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* Graphic 1: reut.rs/2dEIwp9
* Graphic 2: reut.rs/2dEJgdz
* Graphic 3: reut.rs/2dEHHMW
* Graphic 4: reut.rs/2dLG9Vv
By Karen Braun
CHICAGO, Oct 4 (Reuters) - It may seem as if the United States is warned every autumn of the potentially cold winter that lies ahead. But this time, it might be for real.
Everyone from ordinary citizens to energy industry executives appreciates an early glimpse of what winter may have in store, particularly for the Northern, Central and Eastern United States - areas that are most subject to winter’s brutality.
Cold is not something that the United States as a whole has dealt with over the past year. The contiguous United States is coming off the warmest winter and fifth-warmest summer in 122 years of record-keeping.
Some weather vendors and government agencies have already started to flag the potential for a colder winter ahead, with a particular focus on the latter half of the period. Based on the current state of the atmosphere, there is validity to these forecasts despite a good deal of uncertainty at this early stage.
Records may not be broken this winter, but there is already enough evidence in the Pacific Ocean hinting that Americans residing east of the Rocky Mountains may want to make sure that the heavy winter parka still fits.
Between late 2013 and 2015, a very large, circular pool of warm water - informally known as the “blob” - dominated in the northeastern Pacific Ocean. After about a year-long hiatus, the blob is back in full force.
Located just south of the Gulf of Alaska, the blob and its associated high-pressure ridge create a blocking pattern in the atmosphere. This amplifies the polar jet stream over Alaska, resulting in a more north-south entrance of the jet stream into the Northern United States (reut.rs/2dEIwp9).
An amplified polar jet stream allows colder Arctic air to flow farther south than usual. This occurred during the notoriously frigid winter of 2013-14, which was the sixth-coldest winter in 121 years for the Upper Midwest (reut.rs/2dEJgdz).
The blob is already off to a much stronger start than in the fall of 2013 as it was only beginning to form in early October that year. But last month's sea surface temperature anomalies already equaled those of the 2013-14 blob at its peak (reut.rs/2dEHHMW).
The blob does not guarantee anything other than an open invitation for savagely cold Arctic air to cross into the United States, if it so wishes.
The blob regime has the greatest effect on winters in the Northern United States, though its impact on the Northeast is less direct and highly dependent on positioning of other upper-atmospheric features. The Pacific Northwest and the West Coast tend to experience warmer winters under this pattern.
El Niño-Southern Oscillation or ENSO, characterized by fluctuations in sea surface temperatures in the eastern equatorial Pacific, is aligned with the blob in terms of its likely influence on the upcoming U.S. winter.
Sea surface temperature anomalies in the telling Niño 3.4 region are the coolest since February 2012, and the outlook for this winter is most likely between a neutral ENSO and its cool phase, La Niña.
But the precise forecast matters not. The key point is that El Niño, the warm phase of ENSO, has vanished and most of the northeast quadrant of the Pacific Ocean is considerably cooler than one year ago - save the blob region (reut.rs/2dLG9Vv).
During the warm phase of ENSO, the tropical jet stream gains strength and often transports warmer and sometimes wetter weather into the United States, even during the winter.
But the presence of cooler-than-normal sea surface temperatures greatly reduces the tropical jet’s relevance and essentially hands the control of winter over to the polar jet stream and the Arctic.
Both the blob and ENSO cannot deliver any promises for the winter, though. The amplification of and preference to the polar jet stream is only one part of it. Just how cold the air may be as well as its staying power will largely be determined by ongoing activity in the northernmost latitudes.
This Arctic activity is predominantly driven by pressure differences across the region and it manifests in familiar climatic entities such as the polar vortex and the Arctic and North Atlantic Oscillations.
These entities perhaps have the most direct bearing on winter temperatures in the Northern and Eastern United States and can easily overpower the effects of ENSO, particularly when coupled with a blob-amplified polar jet over Alaska.
But these oscillations operate on a much smaller time scale than ENSO or the blob. Most of the time we can only look out up to two weeks in advance for guidance on the Arctic and North Atlantic Oscillations. The polar vortex may have slightly longer-term predictability, but it too can have a mind of its own and go against all apparent logic.
One more supportive ingredient of a colder winter is a high-pressure blocking pattern to the east to match the blob-associated one to the west. This would prolong cold air intrusions into the United States by wedging the bitter Arctic air between the two bookending high-pressure systems.
However, the eastern blocking pattern is also a shorter-term feature, meaning that forecasters will have to be vigilant all winter long with detecting changes in the upper-atmospheric pressure and thus the influential oscillations, blocks, and vortexes.
But even though it is still too early to predict the upcoming behavior of all the winter ingredients, two key boxes, the blob and ENSO, are already checked along the route to a colder winter for a majority of the United States.
Editing by Matthew Lewis