2013-2014 Winter Outlook
Recent winters have been characterized by frequent weather extremes across the northeast US region; after the much snowier than average winters of 2009-10 in the Mid Atlantic and 2010-11 in the Northeast, the winters of 2011-12 and 2012-13 began with anomalously early snowfalls, in late October and early November, respectively, but the former never saw a wintry pattern emerge while the latter only saw a sustained wintry pattern develop in late January, with a historic February snowstorm in Connecticut and snows continuing through late March. This year has had a relatively normal start in terms of snowfall, with a few light snows in November, but with significant temperature extremes ranging from mild temperatures to near record cold. The anticipated theme for this upcoming winter is for a continuation of these temperature swings, but with average temperature departures for the December-February season forecast to remain close to average.
A. Factors considered for the winter outlook
ENSO – El Nino-Southern Oscillation
The ENSO, El Nino-Southern Oscillation, is represented by two main phases: La Nina and El Nino. Sea surface temperature anomalies near the equatorial Pacific are used to determine if an El Nino, La Nina, or neither are present, with four ENSO regions between 80W and 160E longitude used to determine the ENSO state. Warm SST anomalies represent El Nino conditions, and cold SST anomalies represent La Nina conditions. Region 3.4 is often used to determine the state of the ENSO, with the other regions used to determine whether the El Nino or La Nina is west or east based. Below is a CPC graphic representing these regions:
Officially, an El Nino is classified when the SST anomalies in region 3.4 are at least 0.5°C warmer than average for 5 consecutive 3-month running means. The same applies for a La Nina but with SST anomalies 0.5°C colder than average. When SST anomalies fail to meet the criteria above, it is often called neutral ENSO, representing neither a La Nina nor an El Nino, but may take on some characteristics of either one.
The last three years have been dominated by neutral or La Nina conditions, with the last El Nino dating back to 2009-2010. This makes it the longest period without an El Nino since 1998-2001. 2010-11 and 2011-12 each featured a moderate La Nina; 2012-13 began to have an El Nino developing in the late summer, and further development of the El Nino was initially forecast, but equatorial SSTA’s unexpectedly turned negative, with a neutral-negative ENSO during the winter months. These conditions have persisted through this year’s summer, with the latest 3-month running mean anomaly in region 3.4 during the August-September-October time frame at -0.3C. Over the last few weeks, however, there has been a slow uptrend in SSTA’s in the ENSO regions, with slightly positive anomalies in the western regions 3.4 and 4, and slightly negative anomalies in the eastern regions 1+2 and 3. For this winter outlook, I am siding with continued neutral ENSO conditions, with a gradual leaning towards slightly positive SST anomalies towards the second half of the winter.
QBO – Quasi-Biennial Oscillation
The QBO, or Quasi-Biennial Oscillation, is a measure of zonal equatorial stratospheric winds, which reverses on average every 13-14 months between westerly winds (+QBO) and easterly winds (-QBO). A negative QBO is often associated with a weaker stratospheric polar vortex than a positive QBO with increased probability of sudden stratospheric warming (SSW) events, with significant SSW events in the lower stratosphere leading to a disruption of the polar vortex and correlated with a negative AO and NAO, which increases the probability of, but does not guarantee alone, the possibility for colder than average temperatures in the US. Currently, the QBO is in its positive phase, and has been since at least March. The QBO is expected to remain positive this winter, and at this time it does not appear a major SSW is likely during the December-February time period.
Despite the anticipated continuation of the +QBO and the low likelihood of a major SSW event throughout most of the winter, it alone does not guarantee a mild winter with a lack of cold outbreaks, depending on the many other variables that affect the winter patterns; the winter of 2010-11 for example was during a +QBO phase with no major SSW, yet was still cold and snowy for most of December and January.
AO – Arctic Oscillation
The AO, or Arctic Oscillation, is another variable affecting the weather patterns in the northern hemisphere. A positive AO is often correlated with below normal mid-upper level height anomalies near the polar regions and above normal height anomalies and surface temperatures near the eastern US, while a negative AO is often indicative of above normal height anomalies near the polar regions and may indicate high latitude blocking. There are some variables that offer hints regarding how the AO may shape up during the winter months; a +QBO is often correlated with a stronger stratospheric polar vortex and a positive AO index. Another variable hinting at the potential wintertime state of the AO is the relatively new Snow Advance Index (SAI), relating the AO to the October rate of change of snow cover in Eurasia, with a significant increase often correlating with a negative AO and a slow increase with a positive AO. Last year, there was a significant increase in Eurasian snow cover during October 2012, and the winter of 2012-13 frequently featured a negative AO, especially during the second half of the winter. While this October started with much higher than average snow cover in Eurasia, the increase throughout the month was slow, and would suggest a positive wintertime AO. So far, through late November into early December, the AO has been generally positive, occasionally falling towards neutral; the current forecast is for a mostly positive AO, possibly falling at times to neutral or negative but averaging out positive overall.
EPO – East Pacific Oscillation
While the typical teleconnections used for short range forecasting include the NAO, AO, and the Pacific/North American Pattern (PNA), another teleconnection which can often play a significant role in the pattern is the East Pacific Oscillation (EPO), which in its positive phase is associated with below normal mid-upper level height anomalies near Alaska and the northeastern Pacific and often correlates with above normal temperatures across the US. Conversely, a negative EPO is associated with above normal heights in the same region and can lead to colder than average temperatures in the US. Recent warm winters, such as 2011-12, consisted of a dominantly positive EPO, with the cold and snow centered over Alaska and a mild winter across much of the United States.
This fall, as opposed to the last tow years, the EPO has been persistently negative, which has continued into early December with a strongly negative -EPO with standard deviation near -3. Despite the +NAO/+AO/-PNA pattern observed in November, which would typically indicate a warmer than average pattern in the eastern US, the strongly negative EPO supported several strong arctic air masses surging southwards into the eastern half of the US, with the strongest cold air mass having affected the region during late November with record cold maximum high temperatures set across the area. The strong -EPO ridge can be seen with the 500 millibar height maps as posted to the left, showing initialized 500mb heights on the GFS from the NCEP Model Analyses and Guidance site, showing strong ridging over the northeastern Pacific and Alaska; in the short range, along with a -PNA and western troughing, this pattern is supporting a frigid air mass persisting over the north central and NW US with well below normal temperatures. The current forecast is for the EPO to remain mostly negative this winter, with occasional positive EPO periods likely, such as one possible in the middle of December, but likely remaining transient.
In addition to the above, other variables were also considered for this outlook, some of these include the North Atlantic Oscillation (NAO), which is forecast to remain overall neutral to slightly positive; Pacific/North American Pattern (PNA), which is forecast to average out neutral to slightly negative; and the Pacific Decadal Oscillation (PDO), which is expected to remain negative as with the last few winters.
B. Regional Winter Forecast:
Unlike the last two winters, when most of December consisted of a mild pattern covering most of the US, this December is getting off to a more wintry start largely due to the anomalous negative EPO pattern, with frigid temperatures expected over the northwest and north central US, with less significant cold anomalies towards the eastern US with ridging persisting aloft but with increased likelihood of above average precipitation through mid December, some of it falling as snow across the Northeast. Towards the middle of the month, there are some signs of a pattern relaxation as the -EPO moderates as ridging likely shifts further west, with the Madden-Julian Oscillation (MJO) expected to shift towards phase 3, which favors above normal heights and surface temperatures over the region. Later in the month, however, a more active and stormy pattern is currently forecast to resume, with December overall consisting of near average temperatures in the NYC area with frequent variations in temperatures, above average temperatures in the southeast US, and below average temperatures in the central, north central and NW US. Towards the second half of the winter, with the lack of anticipated blocking in the Atlantic side given the forecast +NAO/+AO, a sustained frigid pattern at this time appears unlikely for the region, but with cold temperatures possible through occasional strong arctic surges, mainly in the central US and occasionally reaching the eastern US. With the frequent temperature variations, a stormy pattern is likely as well, but with the highest probability of above normal snow likely in interior regions, mainly north and west of the NYC area. The overall outlook for this winter in the NYC area is for near to slightly below average temperatures, slightly above average precipitation, and near to slightly below average snowfall, but with these temperatures likely a result of frequent temperature variations.
Monthly forecast for NYC area:
Near average temperatures, slightly above average precipitation and near average snowfall.
Slightly colder than average, Near average precipitation and near average snowfall.
Near average temperatures, Near average precipitation, slightly below average snowfall.
Overall winter outlook:
December: -1 to +1 degrees
January: -1 to -3 degrees
February: -2 to +1 degrees
Temperatures: -2 to +1 degrees
Snowfall for NYC: 18-29 inches (Average: 26 inches)
While there have been some advancements made in long range forecasting capabilities, forecasting winter conditions with high accuracy remains very difficult, and different outcomes compared to those forecast are always a possibility. Snowfall outlooks are especially of high uncertainty, as one significant storm could largely skew snow totals towards much higher amounts than they otherwise would be; an example of this is 2005-06, when the most significant winter storm in February alone made up over 50% of the total winter snowfall, which otherwise would have been below average. An updated winter outlook will be posted in early January, reflecting the verification for December and updating any potential changes towards the second half of the winter.