Hurricane Hilary had the potential to bring record-setting rainfall to Boise, reminiscent of nearly 50 years earlier when the remnants of Hurricane Kathleen blew through the Gem State.
Kathleen, which reached Boise on Sept. 11, 1976, dropped 1.74 inches of rain on the city, including 1.1 inches in one hour.
But as Boiseans braced themselves for Hilary’s predicted deluge over the weekend, not much transpired.
The City of Trees picked up just 0.39 inches of rain over the weekend, falling well short of the initial forecast for the city. But just 75 miles northwest of Boise in eastern Oregon, Ontario got about 1.5 inches of rain from Hilary’s remnants. And north of Boise in McCall, 2 inches of rain fell.
— NWS Boise (@NWSBoise) August 21, 2023
Boise then picked up 0.89 inches of rain during a fierce thunderstorm Tuesday night, but that had nothing to do with Hilary and was the result of a separate low-pressure system that came in behind the named storm.
What happened for Boise’s rainfall to be so drastically different from the initial forecast, and why did other areas still receive a significant downpour? There’s a reason.
Blame the Owyhees
In the southwest corner of Idaho, bordering Nevada and crossing over into Oregon, is the Owyhee Mountain Range. The range spans 6,747 square miles and reaches over 8,000 feet high at its tallest peaks.
That range is why Boise received less than half an inch of rain this past weekend.
“A lot of the moisture ended up being shadowed by the Owyhees right here in our part of the valley,” Stephen Parker, a meteorologist for the National Weather Service, told the Idaho Statesman. “The southerly flow was strong enough and lasted long enough to make that a significant factor.”
Rain shadow is a phenomenon that occurs when storms approach a large mountain range. As moist, warm air approaches the mountains, it cools as it reaches higher altitudes and forms clouds, resulting in rain and snow. But as that air crosses the peak and starts down the other side, the air warms again and dissipates rain clouds.
This is what happened with Hilary and the Owyhees. The air sunk down the north side of the Owyhees and toward the Treasure Valley, inhibiting heavier rainfall, before the air reached the Boise Mountains to the north and cooled again, creating more rain clouds in the mountains.
Hilary had a long period of southerly flow, according to Parker, which meant any areas directly north of the Owyhees — such as Boise — would feel the effect of the rain shadow.
“If the wind direction had been geared around a little bit more out of the southwest, then the shadow would have been over more towards Mountain Home,” Parker said. “And Boise would have gotten a whole lot more rain.”
What could have happened if Hilary hit Boise hard?
In early June, a flash thunderstorm dropped between 1.47 and 1.77 inches in Boise, causing flooding across the city as drainage systems were overwhelmed.
Boise has “100-year” storm drains, according to previous Statesman reporting, which means only a storm with a 1% chance of forming in any given year would be strong enough to overwhelm the drainage system. But parts of the city had brief flooding Tuesday night as well during the severe thunderstorm.
Based on rain totals seen in Ontario and other areas west of the city, significant flooding could have occurred if Hilary had taken a more southwesterly track, moving the rain shadow farther east.
“Most of the western U.S. is not built to handle that kind of rain in a short period of time, just simply because it doesn’t happen very often,” Parker said. “I’ve seen pictures of a lot of just basic street flooding in Ontario. And that would have been the same type of thing we’d have had here.”