This is the updated winter 2012-13 outlook; the main outlook from November, which is referenced throughout this outlook, can be found here. The most recent updated winter outlook, posted on January 2012 for the winter of 2011-12, can be found here.
Updated Winter 2012-2013 Outlook
NYC Area Weather
Since the winter outlook was issued at the end of November, three snow events have affected the area during the month of December, with accumulations mostly to the north and west of NYC while Central Park has only recorded 0.4 inch in the entire month. The final outlook expected December to be slightly warmer than average with slightly below average snowfall; this was the overall theme of December, but the month ended up even warmer than expected. The majority of January, however, is shaping up to be mild for the region, and the outlook for January has been revised to slightly warmer than average for the region.
December Outlook Verification
The winter outlook focused on the developing mild pattern for the first half of December, expecting mild temperatures with little snow through mid December, perhaps beyond, while noting a gradually colder pattern in the second half of December into January and February. The first 20 days of December were warmer than average as expected, but exceeded the expectation; the transition to a colder pattern did occur in the last third of December, with frequent snowstorms in the northern and central US, but most of the snow stayed north and west of NYC, and despite the snowstorms, there has been a lack of sustained cold air, which was unable to reverse the warmth of the first 20 days. December ended up 4 degrees warmer than average in Central Park, exceeding the outlook for 1-2 degrees above average, although there were not many signals from late November suggesting anomalous warmth for almost the entire month. Precipitation was slightly wetter than average, with the majority of the precipitation falling after 12/15. While Central Park had very little snowfall, only 0.4 inch for the month, the rain/snow line was very close to NYC with the three snow events on December 24, 26 and 29, and snow totals sharply increased to the north of NYC with several inches of snow in the north and west suburbs. Newark, NJ recorded 1.9 inches of snow, which is below average, and Bridgeport, CT recorded 7.8 inches, which is slightly above average.
Medium Range Pattern Outlook: Mild Pattern Persists
With the above considered, the pattern generally favors the trough axis near the western US and a ridge along the eastern US, with above average 500mb heights and surface temperatures for the eastern half of the US. This is illustrated above in the 12z GEFS mean at hour 156 for January 10th, from the PSU e-Wall. The MJO, having been relatively inactive near phases 1-2-3, is currently becoming more active while progressing through phases 4-5-6, which also support above average 500mb heights and surface temperatures across the eastern half of the US. Temperatures will not be very mild throughout the entire time period; there will still be transient cool air masses moving through the region, especially in the Northeast, but with the more sustained cold air failing to reach the region while the storm track generally focuses over the Ohio Valley and Great Lakes region, keeping the region mostly in the warm side of these storms with more rain than snow, with temperatures in the eastern US remaining warmer than average. For the NYC area, this pattern is unlikely to produce much, if any snow through the next 10 days aside from snow showers on the 6th.
Going into the 13-18th time period, ridging is expected to return towards Greenland with an east based -NAO, with continued strong ridging in the northeastern Pacific and blocking near the north pole. The trough axis is expected to cover the central United States as well, with an active cold and stormy pattern likely for the western half of the US. The latest GFS runs have been consistently suggesting that a sustained cold pattern develops for the region with several snowstorms in the Mid Atlantic and Northeast regions in this time frame. This is not expected to be the case, however; as mentioned above, the large scale pattern does not support the region entering a cold pattern, especially with ridging persisting in the East. The GFS also frequently tends to speed up pattern changes, as observed several times earlier this winter. Overall, temperatures are likely to remain above average through this time period, with more rain than snow likely for the Mid Atlantic region including NYC, although transient cool air masses are still expected, with the storm track potentially shifting more east with more snow potentials into the Northeast and perhaps parts of the Mid Atlantic.
Late January - February: Potential For Colder Pattern
By the longer range beyond the 20th or so, more signs of potential changes for the East towards a colder pattern are showing up with a major stratospheric warming event, a blocking pattern likely to persist, as well as the MJO likely moving out of phase 6; some models bring the MJO into phases 7-8, which are more favorable for cold in the East, while others bring it into the COD; MJO forecasts especially for the longer range aren't always accurate, however, and this can still change. The long range CFS model, which consistently modeled January to be colder than normal, has been pointing towards the last week of January for much colder temperatures in the East, although it is worth noting that it underestimated the upcoming mild period and significantly exaggerated the cold departures in the first week of January.
As noted with the winter outlook and observed so far, there has been a strong tendency for a negative PNA this season, and this will continue through the medium to long range with ridging focused off the west coast. The pattern is likely to trend colder for the end of January into at least parts of February, although there is still uncertainty regarding whether there is a stronger, sustained cold pattern for the East or more of an up and down type pattern regarding temperatures with the coldest temperatures focused further west. For the updated outlook, I am siding with the latter scenario, with more cold getting into the region towards late January into February along with increased snow potentials, especially in the Northeast, but nothing close to the cold and snowy patterns that 2009-10 and 2010-11 produced across the region.
Summary:
As this is in the longer range, there is uncertainty with this forecast, and it is possible a cold pattern fails to materialize for the region, and near to above average temperatures persist for the rest of the season. However, recent evidence suggest that at least a somewhat colder pattern is likely for parts of the region and especially in the central-western US following the mild pattern through the medium range. For the first 4 days of the month, Central Park averaged about 2.5 degrees colder than normal, although these cold anomalies will be overturned by the warmer medium range pattern. The main question regarding the January temperature departures is with the intensity of the medium range mild pattern and the potential colder pattern towards the long range.
The original outlook for -2 to -4 degree anomalies for the region in January is no longer likely to verify; at this time, I am expecting any cold period late in the month to struggle completely reversing the warm anomalies from the medium range mild pattern, with temperatures at least 1 to 2 degrees warmer than average for January in NYC. Should the longer range cold pattern end up focusing over the region as opposed to the central-western US, however, the possibility may still be there for temperatures near average for the month, perhaps slightly below average for parts of the region. Snow is more of a tough call; December was much warmer than average across the region but also had above average snow for some areas thanks to the late month stormy pattern. Precipitation for January is likely to end up above average, with snowfall ranging from below average in the Mid Atlantic to slightly above average in the Northeast.
Overall, taking the outcome from December and the updated January outlook into consideration, the winter is likely to end up with near-slightly warmer than average temperatures and average snowfall for the area, with most of the cold and potential snow focusing from late January into parts of February. As with the final winter outlook, it is important to note that there is still uncertainty in the longer range as with every long range outlook; the potential is there for the pattern to struggle cooling down in the East, with a mild pattern generally persisting through the end of the winter. Considering that there is a strong polar vortex in this side of the northern hemisphere as well as significant ridging in the northeastern Pacific, however, there is also potential for a colder second half of the winter than currently expected. More information on the longer range pattern will be posted with the medium range pattern outlooks, posted once every 1-2 weeks.
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