Winter 2011-2012 Final Outlook
NYC Area Weather
The winter of 2011-2012 got off to an unusually early start as a late October nor’easter brought widespread moderate to heavy snow across the area, with accumulations reported across almost the entire area over a month before the start of meteorological winter. Some places in the interior parts of the area already over a foot of snow. This early snowstorm did not keep the early winter conditions in place, however, as a much warmer pattern developed for November, with a persistent ridge in the eastern US bringing frequent surges of above to well above average temperatures into the region. The development of this pattern in November, which was not expected at first, caused me to change parts of my winter outlook, but even though we are currently in a mild pattern, this pattern will not prove to be the case for the entire winter.
A. Factors Behind The Forecast
1. La Niña Overview
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 four 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.
Some of the main teleconnections I used for this outlook will be defined and briefly discussed below. These teleconnections, as well as other variables such as the MJO, will be used throughout the rest of the outlook.
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. The graph to the left provided by the NOAA/ESRL/PSD (http://www.esrl.noaa.gov/psd/forecasts/teleconn/) shows a typical +NAO set up, with a trough near Greenland, keeping the cold air over Canada, and a ridge along with warmer than average temperatures in the eastern US. Although a ridge is frequently found in the East with a +NAO, there can be cold air in the East. 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.
Another factor for this winter is the PNA, or the Pacific/North American Pattern. The graph to the left provided by the NOAA/ESRL/PSD shows a typical +PNA set up, associated with a ridge near the West Coast. 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 sometimes 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 does not indicate that warmth is certain in the East, however; the warmth could be overcome by strong blocking associated with a strong -NAO and -AO pattern, as the winter of 2010-2011 featured.
The EPO, or the East Pacific Oscillation, is another factor that is used for long range forecasting. The graph to the left provided by the NOAA/ESRL/PSD shows a typical +EPO set up, consisting of below average heights near Alaska and above average heights in central/SE Canada and the northern US, which results in above average temperatures across the eastern half of the US. A negative EPO pattern would bring the opposite conditions. So far this November, a positive EPO has been present, which combined with the negative PNA resulted in a Pacific pattern unfavorable for cold in the US, keeping the coldest temperature departures trapped over Alaska and northern Canada while above average temperatures were observed in the eastern and central US.
B. Pattern Outlook for This Winter
November Warmth: When making my fall outlooks, I anticipated a changeover to a colder pattern towards the second half of November, leading the way to a colder and snowier pattern for December. Instead, however, a very strong negative PNA pattern developed, combined with a persistent positive EPO. Despite the slightly negative NAO, the –PNA/+EPO pattern dominated the pattern across the US, resulting in a mild pattern for the central and eastern US with only transient cool spells lasting for 2-3 days at most. The colder departures this month have been focused over the western US, where troughs were more frequent, although these troughs didn’t bring any strong cold air into the US yet. The southeast ridge has been another part of this pattern, as a ridge has been stuck over the eastern US most of this month so far. A southeast ridge brings warmer than average temperatures into the East Coast closer to the coastal areas, and depending on its strength, occasionally into the rest of the Northeast and the Ohio Valley. Below is an image from the NCEP model pages showing the 500mb heights on a GFS run from ____, showing the +EPO pattern with the trough over Alaska, as well as a ridge over the eastern US and a weak trough over the western US:
Early-Mid December: During last year’s November, there were also above average temperatures just as there were this year except for the first week of the month and the Thanksgiving time frame, which was followed by a strong cold front on December 1st which started a sustained cold pattern that would last through late January with only a few breaks in between. This year, however, it will not be as easy to get this sustained cold as it was last year. During last November, there was already strong, sustained cold air over southern Canada, and with a very strong west based –NAO block, the cold air was able to pour into the eastern half of the United States along with a western US ridge. The current pattern, however, is far from ideal for a quick and strong pattern change. The +EPO/-PNA pattern has weakened, and the EPO is now slightly negative, with a neutral to slightly negative PNA. Despite the changes in the Pacific pattern, the Atlantic pattern remains unfavorable for cold, with a positive NAO and AO as well as a lack of blocking near Greenland. Last winter, the strong blocking, or a strong ridge west of Greenland, kept the cold pattern stuck in place. The current pattern features very little ridging near Greenland, and any ridge that does form near western Greenland remains transient and quickly moves out. Without the blocking, the cold will not be able to stay trapped over the region, and will remain mostly transient, and the lack of blocking also decreases the likelihood of snowstorms towards the coast, instead favoring interior areas for snowstorms. Although the ridge in the southeastern US weakens towards December 7th, the ridge fails to collapse, and troughs dropping into the north central US flatten out and weaken prior to reaching the East Coast. This pattern will keep near to slightly above average temperatures in the East for the first half of December, with the coldest departures focusing over the central US.
Rest of winter: La Niña years similar to the current one on average tend to bring slightly below average temperatures in December, lasting into January in some cases, with above average temperatures in February. This fall, however, has failed to follow the analog years so far with a much warmer than average November, and December is on track to bring above average temperatures again. The pattern so far consists of a positive NAO, positive AO and a negative PNA, usually found with typical La Ninas. Last year’s La Nina was unusual as it contained a very strong -NAO/-AO block, a scenario rarely observed with strong La Ninas. Although some outlooks show the NAO and AO becoming negative by mid December, there is no sign of any large scale change in the pattern in Canada, with troughs still frequent near Greenland and no significant warming in the stratosphere, which a warming stratosphere would increase the chances of a -AO and, and as a result, the positive NAO and AO are likely to continue through most of December and potentially into parts of January. Meanwhile, the EPO is likely to return back towards positive, along with a -PNA, which brings the potential for a warmer second half of December. Overall, although occasional cold spells will take place in December, with a few Northeast snowstorms if the timing of storms ends up supportive,
C. Monthly outlook for the northern Mid Atlantic
A colder start to December is expected for the region with a ridge in the western US which displaces the polar vortex over Alaska, likely focusing colder temperatures over the central and eastern US around the December 8-18 time frame. There is still a lack of blocking, and with ridging staying off the East Coast but not well offshore, most of the snowstorms will stay inland towards the Ohio Valley, Great Lakes and potentially the Northeast. Although the possibility of at least one snow event closer to I-95 cannot be ruled out later in the time frame, this pattern does not support I-95 snow events. The lack of blocking and a –NAO/-AO will also mean that the cold will not be able to remain stuck over the region for the entire first half of December, but even though occasional moderation in the cold pattern is expected, any warm spells will not be as strong as those of November.
Towards the second half of December, especially after December 15-20, it appears that the pattern may moderate again. There are some hints on the longer range models that a polar vortex may return into Alaska, which as we observed late this month, is not supportive of cold in the East. Along with a +NAO/+AO pattern, a rising EPO and a moderating –PNA, warmer departures are possible for the second half of the month, and the possibility is there for at least one significant warm spell in the second half of December. Occasional cold spells are still expected, which may produce snow for parts of the Northeast if the timing is supportive, although warm spells may become more frequent by that time. The forecast for December was a bit tough as the current pattern supports above average temperatures, but climatology from analog years with conditions similar to those of this year support a colder than average December. November’s analogs also supported a colder than average month yet temperatures ended up well above average, and for December, I’m siding closer to the pattern than the analogs, with slightly above to above average temperatures, near average precipitation and below average snowfall across the northern Mid Atlantic.
NAO: Slightly positive
PNA: Slightly negative
If a pattern change does take place where the East transitions to a more sustained period of cold and snow, January appears to be the more likely time frame for this to happen. The pattern for January and February is more uncertain and will be discussed in more details in separate monthly outlooks issued as these months get closer, but it appears at this time that early January may continue the overall above average temperature potential, with transient cold spells in between. Towards the second half of January, however, the NAO and AO may turn more towards neutral or slightly negative, and the potential may be there for a more winter pattern to affect the eastern US towards mid to late January compared to December and potentially early January. When such a colder pattern sets up for the East is not certain yet, and could range from late December to late January, although at this time, I am siding with the middle of January for the change towards a more winter-like pattern. The temperature departures are more uncertain, as the time frame that the pattern becomes colder, if it does so, would mean either colder or warmer than average departures depending on whether any change happens early or late in the month, although with strong cold air available in Canada, when and if a colder pattern develops for January, stronger cold spells may be possible into the central and eastern US. Overall, I am expecting the month’s temperature to average out near average across the region with near average precipitation and slightly below average to average snowfall.
PNA: Neutral to slightly negative
February is more uncertain due to the time range, but the possibility is there for more of a –NAO and a -AO signal than the first half of the winter. The pattern may continue to favor the Northeast and the Great Lakes regions for the best potentials for snow, although the potential may be there for stronger and more frequent cold in the region with more frequent snow chances as well, although unlike the last two years, snow events would be more likely to mix with sleet and rain due to the lack of extreme –NAO/-AO blocking observed last year which kept almost every storm to the south of NYC. February’s outlook will be discussed in more details in January, although at this time, I am thinking that February may bring slightly below average temperatures with slightly above average precipitation and near to slightly above average snow.
NAO: Slightly negative
AO: Slightly negative
PNA: Neutral to slightly negative
Experimental Monthly Forecast for I-95 cities
New York City
December: +2 to +3 degrees
January: -1 to +1 degrees
February: -2 to 0 degrees
Overall: 0 to +2 degrees
Snowfall: 16-26 inches (Average: 26 inches)
December: +2 to +3 degrees
January: -1 to +1 degrees
February: -2 to 0 degrees
Overall: 0 to +2 degrees
Snowfall: 9-17 inches (Average: 19 inches)
December: +2 to +3 degrees
January: 0 to +2 degrees
February: -1 to +1 degrees
Overall: +1 to +3 degrees
Snowfall: 5-13 inches (Average: 16 inches)