Dec 25, 2012: Final Storm Forecast

Forecast Highlights:

An active pattern continues for the region following a light rain/snow event last night, with a major storm currently in the southern United States making its way towards the Northeast, and is expected to produce heavy rain, strong winds, significant snowfall and ice across the region from Wednesday afternoon through early Thursday. A brief break is expected for Friday before another storm affects the region on Saturday with light snow.


Wednesday – Thursday: Final Storm Forecast

Until this point, there has not been much uncertainty regarding the large scale setup of this storm, with a primary low pressure tracking towards the Ohio Valley, followed by a secondary low pressure developing further east and tracking along the Interstate 95 corridor or just east of I-95. Cold air damming is expected ahead of the secondary low pressure, which will result in widespread frozen precipitation ahead of the low pressure. Uncertainty continues for the minor details, however, with differences between the model guidance regarding the temperatures and the precipitation types north and west of NYC.

The NAM and GFS are currently the warmer models, with the ECM, UKMET and RGEM the colder models. The NAM has very little snow at all in the area, and according to HPC has initialization errors with 850mb temperatures slightly too warm; for tonight’s forecast I am disregarding the NAM as a warm outlier. The RGEM, however, is colder than the rest of the models and shows northern NJ with almost plain snow through the peak of the storm. While it is a possibility the storm may be a bit colder than currently expected, the RGEM is too bullish with the cold and is considered a cold outlier.

For my final forecast tonight, I made minor revisions to the more local forecasts, with a slightly cooler scenario than previously expected. To the left is my final scenario map; the pink zone represents snow to ice and/or rain, with accumulations expected, significant for many. The green zone is for mostly rain and/or light snow accumulations involving mixing, although some snow is expected at the start of the storm especially along and northwest of I-95, with minor accumulations.

Forecast for NYC Area: Precipitation is expected to start developing around 3-5 PM. South and east of NYC, this will be in the form of rain. In NYC, a brief burst of light to moderate snow is possible without accumulations followed by a quick changeover to rain. North and west of NYC and in southern CT, at least a period of snow is expected for the north/west suburbs, up to 1-2 inches, followed by a changeover to rain in the evening. For the immediate NYC area and further east, heavy rain will then develop overnight, with strong wind gusts developing as well, ending by early Thursday morning. For the northwestern parts of the area, in NW NJ and interior SE NY away from Rockland county, the storm will start with snow, intensifying by the evening and early overnight hours as it begins to mix with sleet and freezing rain. Towards the end of the storm, a changeover to rain/freezing rain is expected, with moderate ice accumulations possible. The storm will also end by the early morning hours on Thursday. For more localized forecasts, check the latest 5-Day Forecast.

Storm totals: Overall, at least 1 to 2 inches of rain are expected from the immediate NYC area and further east. Strong winds are expected as well, with gusts up to 50 mph in the immediate NYC area and southern CT, with the potential for gusts above 50 mph in parts of NYC and Long Island, where coastal flooding may be a concern. Windy conditions are expected for the interior parts of the area as well. Further inland, snow is expected, with at least up to 1-2 inches for the north/west suburbs. Towards NW NJ and interior SE NY, at least 4 to 8 inches of snow are expected along with at least 0.1 to 0.3 inch of ice.

There are still minor uncertainties with the exact precipitation types from NYC and further north/west, ranging from the warmer GFS/NAM showing brief snow followed by a quick changeover to rain for most except for the interior NW areas, while the colder ECM has more snow to start with a slower changeover to ice/rain north and west of NYC. It is possible the storm may end up colder than currently expected, in which case up to 2-4 inches would be possible for the north/west suburbs and in parts of southern CT, while northwest NJ and interior SE NY stay with mostly snow during the duration of the storm, with 6+ inches possible, along with more significant ice amounts for some areas. Storm updates will be posted throughout the late afternoon and evening hours on Wednesday.

Next Weekend: Light-Moderate Snow Event

Following the significant mid week storm, drier conditions are expected for Thursday and Friday with breezy conditions. The next storm will arrive in the region on Saturday, with clouds and precipitation expected. While yesterday, it had appeared the potential was there for a larger storm to affect the region, today’s trends have significantly backed from that potential with the continuation of the progressive flow supporting at least moderate impacts from any storm affecting the region, as opposed to a significant nor’easter. While the forecast is still subject to change, and it is still possible it ends up a bit too far east for the area to receive much precipitation, at this time the potential is there for a light to moderate snow event to affect the region including the area, with amounts remaining generally below at least 6 inches. Stay tuned for more information on this storm and its impact in the region.

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