Winter 2011-2012 Preliminary Outlook
NYC Area Weather
With an early season snowstorm expected to affect the region this weekend, bringing the first snow of the season into parts of the NYC area, it is time to start looking at the upcoming winter. Long range forecasting is not exact, and can be incorrect, but it does give a general idea on what we can expect for the winter. Last year’s winter outlook was incorrect in many ways, with the winter turning out to be a historic winter bringing two of the biggest snowstorms in Central Park’s recorded history. Last winter’s forecast, however, was more complicated due to a lack of historical analogs to look back at; strong La Niña’s are relatively uncommon, and most of them produced below average snowfall in New York City. Although there are uncertainties with this winter’s outlook as well, there are more hints and historical references that are providing more clues to what this winter might bring.
A. Factors Behind The Forecast
1. La Niña
The ENSO isn’t the only factor that decides the outcome for the winter, though it does play a significant role. There are three different ENSO states: La Niña, neutral, and El Niño. Sea surface temperature departures near the equator in the eastern Pacific are used to determine whether a La Niña or an El Niño is in place, with that specific region separated into four regions: ENSO regions 4 and 3.4 are to the west, with regions 3 and 1+2 further east. Below is a graphic from the CPC showing these regions:
If ENSO region 3.4 averages at least 0.5 degree Celsius below normal for five consecutive months, there is a La Niña in place. If ENSO region 3.4 is at least 0.5 degree Celsius above normal, there is an El Niño in place. For this winter, we are dealing with a La Niña. Over this summer and fall, a La Niña developed again, making this winter a second consecutive La Niña, after last winter’s borderline strong La Niña. The La Niña is still developing and slightly weakened earlier this month, although it is intensifying again. I am expecting the La Niña to continue to intensify throughout the rest of this fall into the early winter with departures of at least -1.0 to -1.3 degrees Celsius, making this winter a moderate La Niña.
An average La Niña can result in a warmer than wetter than average winter in the Mid Atlantic, warmer and drier than average conditions in the South, and a colder and stormier than average northern US. Not every La Niña is the same, however, as the location of the coldest departures with the La Niña is important as well. Looking back at the graphic above showing the ENSO regions, the La Niña can either be east based, basin wide, or west based. If the coldest departures are focused in the western regions, the La Niña is west based; the opposite applies for an east based La Niña. A basin wide La Niña is observed when the coldest departures are spread throughout the ENSO regions. A west based La Niña, as we saw during the winter of 2007-2008, often results in a ridge near the East Coast with above average temperatures, with the colder departures focused towards the north central/NW US. An east based La Niña, as we saw during last year’s winter, often results in a colder and sometimes snowier than average winter for the East Coast. Currently, the La Niña is basin wide, fluctuating between slightly east and slightly west based, which can result in a scenario in between the two scenarios previously mentioned. At this time, I am expecting the La Niña to remain mainly basin wide throughout the winter.
To create this winter forecast, I used some teleconnections and other factors, with the two important factors discussed below.
The NAO, or the North Atlantic Oscillation, is an index that can either be positive or negative, and its phase is determined by the North Atlantic pattern. A positive NAO is present when there is a trough near Greenland, keeping the cold air over Canada, and often results in a ridge along with warmer than average temperatures in the eastern US. A negative NAO is present when there is a ridge near Greenland, which displaces the cold air further south and blocks it with more frequent below average temperatures in the central/eastern US. A negative NAO alone does not indicate colder than average temperatures, as the negative NAO can be either east or west based. An east based –NAO has the ridging to the east of Greenland, which typically brings less cold into the East US with a further west storm track, while a west based –NAO has the ridging to the west of Greenland, and brings a higher probability of below average temperatures in the East coast. At this time, it is more difficult to determine whether the NAO will be east or west based this winter, but it is a factor that should be kept in mind during the winter months.
Over the last two winters, there has been a strong –NAO signal. This contributed to the colder and snowier than average winters the Mid Atlantic has experienced. Looking back at other La Niña winters, the winter of 2007-2008 featured a positive NAO, which contributed to a warmer than average winter in the East. This summer and fall, however, the –NAO signal has weakened, and a positive NAO is beginning to show up more frequently. Although a negative NAO is expected for this winter, the –NAO isn’t expected to be as strong as the last few years. For this winter, I am expecting the NAO to average out as slightly negative, with occasional periods of a positive NAO at times.
Another factor for this winter is the PNA, or the Pacific/North American Pattern. A positive PNA is associated with a ridge near the West Coast, and a negative PNA is associated with a trough near the West Coast. The PNA often reflects what happens in the central and eastern US; a +PNA often translates into a central/eastern US trough with colder than average temperatures, and a –PNA often translates into a central/eastern US ridge with warmer than average temperatures. A +PNA and –NAO combination is indicative of cold and potentially snowy conditions for the Mid Atlantic and Northeast US, though +PNA/+NAO and –PNA/-NAO combinations are possible as well.
Over the last two winters, the PNA has been mostly positive, which combined with the negative NAO, contributed to the cold and snowy winters the Mid-Atlantic and Northeast US have observed. As with the NAO analogs, the winter of 2007-2008 as well as 1988-1989 featured a negative PNA, which resulted in warmer and less snowy than average conditions in the eastern US. So far this fall, we have seen a mainly negative PNA, with occasional periods of a positive PNA. For this winter, I am expecting the PNA to average out to at least neutral or slightly negative, with some periods of a positive PNA at times.
B. Winter Comparisons
In addition to the factors mentioned above, analog years are also considered when making the winter forecast. For this winter, a weak-moderate La Niña is expected along with a slightly negative NAO, neutral PNA, and a negative PDO. Some La Niñas that can be compared to this winter include 2007-08 and 2008-09. Last winter was also a La Niña, but considering that the –NAO and –AO signal was much stronger last winter, the La Niña was more east based, and was stronger than this year’s La Niña, a repeat of last winter’s extreme snowfall in the northern Mid Atlantic and southern New England is unlikely for this winter.
The winter of 2007-08 has some similarities to this winter, including a –PDO pattern. That winter, however, brought a consistently positive NAO and a negative PNA, with a strong ridge in the southeastern US pushing the storm track inland during most of the winter months. That La Niña was also stronger than the current La Niña, as by this time of the year in 2007, there was already a well-organized, strong and west based La Niña. The La Niña of 2008-09, which brought slightly above average snowfall to the NYC area, has more similarities to this winter, as it was also a second year La Niña, and brought a –NAO and a slightly positive PNA. That La Niña, however, was weaker than this year, and fell short of the criteria for an official La Niña. With all of the factors considered, this La Niña may have some similarities to the two winters mentioned above, but is unlikely to be as warm as 2007-08. At the same time, a less extreme version of last winter is possible, leading to the conclusion that the above average snow zone will likely extend further west next winter, but above average snow is still possible in the region along with slightly below average temperatures.
C. Outlook for the United States
When putting the factors mentioned previously into a single forecast, signs point to another active winter in the United States, but with the focus of the snow towards the Great Lakes, Ohio Valley and the Northeast, unlike the last two winters which focused the snow over the Mid Atlantic. Regardless, with a negative NAO signal likely to continue along with a slightly negative NAO and a neutral PNA, positive at times, snowstorms will still affect the Mid-Atlantic region, with slightly above average snow and slightly colder than average temperatures possible for this winter. The coldest departures for this winter will likely focus over the north central US into the Great Lakes and the Ohio Valley, with warmer than average temperatures over the southern and SW US as well as parts of the SE US. Above average precipitation is likely for the central US into the Ohio Valley, Great Lakes, Mid Atlantic and the Northeast, with below average precipitation for the southern and SW US.
For the northern Mid-Atlantic region, including NYC, a slight colder and snowier than average winter is possible. There is more uncertainty this far out with the month by month outlook, but a cold and snowy start to the winter may be possible as with last winter, with a slightly warmer pattern for the second half of the winter. There will be more frequent warm ups than last winter as some of the storms track to the west of the coast, either near the Great Lakes/Ohio Valley or over the I-95 corridor, bringing rain to the East Coast including the area, but coastal lows are expected as well, which may bring the total snow this winter to slightly above average. Forecasting the more specific snowfall amounts is more difficult, however, as only one big storm could change the entire forecast as last year’s December 26-27 blizzard proved.
Preliminary Temperature Departure Map:
Preliminary Snow Departure Map:
This is only the preliminary outlook, and is still subject to some changes. Stay tuned for the final winter outlook which will be posted around the middle of November, discussing more of the factors used for the forecast and going more in-depth on the forecast itself, including a month by month outlook.