Do summer thunderstorms help or hurt Northern California wildfire risk?

Summer thunderstorms can spark a fire — even if dreary skies are a welcome sight for dry Northern California..

A few notes: Just because there’s a storm doesn’t mean we’ll get lightning — or measurable rain for that matter. And lightning doesn’t equate to a fire starting.

There are different kinds of thunderstorms and they pose different risks and benefits for California’s landscape. Here’s what to know:

Types of storms

Dry thunderstorms, which produce little rain at the surface, are more likely to set off fires, depending on the conditions and elevations of the surrounding area.

Otherwise known as virga, these storms occur when precipitation falls from a cloud — but evaporates before it reaches the ground “due to a very dry atmosphere below the cloud base,” according to the weather service.

At lower elevation levels, like in Sacramento, dry thunderstorms have a higher chance of setting off a spark, said Idamis Del Valle-Shoemaker, a weather service meteorologist.

“Because the air is so dry at the low levels, any lightning strike that happens could lead to a fire spark,” Del Valle-Shoemaker said.

When coupled with surrounding dry vegetation or fuel, dry storms are more likely to result in cloud-to-ground lightning fire ignitions, according to an article from the weather service.

“Basically, there’s very little to no precipitation, and if there’s a lightning strike that could lead to fire starts because the vegetation is also very dry,” Del Valle-Shoemaker said of Sacramento’s weather conditions.

Dry thunderstorms also have the capacity to produce gusty winds at the surface.

Wet storms, on the other hand, produce a substantial amount of precipitation which limit the threat of wildfire, Del Valle-Shoemaker said.

They typically are associated with a heavy downpour, which yields a significant amount of rain. They can be accompanied by small hail and gusty winds.

However, the distinction between the two storms are on a “rolling scale” when it comes to fire weather conditions, with varying precipitation levels required for each.

“If it has been hot and dry for a long period, rainfall amounts need to be larger in order to moisten the fuel bed and lessen the chances of lightning fire ignition,” the weather service article states.

Sometimes meteorologists describe thunderstorms as a “mixture of wet and dry” when they produce a diverse rainfall pattern.

This story was originally published July 26, 2022 at 3:19 PM with the headline "Do summer thunderstorms help or hurt Northern California wildfire risk?."