(The opinions expressed here are those of the author, a market analyst for Reuters.)
By Karen Braun
CHICAGO, Jan 19 (Reuters) - Last fall, the general consensus among weather forecasters was that most of the Central and Eastern United States would likely endure a colder, snowier winter than it is used to.
But instead, the third week in January - climatologically the coldest week of the year - is ushering in temperatures some 20 degrees Fahrenheit above average across the eastern two-thirds of the country. For example, Chicago may hit 55 F (12.8 C) on Saturday.
The recent milder temperatures have also diminished snow cover to below-average levels everywhere east of the Rocky Mountains - save the Dakotas - and any snow that is left is almost certain to melt over the next week or so.
What has happened is that several of the atmospheric "ingredients" that would power the chilly season have instead opted to go rogue from what the outlooks had originally predicted.
The winter started off cold and snowy for much of the country. December and the first two weeks in January featured repetitive arctic blasts along with several rounds of seasonably large snowfalls - all of which was in line with the early predictions.
But things have abruptly changed course, as temperatures in the eastern half of the country have already been very warm for the last week, particularly in the Southeast, and things are about to heat up even more.
The warmth is expected to top out on Saturday with temperatures up to 30 degrees F above normal for the majority of states east of the Mississippi River. And although things will slowly cool down after the weekend, much of the same region should experience above-average temperatures through the end of the month (http://reut.rs/2jCEFia).
So how did the "cold and snowy" winter suddenly turn into what feels like an early spring? And does this mean that winter is over?
JANUARY TURNS BALMY
One of the key reasons that the majority of the United States will not be hit with any cold blasts for the rest of January has to do with how strong the polar vortex has been over the past month.
When the polar vortex - a spinning mass of air in the upper atmosphere over the North Pole - is very strong as has been the case, this means it is spinning faster than usual, containing the frigid air over the Arctic. The contrary situation would be if the polar vortex was weak, meaning it could potentially break apart and send cold chunks of air down into the mid-latitudes.
The strength of the polar vortex has also been facilitated by how warm the Arctic is relative to normal. This anomaly is evident by the sea ice extent - which is at the lowest values in at least 38 years.
The polar vortex is also linked to the Arctic Oscillation and North Atlantic Oscillation, both of which are large-scale pressure variations that partially control cold- versus warm-air domination in the mid- and upper latitudes of the Northern Hemisphere.
When the AO and NAO indexes trend positive during the winter months, as they both have over the past month or so, cold arctic air intrusions are less likely.
Even if the polar vortex or AO/NAO were to spin off a large mass of frigid air from the Arctic, one of the vehicles for sending it down deep into Canada and the central United States has steadily vanished as the winter has progressed.
This vehicle, sometimes known as "the blob," is a warm pool of water in the Northeast Pacific Ocean/Gulf of Alaska associated with a dome of high pressure in the atmosphere just above. When the blob is strong, it encourages the north-to-south transport of cold air masses over the continent via the jet stream instead of the usual west-to-east movement.
The blob was extremely warm in September, which was one reason that meteorologists felt comfortable with the call for a cold winter. But ever since, the anomaly has fizzled and now predominantly cold water sits off the west coast of Canada and the United States (http://reut.rs/2jCByqc).
The cool-water anomaly in the equatorial Pacific Ocean - known as La Niña - was also a main reason for the cold U.S. winter predictions, as the two events have frequently coincided in the past.
The La Niña signal is still out there, but it has been a little weaker lately than in December, and its focus has been in the central Pacific rather than the eastern Pacific as is usually the case. And there is no expectation for La Niña to re-strengthen to any significant degree.
BUT WINTER IS NOT OVER
Just because the rest of January will have many Americans basking in temperatures more fit for springtime, the extraordinary warmth is not necessarily an omen for the rest of the U.S. winter.
Both the extremely strong warm and cold temperature anomalies that the United States has experienced over the past two months demonstrate just how much seasonal variability can exist. They also prove that the very active atmosphere is more than capable of turning up more cold spells before the winter is over for good.
One possible sign of the potential return to cold is that the polar vortex is forecast to drop in strength significantly as the month comes to a close. Not only could this free up some of the arctic air locked within the vortex, but it would also loosen the vortex's grasp over the AO and NAO.
The AO and NAO forecasts are generally unreliable more than a week out, but the weakening polar vortex could also precede a drop in one or both indexes. And if the AO or NAO turn negative to begin February, this would very likely be associated with a shift to colder weather - especially in the U.S. Plains, Midwest or Northeast.
The active atmosphere this winter has also been friendly to the development of blocking patterns, which are persistent domes of high pressure that linger longer than usual. These blocks often wedge cold, arctic air deep down into the North American continent.
So the fact that the blob's blocking force is gone may not mean that the arctic air has lost its path down into the United States - more blocking patterns could be in the works.
But there is still much uncertainty over what February has in store, which is evident in the U.S. Climate Prediction Center's forecast for next month's temperature anomalies.
CPC placed a cold bias in the northernmost part of the country - including North Dakota, Montana and northern Minnesota - and a warm bias along the entire edge of the Southern Tier - from California to North Carolina (http://reut.rs/2jCPlgz).
But the rest of the country is rated as "equal chances," which means that CPC did not necessarily identify a cool or warm bias for February across the majority of the United States, and this opens the door for just about anything.
Many private weather vendors have a similar take, as most groups have forecasted February's temperatures to be between normal and slightly below normal levels.
February could end up on the cooler side, but either way it is very unlikely to be as warm as the finish to January. So it would be wise to avoid packing away the winter gear just yet.
<^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Graphic- Forecast - average temperature deviation from normal (F), Jan 21, 2017
http://reut.rs/2jCEFia Graphic- Global sea surface temperature anomaly (C), Sep 2016 vs Jan 2017
http://reut.rs/2jCByqc Graphic- U.S. Climate Prediction Center temperature outlook for February 2017
http://reut.rs/2jCPlgz
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^> (Editing by Matthew Lewis)