After numerous bomb cyclones and atmospheric rivers that caused severe flooding in October in B.C., and November in California, the jet stream has seemingly vanished from the area. Now, just weeks later, the Pacific Northwest faces a dramatically different pattern, with rain abandoning some regions entirely.
RELATED: Why a B.C. ridge in December isn’t as sunny as you think
Over the past two months, total rainfall amounts have exceeded 800 millimetres for portions of Metro Vancouver, such as Coquitlam. That is more rain than Montreal, Hamilton, Ottawa, and Toronto typically see annually. Nearly a metre of rainfall fell in Prince Rupert this fall, which surprisingly, is close to normal.
Will this dramatic change in the pattern persist, or will the Pacific storm track return to its typical wintry way?
DON’T MISS: The deadliest risk Canadians take each winter
After a dry start to September for many locales, the storm tap refused to shut off, with places like Ucluelet receiving over one metre of rain.
Above-normal rainfall (+20%)
Normal rainfall (+/- 10%)
What happened?
In what is normally a wetter period for the Pacific Northwest, the jet stream is taking a mighty detour and migrates north. The clues lie in the North Pacific.
Did you know? The jet stream, a high-latitude current of fast-moving air, guides storms across the Pacific Ocean. When this storm track shifts north, drier conditions prevail.
The dominant trough over the Gulf Of Alaska has been replaced by a prominent ridge, with the trough now thousands of kilometres further west.
The East Pacific Oscillation (EPO) describes pressure patterns over the North Pacific, and when it’s in the negative phase, a ridge of high pressure expands across the Northeast Pacific, forcing the jet stream further north.
Negative phase
Positive phase
This pattern can famously flood central Canada with cold air, like right now, but it can also create interesting precipitation anomalies on the west coast of North America.
Looking ahead, a shifting storm track
When does a trough re-emerge in the Gulf of Alaska?
Will the Pacific storm track settle into this particular groove, or is this divide a preview of what’s to come this winter?
WINTER FORECAST: El Niño is a distant memory; will Canada feel winter’s wrath?
Give our Winter Forecast a read, but current guidance suggests that the trough will temporarily return this upcoming weekend, and the ridge will flatten. Until then, the temperature will be within a couple of degrees of seasonal across British Columbia, but be warned, there will likely be a marine layer at times.
Have you spotted the fog?
By mid-December, a more active period is possible once again, which brings a typical onshore flow and renewed stormy conditions along the coast.