What the Pacific ‘blob’ + La Niña means for winter in the US

By Andrew Freedman, CNN

(CNN) — A marine heat wave spanning much of the North Pacific Ocean is already influencing the weather in North America and is poised to make its mark on winter.

The unusually warm ocean water, coined “the blob” due to how it appears on weather maps, has brought milder and more humid air to the West Coast in recent weeks, according to Daniel Swain, a climate scientist at the University of California, Los Angeles.

This has led to unusual thunderstorms in coastal California and may be helping to limit the severity of the late summer and early fall wildfire season, at least for now.

What is clear to meteorologists is that the Pacific heat wave may be a significant player in influencing late fall and winter weather patterns across large parts of the Northern Hemisphere.

But long-range forecasters have another global weather pattern to consider this winter: La Niña has arrived, just announced Thursday morning by the National Oceanic and Atmospheric Administration.

The big question mark for winter in the United States is what the extreme ocean heat, with water temperatures running as much as 9 degrees Fahrenheit above average for this time of year, will do to the jet stream.

“I think it clearly already is affecting the weather,” Swain said of the expansive Pacific marine heat wave. He called its intensity “phenomenal, really remarkable,“ and noted it has been responsible for record warm temperatures over the summer in South Korea, Japan and other areas.

“It is probably going to result in some shifts in the Pacific storm track late this fall and winter,” Swain said. To Swain and other experts, this event is larger than previous Pacific blob heat waves that were more limited in geographic scope.

It is, based on experts’ descriptions of it, the blob to end all blobs.

The jet stream, which is a band of strong winds in the upper atmosphere, steers and energizes storms as they move from west to east across the Pacific and across North America. Its strength comes from the extreme difference between the tropics (very warm) and the Arctic (very cold).

The warm blob diminishes that difference and takes some of the wind out of the jet stream’s sails, according to Swain.

What does that mean for US weather? It could increase the amount of moisture in storms, making atmospheric rivers more prolific rain and snow producers, while also slowing the winds in the jet stream somewhat, which in turn means slower-moving storms, Swain said.

The setup across the Pacific also could give the jet stream a bit of a shove to the north, resulting in a wetter than average winter in the Pacific Northwest, Swain said.

The nascent La Niña in the tropical Pacific, which features cooler than normal water along the equator, also needs to be factored into the winter forecast, experts told CNN.

Typically, seasonal climate forecasters put more weight on conditions in the tropical Pacific than conditions farther north, but Swain said the widespread heat wave occurring to the north will interact with the La Niña in ways that need to be taken into consideration.

The warm Pacific, coupled with warmer than average Atlantic water, too, could result in a mild winter for the Lower 48 states, at least to start with, he said.

“The global oceans are super, super warm right now, so that’s just going to favor … some of that warmth,” Swain said. Colder weather may arrive later in the winter, as has been the case in other recent winters.

Atmospheric scientist Elizabeth Maroon of the University of Wisconsin-Madison told CNN there are large uncertainties in how this marine heat wave will affect the atmosphere above it, but there is no question that the blob will have an impact.

For example, she said there is the question of, “Will the marine heat wave get amplified by La Niña conditions, which is a very distinct possibility?” Those two would then work together to change winter weather patterns, she said, calling it one of several scenarios.

“It’s really hard without running some sort of weather model that has this exact sea surface temperature pattern in it to know how it’s going to impact weather over North America this winter,” Maroon said.

Predicting what this kind of extreme ocean behavior will do to winter weather, she added, is “at the cutting edge of the science we have.”

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