27 December 2016
There are growing signs that Arctic air may return to the U.S. by late next week as the mild weather pattern of the last ten days begins to fall apart. There is still a lot of uncertainty about whether or not this pattern will lock in or if this will be just a temporary blast of cold before milder Pacific air takes back over. The big clue to these changes is the temperatures in Alaska and Greenland. It’s been quite cold over Alaska over the past two weeks, but the long-range NOAA and Euro models show a strong ridge of high pressure in the upper troposphere building back over the area later next week. This will turn winds aloft over Canada back to the north and steer an Arctic air mass southward.
High pressure aloft will also build back over Greenland and this is where we look for some cold and snowy winter weather in the Northeastern U.S. The pressure pattern called the North Atlantic Oscillation (NAO) is heavily dependent on Greenland pressure, and when the pressures go up, the NAO goes into what forecasters call the negative phase. This tends to block the normal west to east storm track and amplify it, with colder air reaching the U.S. East Coast. The charts below show what I am talking about, but it is too soon to say if this will be a new pattern or just a temporary blast of winter.
The low-level temperatures are forecast to be well below normal over much of the U.S. by late next week…
You may be wondering just how the record warmth in the High Arctic and the fact that the Arctic Sea ice remains at record low levels play into this, and that is indeed the million dollar question! There is a developing theory about this, and while scientists agree the rising greenhouse gasses are causing the warming and melting, there is a great deal of discussion about how this may be impacting the storm track/winter temperatures. Chris Mooney at the Wash. Post had an exc. piece on this last week, and it’s well worth a read. You may recognize the name of Dr. Jennifer Francis whose been working on this for several years (I’ve mentioned it here several times).
One last note: Anytime you get true Arctic air in place, a significant snow storm is fairly likely….