Flood risk could remain in B.C.'s southern Interior for some time to come

The potential of large-scale flooding in B.C.'s southern Interior for a second straight weekend is due to a combination of factors that could keep the region in a heightened state of risk for awhile. 

"When we talk about a one-in-200 year flood situation ... in any given year, that means there's only a 0.5 per cent chance this kind of event could happen," said CBC meteorologist Johanna Wagstaffe. 

"But it's been such an unusual convergence of these different factors that it's probably going to make this a historic year."

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As of 5:30 p.m. PT Thursday, the River Forecast Centre had issued a flood watch for the Central Interior and Thompson Region, Shuswap Region, Okanagan, Similkameen, Salmon River and Boundary.

It comes a weekend after two people died in flooding events — and there's likely to be more flood watches in the weeks to come, regardless of how the next few days play out. 

High snow water index

At issue is the large amount of snowpack that has accumulated in the southern half of British Columbia over the last six months: while virtually everywhere has accumulations above normal heights for this time of year, it is highest in the Okanagan (147 per cent as of May 1) and (146 per cent) Similkameen regions.  

"April saw 300 per cent of normal precipitation in the southern Interior, with temperatures between half and one degree cooler," said Wagstaffe.  

"Then we went from cold and snow to, all of a sudden, three days of 25-plus degree temperatures in a row, and that led to rapid snowmelt with more snow than should have been there." 

The biggest risk this weekend is the threat of thunderstorms — which means the regions most at risk are hard to predict. 

"I know officials have been vague who's hit the hardest, but because of isolated areas, it'll bring pockets," said Wagstaffe.

"So, we can't say where we're going to see the worse of not only river flooding, but also overland flooding, because the ground is still so saturated because of last weekend."

Relief on the way? 

At the same time, the situation will likely improve after this weekend, according to Wagstaffe. 

"With this cold front that's moving through tonight, temperatures will drop 10 degrees ... it will stop the snow from melting and give the watershed a bit of a break," she said.

"But it's still unsettled, so we still could see unsettled thunderstorms throughout the weekend."

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And if thunderstorms pass this weekend, and you don't see immediate flooding, don't think the risk has passed.

"The Okanagan watershed is quite large. Some watersheds only take a couple hours to respond. But the Okanagan takes upwards of 12 hours," said Wagstaffe. 

"Even though the hardest rain is falling this evening, it might take until the morning hours for people to see how bad this is."