Welcome to La Nina

A very strong Pacific jet stream of over 200 mph is going to help intensify an exceptional fall storm in the northern Plains. Some computer models deepen the storm into the 960mb range which is what you’d see in a sizable hurricane!

The jet stream is extremely powerful and will likely result in some powerhouse storms in the Rockies and midwest over the next couple weeks.

One of the reasons the jet stream is so strong is because a massive closed low that’s located in the north Pacific south of Alaska. This is known as a positive phase of the Eastern Pacific Oscillation or +EPO. A +EPO is typical during strong autumn La Nina episodes and generally results in warmer than normal temperatures across virtually the entire Lower 48.

Here’s the +EPO composite at 500mb and the surface where you can clearly see the Gulf of Alaska low (blue, negative height anomalies) and warm surface temperatures across the U.S. and southern Canada. The +EPO chart also shows the strong jet stream with the strong gradient between low pressure over Alaska and higher pressure north of Hawaii. The result… a massive jet stream across the Pacific like we have now. This effectively transports warm, Pacific, maritimes air across most of North American keeping cold weather bottled up to the north. Until this flow “buckles”, and the jet stream lets up, getting a sustained period of below normal chill into the U.S. is difficult.

One of the keys for the upcoming winter is how long the +EPO regime lasts and whether or not we see any upstream blocking over Greenland (-NAO). Typically strong La Ninas feature the strong Gulf of Alaska low breaking down some during the December, January, February time frame after peaking with a raging +EPO during October/November. Here’s a composite of years with a strong La Nina (MEI <-1) during October/November and strong La Nina during December/January/February and you can see how the upper level weather pattern evolves. With signs of a strong +EPO continuing for the next 10-14 days and a storm track that’s setting up well to our west I expect a 2 week period of well above normal temperatures. Even with a +EPO there can be periods of cold in the northeast if a -AO/-NAO regime develops during the winter.

I do expect there will be an early taste of winter during late November and early December… stay tuned about that.

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