2010-2011 Winter Season Forcast

Technical Discussion

Issued November 12, 2010

The following sections are used in the development of my winter forecast:

Each section will have a detailed discussion and explanation as the reasoning for the upcoming winter.

Comparing Past Seasons

Taking a number of considerations into effect, it is possible to compare past winter seasons with the upcoming one. The term Analog Years is used to compare past atmospheric conditions with the current state and make a possible interpolation of what may be expected for the winter season.

The analog years(winter seasons) I have come up with for the current season are as follows and also show the total snowfall(Dec. to Feb.) during that season for both Washington, DC and Philadelphia, PA.. The Stations used are Regan National and Philadelphia International Airport.

The reasoning as to why these were chosen and their ranking is explained in the Summary and Analysis section.

Climate and Teleconnection Patterns

These patterns are already well understood in the Meteorolgical community. Rather than describe each in detail, only figures, short-term analysis and projections for the upcoming winter


There are multiple ENSO regions and while all are important, the Nino 3.4 region is best one for determining the type of events. For more information on the Nino regions, thresholds and an overview of these values over the past few months, see NCDC's summary. Waters in these regions cooled rapidly through the Spring and continued to do so through the summer months, going from moderate Moderate El Nino to Moderate La Nina conditions in only a five-month period(See the graph). The transition was one of the most rapid of all time(going back to 1950). We have been in moderate La Nina conditions since Mid-August.

Using the Oceanic-Nino Index, the only other year when this happened was in 1998. Perhaps you could also argue 1988 as well, but the transition began early that year. Here are some images showing the rapid transition in the ENSO region, both for 1998 and the current one. The one big difference you'll notice is that the La Nina that began in 1998 was more West-Based, where the current one is spread out in all regions.

As we look at the conditions over the past two months, we can see that the La Nina is firmly established and is not going anywhere, anytime soon. Not only is this seen at the surface, but well below thte surface too. Its very important to look at the depth of warm/cold waters for these events, as that can determine how quickly an event will last and if any weakening/strengthening will occurin the coming weeks.

The forecasts for ENSO continue to show that moderate La Nina conditions will continue for much of the this winter. Its still possible that we could get close to borderline Strong conditions, but in recent weeks rate of cooling has leveled off. In fact, some of the October forecasts show a more rapid weakening of La Nina towards the end of winter. Trying to predict how quickly the events will weaken/stregthen can be quite challenging and can pose big problems for long-range forecasts.

Let's look at the anomalies on a wider scale. There is no east/west based bias with this event. The cool anomalies stretch far, north and south, from those regions as well. A couplet is beginning to form in the Central Pacific, signaling a strengthening negative phase of the PDO. One area of interest is in the North Atlantic, near the Southern coast of Greenland and Baffin Bay. A very large areas of warm SST's exist. This would usually be a good indication that the upcoming winter could see more of a negative AO/NAO pattern. It should be noted that the waters have cooled a bit in those areas through October, compared to September.

There are some big differences in the global anomalies than during the last La Nina event back in 2007. The above average temperatures in the Atlantic, were further south, more confined to the North-Central Atlantic and waters off the Newfoundland coast. The waters had continued to warm through November. This signals more of a positive NAO pattern setup and lead to well above average temperatures in December of 2007. In addition, the coolest waters in the Pacific were farther east in the ENSO 1/2 regions, and in late fall/early winter, it was an East-based event. By the second half of the winter, the coolest waters shifted west towards Nino 4.

The Southern Oscillation Index(SOI) is another method of determining the strength of a particular phase of ENSO. The index is measured by the surface pressure differences between Tahaiti and Darwin, Australia. Negative values indicate El Nino conditions and positive values indicate La Nina conditions. With this current La Nina event, the SOI values have been off the charts. Very positive values have been seen since June. This has not happened often. The only other times it has occured between June-September is 1917, 1950 and 1975. This is mainly an observation and we'll have see how the winter plays out and if there is any sigificane to this. Here are the temp/precip anomalies of those following winters.

In summarizing, the best analogs to compare using ENSO would be seasons with moderate to strong La Ninas and ones coming off a moderate to strong El Nino the previous season. These seasons would be 1998/1999, 2007/2008, and 1973/1974.


The PDO is a longer-term, decadal pattern that originates in the North Pacific Ocean region. A particular phase, warm or cold, can go through 20-30 year cycle; however, there can be brief 2-3 year short intervals where the pattern can change phase. The warm phase is associated with above normal SSTs along the West Coast of North America and the waters off the Alaskan Coast, while below normal SSTs are found in the central Pacific Ocean. This implies stronger ridging and warmer tempertures in Western Canada and the US(+PNA) and colder temperatures in the Eastern US. The opposite is true for the cold phase. In addition, the Pacific Jet Stream is very strong, with storm after storm pounding the West Coast and eroding all the cold air from Canada. Troughs tend to dominate the Western US(-PNA) and the East Coast is warm. Also, the pattern is very progressive, so storms move rather quickly and even if the cold air is in place for a snowstorm along the US East Coast, it will not have time to dump a lot of snow in many areas.

For more information please see the wikipedia page

The beginning of 2010 saw +PDO conditions. However, since June, waters have rapidly in that region and are now in moderately negative values. This implies that a pretty strong Pacific-Jet will dominate in the coming months, leading to a progressive pattern and short intervals of cold and warm outbreaks. See the monthly values

NAO, PNA, and AO

These are probably the most difficult at forecasting on a seasonal basis. These patterns change much more frequently than the ENSO and the PDO. They are more accurate and predictable on a daily, weekly and bi-monthly basis. Basically, If you want to see a lot of snowfall and much below average temperatures, then the following phases of these patterns will need to dominate this upcoming winter: +PNA, -NAO and -AO. I have done research on many of the climate patterns and there is strong evidence that the AO is superior when it comes to forecasting monthly and seasonal snowfall, and major/historic snowstorms. In my opinion, it trounces the other patterns. Just look at last winer. It had the strongest -AO seasonly on record and you saw the results: Massive snowstorms and record monthly/seasonal snowfalls. So what do the past analogs show? Well, its not good if your a snow lover.


Eurasia and North American Snow Cover

Monitoring snow cover during September and October is very important variable in developing a winter forecast. Evidence has been shown that above and much above normal snowfall in both Euraasia and Canada can lead to colder and much snowier winters for the Eastern US. The above snowfall can help bring artic outbreaks and storm tracks much further south and east in latitude. This is something that needs to be monitored almost on a daily basis beginning Sept. 1st of every year. Its a very tricky variable to analyze because there may be well below normal snowfall in Sept and October, but rapidly increase in a matter of weeks through November. This can really affect any early seasonal forecasts that are issued early. September snowfall has been lower in recent years, mainly due to the lack of sea ice during the summer, which increases land temperatures. Its my observation over the past 5 years that we get rapid increases through October and early November. This graph confirms that.

Current conditions show slightly above normal snowfall in both areas, so this season is off to a good start.

September 30th Snow Cover September 30th Climatology October 31st Snow cover October 31st Climatology

Images Courtsey of the Rutgers Snow Lab

September/October Synoptic Pattern Analysis

To get an idea of how the pattern may be setting up for an upcoming winter, watching and analyzing the synoptic pattern in September and October can give a good indication as to what the setup will be like for the upcoming winter season. This includes ridge/trough positions and the mean storm track, and temperature and precipitation outlooks. Storm track is important because it can determine what type of winter events you will see, i.e. all snow, Snow to rain, ice/mixed.

During September, we saw lots of Ridging in the Southwest and south-central US.My thinking as to why this has been occuring is with the large area of above normal SST's near Greenland and a -NAO in both months.

That trend has continued into the first weekend of November as yet another large trough set up along the east coast. But as with all the others, it quickly moved out after a few days. The pattern started becoming more progressive during October as the Pacific Jet began to increase.

September 500mb Anomalies September Temperature Anomalies October 500mb Anomalies October Temperature Anomalies

Summary and Analysis

As is the case each season, how certain phases of the various climate patterns change over the course of the winter will determine how the winter will ensue. I won't be able to see the ENSO forecasts from the various models for November, but if the trend for October continues, we may see some big changes come late winter. The La Nina has pretty much leveled off, even looking at the data into the first week of November. Normally, ENSO tends to peak in Nov/Dec depending on the phase and type, and will start to reverse come the new year. A Weak La Nina is quite different than a moderate/strong event. The big question is does it weaken rapidly or gradually? I believe it will be a gradual decrease, with the possiblility of a rapid decrease come late February, but this will be too little too late to really impact the forecast. Through at least Mid-Feb, we'll be in a moderate La Nina.

The charts below show temperature departures and monthly snowfall for the months in the analog years I have chose. Some things stand out. January was the warmest and had the least snow. February was the snowiest month.

Philadelphia, PA(KPHL)
Season Temperature Departure / Montly Snowfall
December January February
1973/1974 + 4.7, 4.6" + 3.6, 4.1" - 2.2, 12.1"
1988/1989 - 0.1, 0.4" + 5.3, 6.0" + 1.7, 2.4"
1998/1999 + 6.2, 2.0" + 4.6, 4.9" + 5.0, 0.7"
2007/2008 + 0.3, 1.6" + 3.6, 1.0" + 2.1, 3.7"
Washington, DC(KDCA)
Season Temperature Departure / Montly Snowfall
December January February
1973/1974 + 3.8, 11.0" + 7.3, 1.5" + 1.9, 4.2"
1988/1989 - 0.2, 1.2" + 4.7, 2.9" + 0.3, 1.2"
1998/1999 + 5.0, 0.5" + 3.6, 2.2" + 3.7, 0.2"
2007/2008 + 2.3, 2.6" + 5.1, 1.3" + 2.9, 1.0"

Ranking of Analogs and the reasons for it.

  1. 1973/1974- La Nina started a bit earlier in the year based off the ONI; Moderately Negative PDO;
  2. 1998/1999- Fits ONI the best. Moderate La Nina for most of the winter.
  3. 1917/1918- ***Wildcard***. Very strong match with the SOI. In addition, a season with minimal sunspots.
  4. 1988/1989- Strong La Nina
  5. 2007/2008- La Nina continued to stregthen into early February reaching Strong phase as one point. Think this one weakens rapidly come February

The Temperature/Precipitation maps in those years for Dec-Feb.

In conclusion, the theme with moderate to strong La Nina events is much warmer than average temperatures and below average snowfall. As we saw with the 2007/2008 season, the models are going to show many false signals and will try to show big east coast snowstorms beyond 5 days, only to shift North and West with time. Its going to be a west of I-95 and Appalacian storm track with a progressive pattern. Even though September and October didn't see conditions like we normally would across most of the U.S, I think you have to take the stregth of the La Nina for what it is and go from there. I don't think we'll see as much snow as the 73/74 winter nor the extreme warm temperaturs like 98/99, but probably lie somewwhere in-between both seasons. As I already pointed out in the ENSO section, the one thing I'll really be monitoring as an experiment is to see how the highly SOI values over teh past few months plays out. Those seasons I mentioned had a much different outcome than the analogs I have chosen.