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British Columbia’s River Forecast Centre issued an upgrade flood warning for the Squamish River and tributaries including the Cheakamus River on Monday, as another torrent of heavy rain was expected across the province’s South Coast.

A bulletin at 5:15 p.m. said water levels in the Squamish River were rising, with flows reaching somewhere between a two- and five-year return period at a gauge near Brackendale, a neighbourhood north of downtown Squamish, B.C.

With additional rainfall expected in the river’s headwaters, the water levels may continue rising into Tuesday, it said.

Tributaries including the Cheakamus River were also “expected to exceed bank-full flow,” the bulletin said.

The forecast centre has maintained flood watches for the rest of the province’s South Coast, spanning all of Vancouver Island, the Sunshine Coast, the North Shore mountains and parts of the Fraser Valley, including the Sumas River.

Lower-level streamflow advisories are in effect for the Central and North coasts.

River levels were expected to peak in most areas on Monday and Tuesday.

The River Forecast Centre said the “potent” storms had delivered between 80 and 300 millimetres of rain through most of the region since Friday.

Alyssa Charbonneau, a meteorologist with Environment Canada, said Monday that warm air accompanying the series of atmospheric rivers saturating the region is raising the risk of flooding by melting snow in the mountains, causing runoff.

Charbonneau said there may be some breaks in the heaviest rains, but the concerns over flooding come from the “cumulative event” and its extended duration.

The weather office maintained a rainfall warning on Monday covering Squamish, Whistler and other communities near Howe Sound, saying another 60 to100 mm was forecast before the rain eases to light showers Tuesday morning.

Charbonneau said the rain is expected to persist until some time in the middle of the week, perhaps Wednesday night, before easing up.

“We do see things cooling down toward seasonal, and it does look like we’re going to have a stretch of dry weather through the weekend,” she said.

Still, she cautioned, the longer-term forecast for B.C.’s South Coast should be taken “with a grain of salt” at this time of year because it can change quickly.

A bulletin from Avalanche Canada, meanwhile, said rain had saturated and weakened the upper snowpack in several mountain ranges.

The danger is ranked at “high” in the south Chilcotin and Pacific mountain ranges, including Whistler and Pemberton, as well as northwestern B.C.

The forecaster downgraded the rating to “considerable” in mountains in southeastern B.C. and along the boundary with Alberta later on Monday.

The danger is also classified as “considerable” in mountains throughout the Fraser Valley and parts of the central Interior, while it’s ranked at “moderate” along the North Shore mountains, the Sunshine Coast and parts of Vancouver Island.

The forecaster’s map indicates the risk is expected to remain high Tuesday in the south Chilcotin and Pacific ranges, including the Garibaldi area around Whistler.

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