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Surprise thunderstorm breaks Boise rain record. Why even experts were caught off guard

A surprise storm over the city brought enough precipitation to break a weather record in Boise on Saturday night.

The thunderstorm, which featured a dramatic intra-cloud and cloud-to-cloud lightning show, began at 11:15 p.m. in Boise and lasted just an hour. Despite its brevity, it threw a lot of water on at least some parts of the city. The National Weather Service said its official measuring bucket caught 1.46 inches of rain, but the agency recorded as much as 2.42 inches in other places.

That was enough water to officially make this the rainiest August in Boise’s recorded history, according to Anna Lindeman, National Weather Service meteorologist.

This storm, coupled with recent rain from the remnants of post-tropical cyclone Hilary, brought the month’s official total to 2.51 inches. The previous record was 2.37 inches in August 1968.

“We got 0.4 inches of rain in just nine minutes,” Lindeman said. “So it was pouring.”

The sudden storm was caused by an excess of moisture in the air, according to Lindeman. A strong upper flow allowed all that liquid to quickly form into some wild weather.

“Our forecasting models did a really, really bad job with it,” Lindeman said with a laugh. “It’s hard to predict exactly where that much moisture is going to generate another storm.”

The storm was concentrated around Boise, with most of the precipitation falling in the eastern and southern parts of the city, the Weather Service said. At least one North End resident’s rain gauge measured less than one-tenth of an inch of rain.

Thunder, rain and lightning rolled over Boise late Saturday night.
Thunder, rain and lightning rolled over Boise late Saturday night.

Lightning gave Boise a show

Window watchers may have noticed the high quantity of lightning last night. The weather service recorded 78 strikes that hit the ground.

“But there was far more intercloud and cloud-to-cloud lightning that didn’t make it to the ground,” Lindeman said. “And that’s why there were so many flashes that weren’t really making noise.”

Lindeman said the storm’s speed was the reason for all the lightning. The rapid, vertical growth of the thunderstorm meant there was a lot of separation between electric charges within the cloud, according to her. That uneven distribution between charges can cause a lot of lightning within the cloud.

“Basically, it’s really, really flashy,” the meteorologist said.

Lindeman said a cold front that could bring the possibility of another storm on Tuesday. Starting Wednesday, temperatures were forecast to dip to the low 70s with breezy skies.

Business and Local Government Editor David Staats contributed.