Update: Red flag warning issued for Boise as new forecast shows increased storms chances

Editor’s note: This article has been updated to reflect changes in predictions and warnings issued by the National Weather Service throughout the day.

The National Weather Service has issued a red flag warning for Boise from 2 p.m. Thursday until 11 p.m. Friday.

The warning was upgraded from a fire weather watch that was initially issued for Boise on Wednesday morning, as previously reported by the Statesman.

The current warning is for scattered thunderstorms and wind gusts of up to 50 mph.

National Weather Service meteorologist Stephen Parker told the Idaho Statesman that the watch was upgraded to a warning because there is now higher confidence that thunderstorms — which produce wildfire-starting lighting — will move through the Boise area.

Boise was initially only issued a fire weather watch, which is similar to a red flag warning but a step below. Parker told the Statesman that Boise was initially only on a fire weather watch because the chance of “abundant lightning” was not high enough around the city.

But what is a red flag warning, and how does it affect Boise?

Red flag warnings throughout Idaho

The National Weather Service initially had only issued a red flag warning for areas south of Boise, primarily in the Owyhee Mountains and the Interstate 84 corridor through Mountain Home and Twin Falls.

A red flag warning is issued when there is an active fire or when critical weather conditions — such as storms, high winds and low humidity — result in a heightened chance of fires occurring.

The watch was upgraded to a warning for Boise on Wednesday afternoon as models now show storms forming in the mountains east of the city and moving westward.

“Some of our best mesoscale models are indicating that (those storms) will push westward,” Parker said. “As dry as everything is, we decided to go ahead and err on the side of caution.”

What is a red flag warning?

The first requirement for a fire weather watch or red flag warning to be issued for an area is when it has been dry for a substantial amount of time, according to the Weather Service. Boise hasn’t had measurable rainfall since June 18, when a measly 0.05 inches fell — the last meaningful rain Boise received was just over half an inch on June 6.

Most forest fires in the Pacific Northwest are caused by lightning when it strikes an arid area, causing the dry and dead foliage to ignite quickly.

The Weather Service then uses the National Fire Danger Rating System to determine whether certain critical elements pass a threshold to warrant issuing a fire weather watch or warning.

The three critical elements are:

• Sustained winds averaging 15 mph or greater

• Relative humidity 25% or less

• A temperature of 75 degrees or greater

With a lack of rain, a high temperature of 94 degrees on Friday and wind gusts up to 50 mph, Boise and much of southern Idaho are primed for dangerous fire conditions.

“One thing that all of our fire weather partners have communicated to us the last few days,” Parker said, “is because we’ve been so hot and so dry for so long that the fuels out there, all the grass and downed trees, dead stuff, everything is just really ready to burn. So it isn’t going to take much.”