What the are “dry thunderstorms”?

Hold on! Doesn’t a thunderstorm need lots of moisture? Well, it really only needs enough moisture. Essentially, a bit of low/mid-level moisture combined with even modest atmospheric instability for lifting that moisture up in the air to cool and condense into tall clouds can do the trick. The pre-monsoon environment in the southwestern U.S. (May and June) is usually ripe to begin seeing short-lived towering "cumulonimbus calvus" clouds dotting the horizon, especially over favored mountainous terrain that assist the moisture lifting aspect. Although the lack of deep moisture in a 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 in an atmosphere conducive for dry thunderstorm initiation.

First, there is still going to be lightning! Lightning remains a danger with the added threat of wildfire ignitions. The intersection of dry thunderstorms most likely to develop over early summer parched mountain forests is a recipe for disaster. The wildfire season over the Southwest U.S. usually continues until persistent monsoon rains and moisture are established to slow or prevent wildfire growth despite ongoing lightning activity.

Furthermore, you’ll notice dry thunderstorms have very high cloud bases compared to thunderstorms on muggy days. High-based thunderstorms are indicative of drier air in the lower atmosphere. The rain that does fall typically can’t make it to the ground (phenomenon called “virga”). Rain may not have reached the ground, yet it did change the atmospheric temperature profile below the storm by evaporative cooling. Colder air sinks and can sink quite rapidly if there is a significant temperature contrast between the hot air surrounding the rain chilled downdraft air. The result is destructive microburst winds being possible. Even tamer but still erratic gustiness can accelerate wildfire spread (a double whammy in that regard). Additionally, should such strong winds reach desert landscapes then severe dust storms may erupt. Dry thunderstorms are an important weather forecast consideration!

Photo Credit: Jonny William Malloy