Weekend Vancouver storm: why the river overflowed

It was a wet and windy last summer weekend for the South Coast. Heavy rain led to flooding and landslides for areas to the north of the city, and strong winds meant thousands without poweronparts of the Lower Mainland.

What Happened?

A fairly strong jet stream lined right up with the B.C. South Coast Saturday through Sunday morning. It basically acted like a conveyor belt for sub-tropical moisture with its end pointed right at the South Coast. The atmospheric riveralso pulled in a low-pressure system Sunday morning. As the centre of the low crossed over Vancouver Island, very strong wind gusts wrapped around the storm.

The highest rainfall totals happened over the Central Coast, but roughly 150 mm fell in Squamish over the three day event rain that quickly funneled into river systems.

Why no rainfall warning?

However, these rainfall totals did not reach the minimum requirement for rainfall warnings from Environment Canada. The real trouble was a result of the combined rainfall and very dry pre-existing conditions.

Even though the South Coast has seen a few rounds of rain and showers in the past month, the effects of such an incredibly dry summer linger on. The potential for flooding is actually higher when rain falls on dry ground that has become hard, compacted and less able to absorb water.

Instead of getting some water seeping into the ground, all of it ran-off and entered into the river systems. Rainfall rates for Squamish and Pemberton this weekend were up around 8 mm per hour too fast for the ground to take any of it in. Environment Canada had issued a special weather statement for a wet and windy weekend, knowing that impact risks were higher.

What's Ahead?

High pressure is building for the South Coast over the next couple of days which will mean mainly sunny skies. But coastal B.C. will continue to receive a series of low pressure systems this week.

By Thursday, that pattern will once again sink south bringing rain back to Vancouver for the end of the week. Eyes will be on the forecast this Friday with another rainfall event possible.

What we really need is a series of days with steady, but manageable widespread rainfall, falling at a slow enough rate that the ground can absorb it, before it runs off.