Winter is Coming...Sure It Is, Uh Huh

We keep hearing about cold air poised to invade our region of Kentucky soon. While the coldest air continues to dance to our north and expected to penetrate into the western part of the U.S., it sure appears likely that it will overtake us very soon.

Then, just when it seems like this is the cold front to do it, forecast temperatures are now expected not to be that cold, yes cooler than the 15-25 degrees above average we'll be experiencing this week, but not THE cold front to usher in wintertime cold.

What's going on? I have not read or heard too many meteorologists out there talk about the Arctic Oscillation lately, a familiar teleconnection that gave name recognition to the overhyped Polar Vortex from a couple of years ago.

Instead I read about EPO's, which by itself cannot guarantee the cold air we need. It needs to be used in conjunction with PNA and AO and the closely related NAO.

Once these teleconnections align themselves, then we can start talking about cold air. Right now, as one should surmise by now, these teleconnections are not quite aligned yet.

This is the time of year I turn my attention to the Arctic Oscillation index. Because the stormy low pressure area has been centered over the Arctic region somewhat persistently as of late, a rather mild stretch of weather has occurred here while the Arctic cold has been confined to its home base.

But signals are beginning to show a possible flip in that pattern. At the very least, the Arctic region will begin to see higher heights in terms of the average geostrophic 500mb reading. Think of it as high pressure in the Arctic. But a troughy pattern will begin to ensue for the middle latitudes where we are.

Therefore, the strong, stormy winds that were keeping the coldest air locked up in the Arctic will ease thereby allowing chunks of cold air to sink south. Then when the troughy conditions in the middle latitudes come to be, the interaction will help transport more of the real cold stuff toward the northern US.

In addition, a familiar pattern we have seen before will happen again. The Greenland Block is expected to develop and strengthen by the end of the year. This will make cold air transport even easier.

The ideal teleconnection combination involving the EPO, AO, and NAO would be -EPO, -AO, -NAO if you like really cold air here.

Nevertheless, I strongly caution that this is a strong El Nino winter. It is possible that such blocks will not be able to be sustained over a prolonged period of time. I am still expecting much variability in our winter weather.

MS

* The term geopotential should have been used instead of geostrophic.
(12/13/15 - MS)