California weather is likely to be cool and dry as December arrives, owing to a high-pressure ridge over Alaska and Canada.
For weeks, meteorologists have been watching the upper atmosphere flirt with something extraordinary — the earliest recorded sudden stratospheric warming (SSW) on record.
It looked imminent, with upward ripples in the jet stream weakening the polar vortex, a band of strong, cold winds high above the Arctic, and hinting at a massive disruption to the weather pattern in December.
But the atmosphere had other plans.
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New data now confirms the atmosphere never crossed the threshold into a major sudden stratospheric warming event. To qualify, the band of strong winds about 55,000 feet up — the ring that normally circles the pole from west to east — has to fully reverse direction. Models suggest it may briefly flip later this week, but the reversal looks weak, short-lived, and not strong enough to count as a true breakdown of the polar vortex.
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Instead, the disturbance took a different path — a wave-reflection event often called a “Canadian warming,” where the upward surge of energy from the troposphere hits the lower stratosphere and bounces back downward. Such events are not unheard of but do not happen every year. This one peaks on Friday, and it gives higher confidence in a cool and dry pattern for the first part of December in California.
In a Canadian warming event, the portion of that heat that does reach the stratosphere tends to concentrate over Canada, and it nudges or stretches the polar vortex rather than breaking it apart completely (as in a sudden stratospheric warming). A stretched vortex still matters, but it generally works by shaping the jet stream indirectly, especially by favoring the development of a stronger ridge of high pressure over Alaska and western Canada.
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“Practically, I think the sudden stratospheric warming is now irrelevant,” says Judah Cohen, a climatologist at Atmospheric and Environmental Research who has been tracking the event’s evolution. “Instead, it is all about Alaskan blocking.”
In research published this summer, Cohen and colleagues analyzed Canadian warming events. They found that they tend to generate patterns that lock the jet stream into place for a period of time.
Cohen notes that Canadian warmings almost always steer the atmosphere into one of two recognizable jet-stream patterns. In both cases, a strong ridge of high pressure builds over Alaska, forcing the jet to bend north into the Gulf of Alaska before diving southeast across the central and eastern U.S.
Long-range weather models show a jet stream pattern that aligns with what researchers expect following a “Canadian warming” event. A strong ridge of high pressure (red and orange) deflects storms away from California, while a cold and stormy pattern sets up in the east.
Matthew Barlow, a professor at UMass Lowell and co-author of the study, says the current event appears to match one of those two ridge-dominant patterns. “I’d expect that to increase the chance of cold temperatures for the Pacific Northwest,” he says. “And I’d generously include Northern California in that.” But he notes that the influence weakens quickly as you move south, leaving much of California in a cool but largely dry pattern to start December.
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Timing adds another wrinkle. The researchers found that these jet-stream responses to the Canadian warming event typically arrive in short, distinct phases. A “first phase” pattern often settles in for 3–7 days, followed by a slightly longer 2–10 day period where the ridge shifts or weakens. That gives California a window where the jet could wobble just enough to let a few interior-track systems slip through.
For California, that blended pattern sets the stage for the first few weeks of December. By the last day of November, a strong ridge of high pressure is likely to develop in the Gulf of Alaska and close to the U.S. West Coast, keeping California quiet and dry. But about a week after that, a slightly weaker or east-shifted ridge opens the door for colder systems to slide down the interior West. But those kinds of storm systems, which come from inland, tend to arrive as moisture-starved and modest at best — and lack the ingredients that typically drive California’s biggest winter storms. Atmospheric rivers, which come from the ocean, are generally blocked.
And there’s a final wrinkle. This pattern is brittle and potentially just the first act.
Research shows that an early stratospheric disruption such as a Canadian warming event can set the stage for bigger changes down the road. Cohen notes his experience suggests that in the past “a large (polar vortex) disruption in November or December favors a major (sudden stratospheric warming event) later in the winter.”
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It’s a possibility worth watching. The atmosphere chose an unexpected route this time, but that route has placed California in a high-confidence cool and dry pattern for the first part of December.
Whether this is the whole story or just the opening act before a larger stratospheric disruption, which could bring anomalous and extreme weather impacts later this winter, remains an open question.
