Meteorologists at the National Weather Service are calling it a rare event, a slow-moving and potent weather pattern that comes along just once every 10 years.

Nearly 1.50 inches of rain has fallen in Helena since the early morning hours Thursday. While it doesn’t sound like much, the wet weather arrived on the heels of last week’s high temperatures, which flushed snowpack from upper elevations.

The Blackfoot River at Bonner saw flows increase from 3,510 cubic feet per second last week to 9,790 cfs this week. The Big Hole River at Melrose saw flows shot up from 2,180 cfs to 7,250 cfs.

With the addition of rain, the Bighole River has since jumped its banks, prompting the closure of at least one county road. Officials are watching other rivers as well, including the Deerborn at Craig, to see how high the water will climb before Sunday when most flood warnings are expected to subside.

“The nature of the storm we have now is fairly anomalous,” said Dan Zumpfe, a meteorologist with the NWS. “The circulation from this system has inundated us with rain. It’s brought wave after wave of precipitation. We don’t typically see such stagnant weather patterns.”

While most streams in the Helena area were running high, including Tenmile Creek, officials expected them to stay within their banks until the storm system passes.

Diane Tipton, information officer with the Montana Fish, Wildlife and Parks, said officials are closely watching Headwaters State Park, where the full-flowing Madison, Gallatin and Jefferson rivers converge.

“The water was real high,” Tipton said. “They were thinking they may have to restrict certain areas of the park, so that’s one of the areas we’re watching as well.”

Officials believe water levels on the Missouri River below Holter Dam could increase dramatically over the next few days, especially as the Bureau of Reclamation begins releasing water to help control the rate of fill.

Beginning Monday, the release rate from Canyon Ferry is expected to jump from 5,700 cfs to 11,000 cfs. The Missouri River at Toston, several miles above Canyon Ferry, is currently at 9 feet. Officials are expecting the river to reach 10.5 feet there on Monday, which is just below flood stage.

Lewis and Clark County Sheriff Cheryl Liedle said county officials are watching and waiting to see what the next few days hold. Currently, however, no flooding has occurred.

“We’ve been watching the smaller streams,” Liedle said. “Although they’re high, and we may see some spot flooding, especially in areas where there may be some drainage problems, I haven’t heard of any problems yet.”

Not everyone minded the rain, and business around town went on as usual Friday. Kristin Buchan and Andrea Simons, who both completed their freshman year at Carroll College, packed their car between downpours before heading off to Idaho Falls for the weekend.

“Getting this much rain is weird,” Buchan said. “I feel like we’re living in Seattle.”

I like it because I’m a farmer’s daughter,” added Simons. “The rain is always good.”

Reporter Martin Kidston: 447-4086 or mkidston@helenair.com