limited subtropical moisture during the pre-monsoon environment (May and June), makes an ideal environmental setup to see short-lived dry thunderstorms, especially over favored heated mountainous terrain that assist lifting moisture.

First thing you’ll notice is that dry thunderstorms stand out by having very high cloud bases compared to thunderstorms on muggy days. High-based thunderstorms are indicative of limited moisture in the lower atmosphere. The rain that does try to fall from such higher altitudes into an air mass with limited moisture below typically can’t make it to the ground, hence the “dry” storm connotation.

 

Rain may not have reached the ground (a phenomenon called “virga”), yet it did change the atmospheric temperature profile below the storm by strong evaporative cooling. Colder air sinks and may sink quite rapidly if there is a significant temperature contrast between the hot air surrounding the rain chilled downdraft air. The result is the possibility of destructive microburst winds slamming into the ground. Furthermore, should such stronger winds reach lower desert landscapes then severe dust storms may erupt.

Also remember, too, that a dry thunderstorm is still going to generate lightning! Importantly, lightning is not only a direct threat to people and infrastructure, but introduces another hazard concerning new wildfire ignitions!

The intersection of dry thunderstorm outbreaks occurring over early summer parched mountain forests before heavy monsoon rains arrive is a recipe for disaster. This is why the duration of a wildfire season on any given year is quite variable dependent on when persistent monsoon rains and abundant subtropical moisture get established over the region to either slow or prevent wildfire growth, despite ongoing lightning activity. Furthermore, erratic gustiness caused by virga under dry thunderstorms can accelerate spread of new or existing wildfires, a double whammy in that regard!

Photo Credit: Jonny William Malloy