A dry thunderstorm is a thunderstorm that produces lightning but little or no rain at the ground, because rain falling from the cloud evaporates in a very dry layer below the cloud base before it lands. In fire weather work, a storm is commonly treated as dry when it produces less than about a tenth of an inch (about 2.5 mm) of rain.
Why the rain disappears
The storm itself still makes rain. The difference is the air beneath it, which is dry enough that the rain evaporates somewhere between the cloud base and the ground.
That evaporating rain is often visible as virga, streaks hanging below the cloud. The NWS Albuquerque office notes that virga can bring sudden and erratic wind gusts, the same family of downdrafts covered under dry microburst.
Why fire weather forecasters track them
Cloud-to-ground lightning can start new wildfires, and it can strike outside the area where any wetting rain falls. Some lightning starts go undetected for a couple to a few days.
Lightning and gusty outflow together can spread new fires quickly. The NWS Albuquerque office also notes that measurable rain of a hundredth of an inch does not moisten the fuel bed enough to matter.
How they show up in forecasts
The Storm Prediction Center's Fire Weather Outlook marks Isolated or Scattered Dry Thunderstorm areas, both based on fuel conditions and cloud-to-ground lightning with generally less than a tenth of an inch of rain. Scattered areas carry more strikes and rank at the critical level.
Dry lightning can also meet local Red Flag criteria. The NWS Grand Junction office, for example, counts widely scattered dry thunderstorms, 15 percent coverage or more, as a Red Flag criterion, and says Red Flag events normally also require critical fuels.
Season and limits
In New Mexico, the Albuquerque office says dry thunderstorms are most common in May and June, and fuels can stay dry into the early monsoon, June through early July. Timing differs in other regions.
The tenth-of-an-inch line is a guide rather than a hard rule, and one Albuquerque office article ties it to closed-canopy forests, where more rain is needed to reach the fuel on the ground. Rain gauges under one storm also say little about the dry ground nearby where its lightning landed.