So, what's going on? Well, I have a few hypotheses and a theory about that and it actually finds it's roots in my Winter Outlook when looking at the broad key players this winter.
Ok; then what am I posting about, what are the basic key players, the hypothesis's and resulting theory?
The key players or passengers as you will, in our jet stream "vehicle" since November: NAO/AO, North-Central Pacific Water Temperatures, EPO and El Nino (in order of precedence).
NAO/AO: North Atlantic and Arctic Oscillations
Should be no question about this one, the main driver or pilot in our weather this late fall and early winter. Or is it? Is there another, while not the main driver - a "back-seat" driver?
First, note the NAO track record thus far this cold season: it's mainly encompassed in a positive phase. Also, see the NAO remains weakly positive thru most of January; but what of the AO?
Now the AO, Arctic Oscillation did show some life back in November and early December with negative phases and our temperatures in Southeast Lower Michigan acted accordingly by falling below normal. Note the decline or negative phase estimate the remainder of the winter and early spring. I wonder if that trajectory will verify?
North and Central Pacific Water Temperatures:
In my opinion or hypothesis, this and not the weak El Nino has been the co-pilot or "back-seat driver" thus far during the cold season and the "co-conspirator" (along with NAO/AO) as to why December has been mild and uneventful. The wide area of above normal temperatures across the general Pacific has been a major contributor the mild month.
In my Winter Outlook, I stated that the second area of warmer than average water temperatures stretching across the Pacific, north of the El Nino may be just as important as the up and coming El Nino. Here is my explanation and Map from the Outlook:
"It is still assumed however, that the weak El Nino will affect the cold season upper and surface wind flow by aiding and abetting the subtropical flow into the country. In addition, another item of importance is the large area of above normal water temperatures seen out in the mid Pacific north of the standard El Nino. I feel this too will also influence the sub-tropical jet, especially the late Autumn into early winter anyway (see Nov map below). By mid to late winter, the warmer waters all around the central Pacific are understandably projected to wane (Feb map).
The below Pacific water temperatures pattern is now reminiscent of a positive PDO /PDO+/ along with the weak El Nino. Recently, negative PDO conditions have held sway out over this region. Below are the current November water temperatures including the positive PDO with warmer waters surrounding cooler waters in a horse-shoe shape".
Check out the dominant flow across that region and the U.S. this past 30 days in the boxed-in areas.
Mid November to mid December's 500H trajectory below shows very well the strong Pacific jet and also the cold polar vortex /500 Upper Low/ over eastern Canada. Both of which we've had a taste of since November...and battle is ON!
Meanwhile our third player or passenger /EPO/ has remained rather quiet this winter /Dec/ trip thus far, being mainly in a positive phase but it too seems to becoming restless.
EPO: Eastern Pacific Oscillation
Unlike last winter and main reason we were so cold last winter, the EPO has not been in a dominant negative phase. When in a negative phase, it encourages ridging along the west coast of Canada which connects with a cross polar projection of Arctic air and frequently works in conjunction with the NAO/AO but this conjunction as we saw last winter, is not always needed for a Arctic air delivery to U.S.
Note the phases of the actual observance of the EPO phase this fall and early winter on the top panel, of the EPO Index. However, more interesting developments are the change that is projected (and is actually happening as I write) on the successive 4, 7, 10 and 14 day forecast below. Check out the dates on the bottom then look again at the top panel. A strongly negative phase corresponds well with much of November, the cold month, the well below normal temperature month we just felt. See the change in the phase index in December? Nearly the whole month has been positive and that to a lesser extent, along with a positive NAO/AO and the warm waters in the Pacific; have all helped keep the bitter cold air in mainly Canada since early December.
The fourth "passenger" in this winters jet stream "vehicle" is of course the one everyone makes a fuss over, the boy child; El Nino. While he's been squakin and fussing some, he's terribly weak and really not even at "official" El Nino status yet.
From the Climate Prediction Center
Although the SST anomalies alone might imply weak El Niño conditions, the patterns of wind and rainfall anomalies generally do not clearly indicate a coupling of the atmosphere to the ocean. Therefore, despite movement toward El Niño from one month ago, the combined atmospheric and oceanic state remains ENSO-neutral.
That is another great reason why this very mild and uneventful December is actually a "psuedo El Nino", there really isn't one but it's been like one. El Nino really hasn't had an influence on the States by most accounts, which leads me back to what I feel are the main drivers in this winter thus far: NAO/AO and the mild Pacific waters and strong jet across them.
Why Have the Models Been So Erratic (Lousy)?
So what gives for the immediate future and why have the models been all over the place with storm development and cold air delivery timing? Well just look at what they are up against, two or three "passengers" trying to gain control over the weather pattern into the country. The "main driver" or pilot; NAO/AO is fighting against a stubborn "backseat driver" or co-pilot the Pacific Jet and mild above normal water temperatures. Yes the analogies are somewhat silly but very "apropos" given the radical and inconsistent behavior of the forecast models dealing with them. After all, when people think about the chaos of driving and other "drivers", it is a common image yet it really does capture the multifaceted battle in the atmosphere. ;-)
Near Term Outlook
As you can tell by some of the prognostic data presented here there is a change afoot this week anyway; and when there is a change in the overall pattern that usually involves a storm and colder air. Well again the models are dancing around with possibilities of a New Years weekend storm /3rd-4th/ and again it's everything from a major snowstorm to rain or freezing rain to nothing for us. Stay Tuned! Also, after we'll look into what January is likely to have in store and compare its past - analogues - with this December and into January.
New Years Eve looks good weather-wise right now with only the drinkers and subsequent drivers to be concerned about!
Have a Happy, Healthy and Prosperous New Year for 2015!
Bill Deedler -SEMI_WeatherHistorian