Friday, January 6, 2012
Updated Winter 2011-2012 Outlook
Note: Minor revisions have been made to this outlook on January 7.
As of the time that this was posted, on January 6th, New York City still has not seen snow since October, making this year a likely candidate for the latest first measurable snowfall in the December-February time frame in recorded history. By this time of the year in 2010-2011, the area has already seen a major blizzard, and a cold and snowy pattern was just beginning to unfold. This year, however, the majority of the winter so far has sen warmer than average temperatures, in some cases much warmer than average, and there is still no snow event in sight for the next 7-10 days at least. There are changes unfolding in the pattern, and NYC will see snow this winter, a few inches at the very least, although t he winter will most likely end up with below average snow.
Pattern Change Development
The focus of my winter outlook created in late November and published in early December was the potential for a pattern change towards the middle of January, with a mild pattern through early January with below average snow across the region. The outlook also mentioned that although a colder and snowier pattern could develop afterwards, the best risk of snowstorms would still stay to the north of the area, with some snow events for the area but also including more frequent mixing with sleet and rain. Through early January, the pattern so far has generally followed the expectation, with a warmer than average East and only a few snowstorms across the entire Northeast region, even up to the Great Lakes and northern New England region which typically see much more snow by this time of the year.
The pattern has been very stubborn over the northern hemisphere, with a consistently cold stratosphere, near record cold at times, with a sustained polar vortex near the Greenland and Alaska regions preventing the development of Greenland blocking and a -NAO, needed for Mid Atlantic cold and snow, and an absence of ridging near the west coast and Alaska, needed to provide amplification in the pattern which results in a better chance of cold and snow in the East. There has not been much amplification with this pattern as it remains very progressive, with cold spells staying flat and quickly moving out. The most amplification the East has seen so far this winter was with the cold spell on January 3-5, which also was able to quickly move out and weaken with yet another surge of warmth spreading east. There have been minor changes with the pattern in the US, as the ridge in the southeastern US that resulted in the very mild temperatures throughout November and most of December has significantly weakened. Temperatures are still mostly warmer than average, although significant warm spells with temperatures surging to 55-65 degrees are nowhere near as frequent as they have been earlier in the winter. The large scale pattern, however, has not changed yet, so the eastern half of the US is still seeing few significant snow events with generally above average temperatures.
A change is starting to unfold, however. A round of sudden stratospheric warming began to take place not too long ago, although for now the warming is mostly confined to the 1-10 hPa regions. The reaction from the stratospheric warming to changing the pattern to a cold and snowy one is not instant, especially if the stratospheric warming backs off slightly, which is a possibility. At the same time, the pattern that is in place is still firmly in place, with many indications supporting this, ranging from the MJO struggling to move into the phases favorable for eastern US cold/snow, the lack of ridging near Greenland due to the stubborn polar vortex, and the frequent polar vortex over Alaska as well helping to keep the EPO positive. As the original winter outlook showed, a positive EPO tends to result in above average temperatures, especially for the northern US including the northern Mid Atlantic. Considering that the pattern is still generally locked into place with only occasional minor changes, it will take time for any pattern change to unfold, and it is appearing more likely at this time that while improvements may very well take place with the pattern, the current pattern may never completely fall apart, with signs of a Greenland and Alaskan polar vortex still returning along with difficulty to establish -NAO and blocking. Despite this, however, the changes that are unfolding will help to make the pattern a colder and snowier one, especially with ridging developing near Alaska and more stratospheric warming, along with the potential for slightly negative NAO/AO at times towards the longer range, which will help to bring more cold into the northern US along with more snowstorms for the Great Lakes into the Northeast and the NYC area occasionally.
Another factor to consider is the historical analogs. As I have stated in some of my recent daily discussions, there have only been two Decembers without snow in Central Park in recorded history; 2006 and 1877. Both of these winters ended up with well below average snowfall, with the total winter snowfall ending up near 12.4 inches in 2006-07 and 8.1 inches in 1877-78. The fact that only two other winters had no snow in December in addition to this year shows just how unusual this pattern is; furthermore, both of these winters did not have snow events prior to December, while this year there was an October snowstorm. The fact that there was a significant snowstorm in October but no snow in December in NYC is yet another evidence that so far, this winter is unusual. In addition, in the three other winters in recorded history when snow accumulated in October in NYC, accumulating snow fell in both November and December as well. Only two winters with no snow in December is a small list of analog winters, but even when looking through the 15 winters that had a trace of snow in December, all of them except for one had below average snowfall, with more than half of them ending up with less than 20 inches of snow for the entire winter. While history does not always have to repeat itself, these historical references strongly suggest that probability of reaching average snow this winter is low.
Outlook for January - March
With the factors mentioned above, for now I am thinking that the pattern begins to change in the second half of January, especially towards the end of the month, but with the pattern still locked into place, it is very possible that the pattern as it is now, although changing towards more cold and snow, fails to completely fall apart, with the result ending up as a "gradient" pattern, where the Northeast and Great Lakes are favored for snow events while the northern Mid Atlantic sees some mixed precipitation events but more frequently rain and/or a wintry mix instead of snow. These changes in the pattern will also bring some snow events to the NYC area and the northern Mid Atlantic, especially between the end of January into March, although snowstorms are more likely to stay on the moderate side, with a lower probability of any significant snow event like those of the last two winters. Possible variables of this pattern include a neutral/negative EPO, neutral/negative PNA, and a neutral to slightly positive NAO/AO, dropping to negative at times but at least as of now, not likely to stay negative the majority of the time from late January through February. The changes in the pattern are likely to stick around through February and possibly into March for parts of the region, and although the area will see some snow events, this winter is most likely on track to end up warmer than average with below average snowfall, potentially well below average in the scenario that no significant winter storm affects the northern Mid Atlantic or closer to average if the pattern change is stronger than currently expected.
For the second half of January, gradual changes in the pattern are likely, with the first snow of the meteorological winter expected sometime around this time frame for NYC, along with temperatures potentially ending up near average, possibly slightly below average should more cold be able to drop south from Canada; the near-slightly below average temperature forecast assumes that there will be some changes, although in the case that the NAO/AO remain solidly positive, slightly above average temperatures would continue. With the changes sticking through February and March, temperatures for both months may end up once again averaging near to slightly above average for the NYC area, although more cold and snow is likely for the Great Lakes and Northeast regions, with occasional periods of cold and snow also affecting the NYC area in February and March.
Considering that this is still in the longer range, there are still uncertainties in the forecast. It is possible that the pattern stays similar to where it is now through the rest of the winter, the result being very little snow for the entire winter in the Mid Atlantic region, including NYC, with only a few snow events from now through the rest of the winter. Such scenarios, although definitely uncommon, have happened with some of the winters where the patterns were also mild with little snow. In the other scenario, more significant changes with the pattern would take place, and while a cold and snowy pattern like that of the last two winters would not unfold, more snow and cold would take place, with the total winter snow ending up closer to average. The latter scenario is not out of the possibilities, and could happen, although for now I am going with a less extreme change in the pattern given the situation the region is seeing.
Overall, however, the winter is likely to end up with above average temperatures and below average snowfall. The only snowstorm Central Park has seen so far, the late October snowstorm, places this winter's snow to date at 2.9 inches; assuming that some more cold and snow takes place, I am expecting this winter's snow totals to end up in the 13 to 22 inch range. At this time, it appears that the total may lean towards the lower end of this range, although I kept the higher end of the range near 22 inches accounting for the possibility that a colder and snowier pattern than currently expected does set up, in which case total winter snow may be near or possibly slightly above 22 inches, although at least with the latest indications, it appears unlikely that Central Park sees average snow this winter. More information on the medium to long range pattern will be posted with the daily updates throughout the month, along with a few updates in the "Long Range Forecasts" page.
Posted by NYC Weather at 11:13 PM