The goal here is to lay out the reasoning behind the forecast ideas, though admittedly one can get lost in all the numbers. There are a number of large-scale oscillations that affect our weather over time scales ranging from days to weeks to months (if you're really the curious type, you can find some information on a few of those here). The goal is to identify the ones that match up with past similar weather patterns. Inherently we can’t simply assume that the weather will behave the exact same way this time just because it did before, but they can help steer us in the right direction.
Dramatic Flip
One of the most glaring signals to look at is the dramatic development of La Nina conditions over the last several months. This means the waters in the central Pacific are colder than normal. When the water is warmer than normal, it's called El Nino. Either condition affects air pressure and wind over much of the equatorial region, and the ripple effect is often seen across the globe.
El Nino conditions developed in the summer of 2009 and peaked around February of this year. Since then, the MEI or Multivariate ENSO Index (an overall number used to assess the strength of El Nino or La Nina) has seen a record-setting drop in the last 6 months. In fact one has to go clear back to 1955 to find any lower MEI values. As of early November, the MEI has stabilized after dropping for seven straight months.
Going back in climate records I looked for other years in which there was a rapid and dramatic reversal from El Nino (warm) to La Nina (cold) conditions. While none are exactly the same there are similarities, particularly in 1973-1974. I then looked at temperature and precipitation data for the fall (September, October, November) and winter (December, January, and February) following the drop. There were some interesting similarities; namely, it was nearly always warmer than average.
In addition precipitation was usually slightly higher than average and snowfall was near or above normal. There is no evidence, however, that a dramatic flip from El Nino to La Nina prior to winter is any more significant than if we are already in a well-established La Nina.
Tropical Activity
A significant player is tropical activity. An active tropical Atlantic indicates overall lower pressures and increased vertical motion there, which in turn affects how the jet stream responds on either side of the Atlantic. So, I searched for years with high tropical activity and moderate or strong La Nina conditions. There are a few analog years I looked at, which are in the table below.
SeasonTemperaturePrecipitationSnowfall1955/1956
Nov/Dec 1955
Jan/Feb 1956 
Nov/Dec 1955
Jan/Feb 1956
17.9"1964/1965
Nov/Dec 1964 
Jan/Feb 1965
Nov/Dec 1964 
Jan/Feb 1965
12.4"1995/1996
Nov/Dec 1995 
Jan/Feb 1996
Nov/Dec 1995 
Jan/Feb 1996
23.4"1998/1999
Nov/Dec 1998 
Jan/Feb 1999
Nov/Dec 1998 
Jan/Feb 1999
14.9"1999/2000
Nov/Dec 1999 
Jan/Feb 2000
Nov/Dec 1999 
Jan/Feb 2000
7.7"Of these years, 1955/1956 seems to me to be the closest analog with relatively high tropical activity and a strong El Nino. Winter got off to a fast start but the second half ended up fairly warm. Snowfall was slightly under average.
