Dec 6, 2011: Heavy Rain NYC, Snow Inland

As a slow moving cold front stalled over the region today, widespread showers were observed across the area with unusually mild temperatures once again, with high temperatures peaking in the upper 50s to lower 60s inland, lower 60s across the immediate NYC area, and the upper 50s across Long Island and southern Connecticut. Occasional periods of rain will continue throughout the day tomorrow, although the highlight of the forecast will be on Wednesday night, when a developing coastal low brings heavy rain to the central and eastern parts of the area with several inches of snow inland.

Wednesday – Thursday: Wet Start Across Area, White Ending For Some

Forecasting this storm has been difficult, as the model performance for this storm has been poor so far. Over the last several days, the models were not handling the 500 millibar set up correctly, constantly changing the set up with each run. Although yesterday’s models did mostly favor snow for the immediate NYC area, there was a clear trend for a more amplified storm more attached to the system, spelling out a storm closer to the coast with less cold air initially in place and thus more rain. Although I sided with a wetter scenario for NYC with yesterday’s outlook, the model trends were once again underestimated, as today’s models once again shifted west, now portraying a heavy rain scenario for most of the area.
With the model trends considered, the storm will certainly not end up like yesterday’s models, which some of them portrayed a snowstorm for NYC. There were slight changes with today’s set up from yesterday’s model forecasts, showing that even 24 hours out the models could not completely settle on a single solution. With these minor differences taken into consideration, the storm continues to slowly trend more amplified with less cold air ahead of it, resulting in more of a rain to snow scenario across most of the Northeast, even in areas that see more significant snow accumulations, instead of a plain snowstorm as was shown yesterday. In the scenario map posted above, the axis of moderate snow is expected to end up from West Virginia through central/eastern PA and far NW NJ, eastern NY, and central/northern New England. Even within the axis of moderate snow, some rain is expected at the start of the storm in most places as there is no cold air mass ahead of the storm, instead the cold air will proceed east throughout the duration of the storm.

For the NYC area, occasional period of rain are expected throughout the day on Wednesday, with totals by Wednesday evening between 1/2 and 1.25 inch across the area. As the low pressure develops near SE Virginia and moves northeast just off the coast while intensifying, heavier precipitation will develop over West Virginia/Virginia and move northeast towards the area. Steadier rain will develop across the area by at least 10 PM with steadily dropping temperatures, with snow expected to begin falling in NW NJ and Orange County towards 1-3 AM. The storm’s deformation band is expected to end up near eastern Pennsylvania and far NW NJ/Orange county, placing the best risk of snow in these areas. Between 12 AM and 4 AM, rain will continue to change over to snow inland, falling heavily at times, while moderate to heavy rain falls in the immediate NYC area with moderate rain in Long Island. As the storm quickly moves out, the rain/snow inland will end by 5 AM, with rain lingering across the rest of the area until 6-7 AM. A few snow showers may mix with the rain in the north/west suburbs of NYC and southern CT, but little to no accumulations are expected.

By the time that the storm ends, at least 1.5 to 2.5 inches of rain are expected in the immediate NYC area and southern Connecticut, with slightly lower amounts in Long Island. For the interior parts of the area, the storm will start out with moderate rain, although a changeover to snow is expected later on, with at least 1 to 3 inches of snow expected. In the snowier case scenario, far NW NJ and Orange County would end up with 4-5 inches of snow. There is still some slight uncertainty with the scenario, however, and based on my latest analyzing of the storm set up, it is possible that the storm ends up even more north/west than shown on the scenario map above, with plain rain for NYC and more rain to start for NW NJ before changing over to snow. Stay tuned for a brief afternoon update tomorrow, as well as a final storm discussion in the evening.

Longer Range: Temperatures will be colder for Thursday and Friday, peaking in the upper 30s to mid 40s across the area with more sunshine. The coldest temperatures will be on Saturday and Sunday, with highs peaking in the mid to upper 30s inland and upper 30s to lower 40s for the rest of the area, and Saturday night will bring cold temperatures, dropping into the upper 10s to lower 20s inland and the mid to upper 20s across the rest of the area except for NYC, with lows in the lower 30s.

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