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Recent rainfall breaking some daily records
Anyone who lives in the South Okanagan who has stepped outside over the past six weeks will know that this year has been exceptionally rainy.
It’s actually broken some daily records, according to Environment Canada Meteorologist Matt MacDonald.
“It’s been much wetter than normal,” said MacDonald. “It’s been a very wet month.”
April precipitation in Osoyoos is typically 9.2 millimetres (mm) for the entire month, said MacDonald, but as of April 13 we were already up to 47.5 mm and the month wasn’t even half over.
That includes last Wednesday’s rainfall of 15.5 mm, which broke the previous April 12 record of 11.4 mm in 1992.
But the biggest rainfall by far came on Friday, April 7, when a downpour caused Hwy. 3 to flood next to the Osoyoos Visitor Centre.
That day, Osoyoos received 24.6 mm – more than double the previous April 7 record of 9.9 mm recorded in 2013.
Heavy precipitation, especially in the south of B.C., has pushed up snowpack levels at higher elevations to closer to normal, after levels were well below normal earlier this year.
According to the April 1 Snow Survey and Water Supply Bulletin, which was released by the B.C. River Forecast Centre on April 6, the Okanagan snow basin index was at 105 per cent of normal for this time of year.
The Similkameen index was at 95 per cent of normal.
“Cool and wet weather through March has led to a significant increase in snowpack conditions across the province, with the biggest increases being observed in south and southeast B.C.,” said the bulletin.
MacDonald said the south of the province has been stuck under a persistent and stable jet stream, which has moved very little.
“It’s what we call a blocking pattern,” he said. “The jet stream has been aimed at the southern half of the province and we’ve seen a continuous onslaught of weather systems, not only through April, but March was very much the same.”
During March, it rained 13 of 31 days, compared to a normal for March of just nine rainy days.
The jet stream is an air current high in the atmosphere that normally oscillates in a wavelike pattern through a series of ridges and troughs.
The ridges bring nice, dry weather and the troughs bring unsettled, rainy or snowy weather.
“For the past month or so, we’ve been stuck in very ‘troughy’ patterns, as we call it,” said MacDonald. “There’s been this trough anchored over the west coast of Canada. We haven’t really yet seen the long persistent ridges that we all hope for.”
While climate scientists are reluctant to attribute particular daily weather to climate change, the trend in recent years for the jet stream to become blocked may result from climate change.
MacDonald points to studies showing a reduction of sea ice in the Arctic is causing a warming trend in the north.
This decreases the temperature gradient – the difference between temperatures at the poles and the equator.
“That temperature difference is actually what drives the intensity of the jet stream,” he said. “So as you get the weaker temperature gradient north to south, you end up with a weaker jet stream, and these tend to become blocked more frequently than stronger jet streams.”
Another factor influencing weather trends is water temperatures in the equatorial Pacific Ocean.
The winter of 2015-16 experienced warmer temperatures that continued into spring because of El Niño.
This winter, by contrast, we were in the opposite La Niña period, which brought colder-than-normal temperatures.
Around mid-February, the influence of La Niña fizzled, MacDonald said, and this has brought a more neutral El Niño period, properly referred to as El Niño Southern Oscillation (ENSO).
“We’re returned to neutral conditions for the first time since the fall of 2015,” said MacDonald. “That means waters are back to near normal temperatures.”
MacDonald cautions that long-term forecasts have to be taken with a grain of salt, but he says it appears the rest of April will only be slightly cooler than normal.
Then, heading into May and June, average temperatures should be closer to normal.
But while meteorologists can predict temperature trends, it’s much harder to predict precipitation. At most, they can forecast rainfall trends over one or two weeks, MacDonald said.