(Repeats to media)
* 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 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
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
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.
THE BLOB IS BACK
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.
ENSO ET AL
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
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)