Why dry thunderstorms pose wildfire, dust storm risks

Lightning without rain can be a recipe for disaster

Residents of the western United States frequently see the term "dry thunderstorm" in their weather forecasts – particularly from late spring into summer – referring to lightning-producing storms with little or no rainfall. But these storms pose a problem: Lightning without rain can be a recipe for disaster.

A dry thunderstorm has a high cloud base, which is a key component of this type of storm because it requires the rain to fall through a very deep layer of the atmosphere before it can reach the ground.

In the western U.S., the air is typically very dry. Since the raindrops have such a long way to travel from the high-based cloud down to the Earth’s surface, they will evaporate in that dry air before making it to the ground, so the thunderstorm brings very little, if any, rainfall to the surface. Meteorologically speaking, this is called "virga."

Thunderstorms don’t need rain to produce lightning, however. Cloud-to-ground lightning can strike the surface and spark a wildfire, especially if the vegetation is dry. Gusty winds associated with the thunderstorm can then fan the flames and cause the fire to quickly grow out of control.

NOAA’s Storm Prediction Center produces forecasts for dry thunderstorms in its fire weather outlooks. The "true" National Weather Service definition of a dry thunderstorm is a storm that produces at least one cloud-to-ground lightning strike with less than 0.10 inches of precipitation.

However, the SPC also considers up to 0.25 inches of rainfall to be a dry thunderstorm, as long as it’s accompanied by at least one cloud-to-ground lightning strike. Storms with greater than 0.25 inches of precipitation tend to produce "wetting rains" and are then considered "wet thunderstorms," the SPC said.

The fire risk depends on weather conditions before and after a dry thunderstorm. If soils are dry, there’s a higher chance that a dry thunderstorm could spark a wildfire. The odds are lower if recent rains have dampened the vegetation.

It should be noted that any thunderstorm – dry or wet – brings fire concerns because of the cloud-to-ground lightning it can produce.

"Lightning can and does start wildfires virtually anywhere outside the western U.S. from what would not normally be considered a ‘dry storm,’" the SPC said.

Any lightning that strikes the ground outside the main area of rain (or evaporating rain) from both wet and dry thunderstorms can also start wildfires.

Additionally, a dry thunderstorm can sometimes be the catalyst of a dust storm – also called a haboob – if its winds are strong enough to pick up large amounts of dust.

Inside a dust storm, the sky typically turns a dark-orange color from the dust, and visibilities can quickly drop to near zero, causing extremely dangerous conditions along freeways and highways.