Summer-like weather for South Coast this weekend raises flood risk and fire danger

The calendar says May, but it'll be feeling more like July this weekend across B.C.'s South Coast — and that's raising concern over flooding and wildfires.

Temperatures in Metro Vancouver are forecast to climb well above 20 C by Saturday, while the Fraser Valley could see temperatures approaching 30 C by Sunday, making for an unusually balmy Mother's Day.

The early taste of summer is thanks to a very strong ridge of high pressure that began building across the South Coast of B.C. on Thursday, bringing with it mostly clear skies and calm winds.  

As the weekend begins, this same ridge will begin to bring up very warm air from Washington state, putting a few daily high temperature records in jeopardy.

Abbotsford, for example, is forecast to reach 28 C on both Saturday and Sunday, which would smash the previous daily temperature records for May 9 (26.7 C, set in 1946) and May 10 (25.3 C, in 1993).

On Saturday, Vancouver International Airport is likely to reach the 20 C mark for the first time since Sept. 11, 2019.

Elevated flood risk

While the warm spell coincides perfectly with the weekend, the sudden spike in temperature will bring with it some challenges as well.

When temperatures increase sharply in spring, so, too, does the risk for flooding, as rapid melt of the mountain snowpack causes a surge of water to flow into rivers and streams.  

The consequences of this have already been seen in the Cariboo and Chilcotin regions last month, as flood warnings and local states of emergency were declared.

Based on measurements taken in April, the mountain snowpack across the South Coast is slightly above average (103 per cent of normal) while the Lower Fraser is near average (97 per cent of normal).

However, the overall snow basin index for the entire Fraser River basin, upstream of the Lower Mainland, is at 116 per cent of normal, with the snowpack in watersheds not regulated by dams currently among the highest on record.

It means a significant rise in streamflow by next week is likely.

Below average precipitation increases wildfire risk

With no rain in the forecast until after Mother's Day, Metro Vancouver's unusually dry spring will get even drier — and that spells risk of wildfires.

In a normal year, 200 millimetres of rain falls on Vancouver between March and April alone. This year, the city's airport only received 64 mm, less than a third of the normal amount.

These abnormally dry conditions were a significant contributing factor to the Magee Road wildfire that began on April 15 in B.C.'s Squamish Valley, which reached more than two square kilometres in size.

Cooler and cloudier conditions throughout early May, combined with occasional rainfall, has significantly lowered the fire danger rating across the South Coast since mid-April.

But with a multi-day stretch of hot and dry conditions on the horizon, it is likely to increase once again as the ground dries out and makes it easier for surface fires to ignite.

Early forecast indications suggest showers and cooler temperatures will return to the region early next week.