Oct 28, 2011: Significant Snowstorm Tomorrow

Behind a wave of low pressure that brought light rain to the area and light snow across the Northeast with light accumulations, today brought mostly sunny skies across the area with chilly high temperatures, peaking in the upper 40s to lower 50s across the area, colder than the average for this time of the year. Temperatures are currently holding steady across most of the area, slightly dropping in some places, as clouds move in from the southwest. A weak area of low pressure is currently bringing areas of light rain to the southern Mid Atlantic with accumulating snow observed in the mountains of West Virginia and western Maryland. A set up highly unusual for October will bring a stronger coastal low pressure up the coast, however, which will result in an unusually early snowstorm affecting the region with significant snow accumulations, even in parts of the NYC area.

Final Storm Discussion:

Following the model guidance for this storm was a challenge. From when the storm first appeared early this  week, the model guidance quickly trended offshore for two days, and the only models that held onto the storm were the ECMWF and the UKMET models. While it appeared that these models would trend east to join the model agreement, the other models trended west significantly yesterday morning, showing a significant snowstorm for the NYC area. Today’s models trended west once again, but there is now a split between the eastern and the western models.

The main concerns at this time regarding snow potential in the immediate NYC area and closer to the coast include the intensity of the storm, as the models have been trending slightly weaker with the storm. As I will discuss in the scenario section, the main reason we are seeing this snow potential and not just a rainstorm is because of the dynamics of the storm, including a rapidly intensifying low pressure pulling in cold air close to the storm center and heavy precipitation allowing for a heavy wet snow to fall. A weaker storm will result in weaker dynamics, and thus a warmer storm producing more rain and less snow for the immediate NYC area. This weakening trend, however, likely won’t lead to any significantly lower snow accumulations if it continues, as a significant snowstorm is still expected for interior areas. The other issue is with the location of the storm, as the models are currently split into two camps; the first set of models, the GFS, CMC and the SREF, are showing a further west storm, bringing more rain and less snow across the area. The other models, the UKMET, ECMWF and the NAM, are siding with a colder and further east scenario, where NYC sees some rain changing over to heavy wet snow, with significant accumulations as far southeast as the immediate north/west suburbs of NYC. Currently, I sided with a solution in between, as the NAM may be too cold, but the GFS may be too warm as well, and the latest SREF run backed slightly to the east.

Storm Scenario: This scenario is highly unusual for October, and something that we would typically see in December. For early and late season snowstorms to happen, everything has to end up just right, and in this case, all of the right ingredients are in place. There is a high pressure to the north, although it weakens slightly and moves to the north. There is a cold air mass in place before the storm, and as the storm intensifies, it will create its own cold air and pull the cold air towards the coast, which combined with a track off the coast and heavy precipitation, will produce snow in places where snow rarely falls in October. At first, the low pressure will still be weak tomorrow morning and afternoon, likely around 1006 mb, but by the afternoon and evening hours, the low pressure will intensify faster and move towards the NJ coast while heavy precipitation develops in the southern Mid Atlantic and moves north, reaching Pennsylvania and NJ by the afternoon. The low pressure will then continue to intensify, deepening to at least 992 mb by Saturday night while moving close to our just southeast of Cape Cod.

Forecast for the NYC Area: Light precipitation is expected to develop in the area late tonight, with light rain from NYC and further east and a light rain/snow mix inland. By the morning, light snow will fall towards NW NJ/SE NY, with light rain elsewhere. Temperatures will peak early in the day from NYC and further west, reaching the mid 30s inland and the lower 40s across most of the immediate NYC area, getting warmer further east. As precipitation intensifies, temperatures will drop across the western half of the area. By at least 2 PM, moderate to locally heavy snow is expected to fall in Sussex/Orange counties, with a rain/snow mix in the north/west suburbs of NYC and rain, locally heavy, from NYC and further east.

As cold air continues to be pulled into the storm, temperatures aloft will cool down by the late afternoon and evening hours, and by at least 6-7 PM, most of northern New Jersey and SE NY are expected to change over to a heavy wet snow, while NYC sees a rain/snow mix and Long Island continues to see heavy rain with increasing winds, gusting up to 50-60 mph. The snow will have issues accumulating at first, especially due to the warm ground and wet conditions after the rain that falls earlier, but with the snow expected to fall heavily, it will eventually stick and begin to accumulate. A period of at least 6 hours of moderate to heavy snow is expected for the immediate north/west suburbs of NYC and southern CT, which will result in light to moderate snow accumulations in these areas, and with the mix changing over to a wet snow in NYC by the early overnight hours, light accumulations are very possible in NYC itself as well. Southern CT will change over to heavy snow by the early overnight hours, around the same time that parts of Long Island will mix with wet snow. As the storm moves east, snow will end from west to east, around 12-2 AM from NYC and further west, and 1-3 AM east of NYC.

Snow Accumulations: Although the region has seen these snow amounts many times, this storm is historic considering the time of the year; Central Park only had accumulating October snowfall in three occasions since records were kept in the late 1860s. As long as there is no sudden change in the outcome, this will most likely be the 4th occasion, and if Central Park sees near or over 1 inch of snow, which is an increasingly likely scenario given the recent trends, this has the potential to be the snowiest October for NYC in recorded history. The last significant October snowstorms affected the area in October 2008 and 2009; these storms, however, only brought elevation dependent moderate to locally heavy accumulations in NW NJ, and no flakes fell in NYC.

At this time, considering the potential weakening trend of the storm, I went with a solution closer to the eastern models, with the heaviest snow likely falling from NE PA/NW NJ into interior SE NY, NW CT, western Massachusetts, and into southern New Hampshire and parts of southern Maine away from the coast. In this corridor, most areas should see over 7 inches of snow, and the potential may be there for up to a foot of wet snow in some areas. I went with slightly lower snow totals than shown by some models as the slightly weaker storm I am anticipating may lead to slightly less snowfall further west, but if the snowier case scenario verifies, the 7+ inch zone may be larger, extending more to the northwest, more towards the SW into central PA, and slightly more to the SE. A slightly weaker storm, however, will bring a smaller 7+ inch zone.

In the immediate NYC area, up to 1 inch of snow is expected for most of Long Island, with most places likely seeing a trace. In NYC, up to 1 inch of snow is expected, but in the colder case scenario, up to 2 inches may be possible in parts of the city. A sharp gradient is then likely to the north and west of NYC; the immediate north/west suburbs along with southern CT will likely see 2 to 4 inches of snow; further inland towards western Rockland and western Bergen are expected to see 4 to 7 inches of snow, and in NW NJ and interior SE NY, 6 to 10 inches of snow are expected, with amounts potentially up to 12 inches in isolated areas.

As I previously mentioned, a slightly weaker and further east storm may be possible for the area. This is only a recent trend on the models, and with the storm less than a day away, my focus during tomorrow’s storm updates will be on current observations and short range models, to determine if a further east and weaker storm is more likely. If this is the result, the snow may not be as heavy in the area, but accumulations may still slightly increase, bringing 1-2 inches in parts of NYC and 3 to 5 inches in the immediate north/west suburbs and southern CT. A weaker storm, however, would mean that places in the interior Northeast would see less snow than expected, and some of the winter storm warnings may be downgraded to winter weather advisories. This is still only a potential, however, and it is very possible as well that the storm does not end up weaker than currently expected and follows the current forecast.

Regardless of how much snow falls, this will be a dangerous storm, bringing heavy wet snow capable of downing trees and resulting in widespread power outages. For coastal areas, strong winds are expected, gusting up to 50-60 mph in parts of Long Island, combined with heavy rain of up to 1-2 inches. Stay tuned for storm updates throughout the day tomorrow on this historic late October snowstorm.

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