Weather is global, and events that happen in other parts of the world impact what we experience here in North America. The developing La Nina in the Pacific Ocean is one factor. It has been talked about at length for months, but still has not developed fully. Though sea-surface temperatures near the Equator east of the Dateline have been somewhat below normal, they have not crossed the line to be considered a true La Nina state just yet.
However, during the last couple of weeks, those temperatures have lowered more rapidly. Due to waves in the ocean bringing up cool water from below, and stronger trade winds pushing warm surface waters westward, the weekly sea-surface temperatures have fallen a tick below the La Nina threshold as of Dec. 16, reported by the Climate Prediction Center, a division of NOAA. It takes time and is still yet to be technically a La Nina, but the cooler waters are starting to make an impact, and setting up a response for a changing jet stream in the Northern Hemisphere.
At the same time, a burst of thunderstorms that travel east along the Equator is also going to start shifting the weather pattern. Dubbed the Madden-Julien Oscillation, or MJO, the thunderstorms redirect and add energy into the atmosphere that changes the nature of the jet stream. The MJO will be working through the Americas, into Africa and back into the Indian Ocean in early January.
When it does so in Northern Hemisphere winter, it typically leads to a ridge forming up around Alaska and northwest Canada. That ridge pokes up north of the Arctic Circle and in response, a trough typically follows to the east, across Canada and potentially down into the U.S., bringing a burst of frigid air from the North Pole. The magnitude and placement of this ridge will be key, but long-range models are suggestive that this ridge will set up in the right spot and with enough magnitude to cause a disruption.
One final piece to the puzzle will come through via a large high-pressure dome moving into the Himalayas and Tibet. Dubbed a negative East-Asian Mountain Torque event, or EAMT, this has the effect of slowing down the jet stream to its east, allowing it to become much more amplified across the Pacific Ocean. The end result is to enhance a ridge across western North America and Alaska; this will aid the MJO in producing this ridge.
Timing is everything, and that is still being worked out. But these factors seem to be colliding, perhaps starting right at the turn of the calendar or maybe a few days later. A storm is likely to bring in the burst of colder air, and models indicate that one will come through right around New Year's Eve or Day. This type of system will generally spread cold air from the West through the East over a period of days, but will tend to last. It may only be a precursor to the colder air that should persist through the middle of January, coming in waves, so not every day during this colder pattern will be below normal, much like not every day in the current warm pattern has been above normal.
What is unusual, and would have been the ultimate kicker, is that this event is not being aided by a disruption to the polar vortex. Typically, polar vortex disruptions lead to long-lasting cold weather across Asia, Europe, or North America, but this cold event will not be influenced by this feature. Had it been the case, it would have meant much more significant cold. So, while this event upcoming will be a cold one, it could be worse.
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