"Much of the nation can expect some frigid weather this winter."
Hello everyone, and welcome to The Weather Centre's Official 2013-2014 Winter Forecast. This forecast will examine the indices most likely to play a significant role in the coming winter, as well as several hints and clues we can pick up from the weather we experienced this past summer. Bear with me- this post is quite long, but I promise you it will still be easy to understand no matter how into weather you may be.
We will begin with an analysis of one of the main drivers of the seasonal climate, the El Nino-Southern Oscillation (ENSO).
Statistics from Americanwx user donsutherland1 indicate that when the Nino 1+2 region is below normal in summer, 83% of cases led to La Nina development in the winter. The other 17% of cases led to Neutral ENSO development, which means SST's are not cold enough to be a La Nina, and are not warm enough to be called an El Nino. The water temperatures are neutral.
In the positive PDO, the Southeast tends to experience above normal precipitation, while the southern Ohio Valley will see slightly drier than normal conditions. In the temperature department, just about everyone east of the Front Range experiences below normal temperatures, except the Northern Plains, which sees above normal temperatures. The negative PDO sees the opposite of all this, with much of the US warm in a -PDO, and precipitation trends finding the Southeast in a dry area, but the lake effect snow belts in the Great Lakes at above normal precip levels. Right now, the PDO appears to be trying to switch phases from negative to positive. At the moment, I'm still expecting a negative PDO for the winter, and the chances of a neutral to slightly positive PDO this winter are low, but not completely zero.
Looking back at the global SSTA chart above, we see rather variable sea surface temperatures in the waters in northeast Canada and by Greenland. This means the positive Atlantic Multidecadal Oscillation (AMO) we have been seeing recently is losing its grip. A +AMO is defined by above normal sea surface temperatures in the Arctic Ocean, and a negative AMO sees the Arctic Ocean in cooler than normal temperatures. A +AMO tends to enhance the likelihood of a negative North Atlantic Oscillation, which then helps cold and snow enter the Northeast and overall Eastern US. If we see that the +AMO is weakening (which it has been over this past spring), it does not bode well for the chances of a negative NAO this winter. Luckily, the AMO is just one of many factors that go into the chance of a negative NAO, many of which we will discuss below. For this winter, I anticipate the AMO to be rather neutral, maybe a bit negative.
To round out this SST discussion, research I have conducted does indicate that there is a solid connection between sea surface temperatures and high/low pressure anomalies. For example, positive SSTA's lead to higher pressure, while negative SSTA's lead to lower pressure. Because sea surface temperatures take a prolonged period of time to cross the anomaly spectrum (from below normal to above normal or vice versa), we can begin looking at global SST's to find potential locations for mid level height anomalies. Right off the bat we see strong positive SST anomalies across the Pacific. This bodes well for high pressure placement in the Bering Sea and general North Pacific, something I will discuss further in my analog package discussion below. A concerning item is the above normal SST swath off the East Coast. If this body of +SST anomalies leads to high pressure formation, it could happen that we see periods of anomalously warm weather along the Eastern Seaboard that could then maximize cold weather in the Plains. But that's still a while away.
Here's the basis of why I'm issuing my winter forecast so early: my winter analog package. I took into account many factors, including the PDO, ENSO, AMO, and sunspot numbers, with many others also adding to the mix.
The years I took into account were 1962-1963 and 2008-2009. I did drop my previous analog year of 1951-1952 due to an uneasiness I had using it. These years were chosen after an initial check back in the spring for my Preliminary 2013-2014 Winter Forecast, and were recently put through a rigorous examination to assure these are the two years that fit the bill for my analog package.
I: The Polar Vortex is disrupted.
Take a look at the North Pole. We do not see any significantly below normal height anomalies that are composed in one fragment over that region. Rather, we see the two aforementioned positive height anomaly areas disrupting this negative height anomaly area. Thus, the polar vortex, which encompasses a massive amount of cold air from the Arctic, is permitted to weaken and slide to lower latitudes. If the analog package verifies this winter, it is very possible the polar vortex may shift into North America.
II: Sudden Stratospheric Warming (SSW) Chances Increase.
It is well established that the Bering Sea is a favorite place for the origination of sudden stratospheric warmings (SSW's), a phenomenon that involves rapid warming in the stratosphere that then allows unusually cold air to reach the surface and lower latitudes just a couple of weeks later. Last winter, we saw a magnificent SSW originate from the Bering Sea, and other warmings attempt to take place in that same vicinity. With positive height anomalies stationed over the Bering Sea, intrusions of warm air by these positive height anomalies into the Arctic are more easily accomplished, thus increasing the likelihood of SSW events. Due to very strong +SST anomalies across the Northern Pacific and extending into the Bering Sea, chances of persistent high pressure in this area would drastically rise if the current water temperature pattern does not significantly change by the time winter comes.
III: The North Atlantic Oscillation is Negative.
The North Atlantic Oscillation (NAO) is a phenomenon that involves height anomalies across Greenland. In the negative NAO, high pressure is stationed over the land mass in a move that permits cold air and Nor'easter's to affect the Eastern US. On the other hand, a wintertime positive NAO results in a warmer United States with lower probabilities of big snowstorms for the nation as a whole. The analog package above suggests that the upcoming winter NAO will tend to be on the negative side.
Let's take a look at why the polar vortex is so depleted here. We have already established that we have a positive QBO in place, which helps to strengthen the polar vortex. It is currently rounding out from its mature phase, which was seen in mid July. The issue here is that both of my analog years had a positive QBO, while a +QBO for this upcoming winter is to be decided by how quickly the current +QBO pulse wants to progress. If we do have a negative QBO come winter, chances are the polar vortex would be stronger than what my analogs are predicting above, but if persistent high pressure sets up in the Bering Sea, I wouldn't be too concerned. It does appear the QBO is now trying to move to a negative phase, which would allow for not as strong a polar vortex this winter. Chances are modest that we will embark upon a negative QBO by the start of winter, but the probability of getting to a -QBO in the second half of winter is much more favorable.
Alright, enough of the explanations. Let's get to the forecast.
Bear in mind this is all subject to change. If you have questions, put them in comments below and I will try to answer them when I find the time. The final winter forecast will most likely come out in late October.