B.C.'s hot, dry summer has come to a sudden end, with a deluge of heavy rain over the coming weekend threatening to trigger flash flooding on the South Coast.

The rain began overnight Thursday, with more expected over the next four days in the Vancouver area than had fallen all summer, says Matt MacDonald, an Environment Canada meteorologist.

A low-pressure system that has been sitting off the West Coast will make its way inland, picking up even more moisture from a tropical storm, which struck Hawaii earlier this week.

Forecasters expect from 80 to 120 millimetres of rain between Friday morning and Monday afternoon, compared to the 41 millimetres between June and late August.

"To see this at the end of August is pretty impressive," he said. "This is definitely substantial. This kind of thing we usually see in October or November when we get to our storm systems."

About 20 to 30 millimetres are expected to carry over into the mountainous regions of the Interior B.C. during the same period.

Flash flooding possible

MacDonald says the next three days will look more like late October than late summer, and if the rain falls too hard and too quickly, there is a potential for flooding or landslides.

That's because the hot, dry conditions, which forced municipalities to impose water conservation measures, dried out large swaths of southern B.C., make it more difficult for the ground to absorb water.

"When we see such a prolonged dry period, the soil loses its capacity to absorb moisture," says MacDonald. "So instead of infiltrating into the ground, it actually just runs off."

Contaminants that normally get washed away are still on surface roads, parks and beaches. When the rain finally falls this weekend, instead of being absorbed into the ground, the water could run off ground surfaces, picking up debris, clogging culverts, rivers and streams.

Up to 120 millimetres of rain are expected to fall in Metro Vancouver over a four-day period starting Friday. (Jonathan Hayward/The Canadian Press)

"All this will be flushed into rivers and the ocean," says MacDonald. "The rivers will be running pretty dirty."

Environment Canada is issuing weather statements to municipalities in Metro Vancouver and Vancouver Island to allow them to prepare for the heavy rain.

These statements are one step below weather warnings. In Metro Vancouver, for example, the criteria for a weather warning include a rainfall forecast of 100 millimetres over a 24-hour-period.

Still, MacDonald says, the current forecast is significant and "worth warning about."

Water restrictions unlikely to change

Reservoirs on the North Shore were at 55 per cent as of Aug 23, and it's not clear how much the projected rain fall will raise that level, said Marilyn Towill, division manager of transmission operations for Metro Vancouver's Water Services.

"The rain will be beneficial, but it's not going to be enough yet for us to consider moving off Stage 3," she said, suggesting that current restrictions will likely remain in place until early October.

To hear the interview with Marilyn Towill, listen to the audio labelled: Metro Vancouver rain won't lift restrictions.

To hear the interview with Matt MacDonald, listen to the audio labelled: Flood risk follows Metro Vancouver's dry summer.