Reasons behind flooding — and why this week’s weather is not good news

The amount of rainfall in the past three months was 185 per cent of normal, according to figures at Kamloops Airport.

Cam Fortems – Kamloops This Week

The recipe for flooding conditions that have swept away roads and small bridges in the Southern Interior has been building since February — a mix of cold and wet conditions that suddenly turned warm.

Add some rain into the mix and it’s a predictable outpouring into small tributaries that cannot handle the freshet influx.

The amount of rainfall in the past three months, February through April, was 185 per cent of normal, according to figures at Kamloops Airport. Just over 73 millimeters of rain fell during the period.

Environment Canada Meteorologist Matt MacDonald said the period was also 1.5 C colder than normal.

Those conditions set the stage for last week, with a spike in warm weather — as much as 6 C higher than normal — followed by thunderstorms and rain.

As of April 1, the regional snowpack in the South and North Thompson watersheds was only slightly above normal. But MacDonald said that snowpack continued to build later in the month.

“The snowpack typically reaches its maximum in mid-April, but it continued to grow until last week.”

B.C.’s river forecast centre released statistics Monday showing the snowpack in the South Thompson watershed at 115 per cent of normal and North Thompson at 112 per cent.

April was the 13th wettest on record in Kamloops, with data dating back to the 19th century.

MacDonald said this week’s forecast “looks like an exact repeat,” with temperatures reaching a sunny 27 C on Wednesday, followed by several days of showery weather. Depending on snowpack levels, that could again bring cresting of local tributaries.

The river forecast centre said it does not expect flooding in the Fraser system, but warned that persistent wet conditions during peak flows could cause more flooding.

Much of the province is within the normal range for runoff forecasts, with the exception of Nicola, Similkameen and Okanagan — all greater than 120 per of normal for snowpack.