08/27/2015 18:49 EDT | Updated 08/27/2016 01:12 EDT

Heavy rain on its way to Metro Vancouver

Metro Vancouver's hot, dry summer is about to come to a soggy end.

A lot of rain is on the way starting Friday morning and it's forecast to continue all through the weekend.

More rain is expected over the next four days in the Vancouver area than fell all summer long says Matt MacDonald, an Environment Canada meteorologist.

Forecasters expect up 150 millimetres of rain between Friday morning and Monday afternoon compared to the 41 millimetres of rain that fell between June and late August.

MacDonald says the next three days will look more like late October than late summer.

"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."

A low pressure system that's 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.

Dried earth can't absorb big rainfall

But all that rain, falling all at once could cause problems.

The hot, dry conditions, which forced municipalities to impose water conservation measures, dried out large swathes of southern B.C., making 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," MacDonald said. "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.

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