October 29-30, 2011 Storm Summary

It’s the year of 2011, when almost any weather extreme can happen, ranging from two blizzards in January to a heat index near 120 degrees in July, a hurricane affecting NYC in late August, and over 20 inches of rain in a 30-day period. The year of extremes holds its title once again, as the biggest and one of the only October snowstorms on record affected the NYC area. This rare October snowstorm brought over a foot of snow inland, 2.9 inches of snow in Central Park, and a rare white Halloween across most of the area, which all earned this storm its place in history. The forecasting of this storm was difficult with a lack of historical references, however, as never in the nearly 143 years of recorded history in NYC was there any October snowstorm like this one.

Storm Impact Across the Region:

Radar image of storm at 11:48 AM

This snowstorm was the result of a coastal low pressure intensifying while moving from eastern North Carolina to just south of Cape Cod, which combined with a cold air mass in place, heavy precipitation, and a high pressure to the north of the storm, resulted in a rare October snowstorm for the region including the I-95 corridor. Light rain spread across the NYC area between 9 and 10 AM, as snow fell in Pennsylvania and began to mix with the rain to the west of NYC. By 10 AM, with moderate precipitation starting to fall across the area, temperatures quickly dropped to near freezing as precipitation changed over to light to moderate snow across most of the area, and with the rain/snow line near central New Jersey and heavier precipitation moving north, by 11 AM-12 PM, widespread heavy snow fell across most of the area. The snow at first did not accumulate, but once the heavy snow started to fall, the snow quickly stuck on the ground and accumulated, with rates of 1-2 inches per hour. Due to temperatures staying just above freezing, the snow to liquid ratios were much lower than usual, which kept accumulations lower than what they could have been had the snow fell with below freezing temperatures.

Radar image of storm at 5:58 PM

As the rain/snow line advanced further north with the heavier banding moving into southern New England, Long Island changed over to rain by Saturday afternoon, with a rain/snow mix falling in NYC. Snow continued to fall across Connecticut, SE New York and northern NJ, but was lighter than the late morning heavy snow. Meanwhile, a deformation band of heavy snow set up in Pennsylvania, and began to move further east, affecting western New Jersey. The forecast was for this band to move into the NYC area to produce another round of heavy snow; instead, the band weakened, and only brought additional moderate snow to northern NJ and SE NY by the evening hours while bands of moderate to heavy snow affected Connecticut. The rain/snow line began to fall south in the evening hours, and as a result, NYC and Long Island saw snow mixing with the rain. With the deformation band not as strong as originally expected, no significant increase in accumulations was observed overnight, with moderate snow falling across most of the area ending west of NYC by 12 AM and east of NYC by 3 AM.

Forecasting The Storm:

The first signs of this storm showed up on Sunday, 10/23, a day less than a week before the storm, when the GFS model was the first to show weak energy reaching the Southeast US as a coastal low, but there was very little indication of what was about to come. The following day, the model guidance strongly trended towards a coastal storm, with the GFS and UKMET bringing rain and snow to the I-95 corridor, the ECM model showing a decent snowstorm event for I-95, and the CMC took the storm inland bringing rain and over 50 degrees to NYC. With that day’s update, I mentioned a stronger coastal low possibility which had the potential to bring rain/wind to the coast and a snowstorm inland. Although a few models did suggest accumulating snowfall in NYC, that potential was quickly ruled out at that time; when considering that only 3 storms in recorded history brought accumulating snow down to the coast, and with a less than ideal set up modeled at that time along with uncertainty about the intensity of the cold air mass, which did not appear to be very strong, it appeared to be nearly impossible to see accumulating snow down to the coast. As the next several days proved, however, the assumption that a major snowstorm could not affect New York City in October turned out to be wrong.

*NAM model run from Wednesday evening, 10/26, reflecting the model agreement on a mostly offshore storm. Click on the image to view it in a larger size.*

Despite the models agreeing on a nor’easter on Monday, confidence on the storm went down on Tuesday and Wednesday, 10/25-26. The majority of the models trended offshore, showing partly sunny skies for NYC with the rain staying offshore. Only one model, however, did not back away from showing the strong storm scenaio: the ECMWF model, which continued to show accumulating snow for most of the NYC area on both of its Tuesday runs. Although the ECMWF is a relatively accurate model, which coincidentally also happened to be the only model showing the December 26-27, 2010 blizzard several days prior to the storm while the rest of the model guidance was offshore, seeing only one model showing the storm was not an encouraging sign for a significant nor’easter.

On Wednesday, the ECM trended east to join the other models showing an offshore scenario. The UKMET, however, trended west on Wednesday, and was the only model to show a significant snowstorm that day. Although the GFS model trended slightly west that day to bring light rain to NYC, it had appeared by that point that a significant storm became less likely. Regardless, there was still some uncertainty as the models still had difficulty with handling the storm’s energy, and with enough cold air in place, it had already appeared by that time that if the storm was to track closer to the coast, heavy precipitation and supportive dynamics would support snow falling in the NYC area. As a result, in my Wednesday outlook, although I mostly sided with an offshore scenario, I mentioned the potential that should the storm trend closer to the coast, snow would fall north and west of NYC. Above, I posted an image of the NAM model from Wednesday evening, a little less than 3 days before the storm. This NAM run reflects the model consensus at that time showing the storm offshore. The green and blue filled shapes represent precipitation, with the blue line representing the 850 mb freezing line. With that scenario, light precipitation (represented by green) inside the below freezing area would fall as rain or light snow, with heavier precipitation (represented by blue) inside that area would most likely fall as snow.

*NAM model run from Friday morning, 10/28, reflecting the new model consensus on a significant snowstorm.*

Short range trends on the models are infrequent, as the models typically converge on a single solution at least 3 days prior to the storm with only minor changes afterwards. In this case, however, a short range trend brought a scenario that seemed nearly impossible to believe just a few days earlier: accumulating snow in NYC in October. With the UKMET still showing a significant storm, the ECMWF and GFS models quickly trended west on Thursday morning, just 2 days before the storm, showing snow in the immediate NYC area, but were still east of the actual result. The NAM and CMC models were still mostly offshore at that time. By Thursday evening, with the NAM starting to show the big snowstorm, there was a model consensus showing a significant snowstorm affecting the NYC area, supported by temperatures aloft staying below freezing along with heavier precipitation resulting in heavy snow. The CMC model was the only one going with a further east scenario by Thursday evening. With an ideal set up for snow, I followed along that night, and although there was still uncertainty on how far west the storm would come, resulting in uncertainty on whether NYC would see accumulating snow or not, my Thursday night update mentioned the potential for this to become the 4th storm on record to bring accumulating October snowfall to NYC.

With only a day left until the storm affected the area, the models continued to trend even more west on Friday, with the GFS going as far as showing heavy rain falling in NYC for several hours prior to the snow. By Friday afternoon, there were two main model solutions: the GFS, GFS ensemble mean, and the SREF ensemble mean, siding with more rain and less snow in NYC, and the NAM, ECMWF and UKMET, siding with a significant snowstorm in NYC. By that point, it had appeared that the GFS was too far west, and it ended up trending slightly further east and a little colder to support a significant snowstorm in the NYC area as well. Even though the track of the storm was nailed down by that time, there was still uncertainty with the timing of the storm, as the model guidance showed the storm starting out as rain, with the heaviest snow falling in NYC during the evening hours due to a deformation band. Instead, the heaviest snow fell in the morning and early afternoon hours, with the evening deformation band failing to become as strong and widespread as the models showed. Due to this change in the forecast, forecast accumulations were adjusted upwards on Saturday morning, during the day of the storm.

Impact Across the Region and the NYC Area:

The timing of the snowstorm, in late October, as well as the accumulations observed and the places where snow accumulated, made this storm a historic October snowstorm. Typically, during years when October snowstorms are observed in the Northeast, the snow typically falls in the interior Northeast in the higher elevations, with rain falling in the I-95 corridor and closer to the coast. The most recent examples of October snowstorms include 2008 and 2009; during these two storms, several inches of snow fell in the interior Northeast, focusing on NW NJ and the higher elevations of New York in 2008 and in Pennsylvania in 2009, while the immediate NYC area saw rain with some flakes mixing in north and west of NYC. With this storm, however, the I-95 corridor from Washington DC to Boston saw snow falling with accumulations from Philadelphia and further north, while even places with lower elevations such as northeastern New Jersey and southern Connecticut picked up as much as 6-8 inches of snow. The accumulating snow near the coast as well as significant accumulations in the interior Northeast made this storm a category 1 storm in the Northeast Snowfall Impact Scale (NESIS), which ranks the top 45 high impact snowstorms in the Northeast US. To further suggest that this is a historic event, not only is this the only October storm in the list, but this is also the only storm prior to December, the official start to winter, on the list.

Across the region, significant snow accumulations were observed in parts of the interior Northeast, with 20-30 inches of snow in parts of western Massachusetts. Towards the NYC tri-state area, significant snow amounts for this time of the year were observed. Using the National Weather Service storm reports from across the region, I created an estimated snow map to the left, showing an estimation of how much snow fell. Across the area, between 8 and 19 inches of snow fell towards NW NJ and interior SE NY, with the higher elevations seeing the highest accumulations. 4 to 8 inches of snow were observed in the north and west suburbs of NYC, approaching 10-12 inches in northern Westchester county and interior SE CT. Further east, 1 to 3 inches were observed across most of Long Island with lighter amounts further east and near the southern coast and higher amounts closer to NYC, and up to 3-4 inches were observed in NYC, with a report of 6 inches in Bronx and an official measurement of 2.9 inches in Central Park. Amounts in southern Connecticut were much more variable, increasing from east to west and from south to north.

Although this was a significant snowstorm for October, this is not the first time that widespread 12+ inches of snow fell in the Northeast in October, as parts of the Northeast have seen significant October snowstorms previously, such as in 2008 when parts of New York state saw over a foot of snow. In New York City, however, accumulating snow in October is very rare; since records were kept in New York City starting in 1869, only three other Octobers had accumulating snowfall in NYC, and all of these were under 1 inch. The previous record for the most snow in Central Park in October on record was in 1925, when 0.8 inch of snow fell. This storm’s heavy snow easily broke that record, with the new record of 2.9 inches set this month more than triple the previous record.

Summary: This storm proved to be historic across the region, in some areas even producing more significant damage than Hurricane Irene did in late August. Especially considering that many of the trees still had leaves when the snowstorm hit, there were many downed trees across the area and the region, which resulted in widespread power outages that lasted for days after the storm hit and are still ongoing in some areas. The impact of the snow was notable as well, with a record 2.9 inches of snow falling in Central Park and 19 inches of snow in West Milford, NJ. Although the winter of 2011-12 has not officially started yet, regardless of the outcome of the winter, even if we see another 20+ inch snowstorm affecting NYC, this storm already has its place as the highlight of the winter of 2011-2012.

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