Nov 6, 2012: Snow and Wind Expected Tomorrow

Forecast Highlights:

As the nor’easter that has been significantly complicated by unusually large differences in the model guidance approaches, there remains some confusion regarding exactly what it will end up doing, with the latest model guidance just 12 hours before the storm still not showing signs of agreement regarding the exact solution. Despite the uncertainty, there is more confidence that the area is dealing with a moderate to significant nor’easter tomorrow with strong winds for the coast and a wet snowstorm for interior parts of the area.

 


This storm has been a very challenging one to forecast. While it had appeared that the region would likely see a significant nor’easter, the model guidance last night began to trend to the east, with all of the models keeping most of the precipitation east of NYC early today. Since then, however, another abrupt trend in the model guidance began towards more impact with the storm, representing a poor handling of the storm as typically, the models reach a consensus on a forecast this far out, not diverge from one. After reviewing what went wrong and the latest observations with the developing nor’easter, while the forecast is still not certain, there is more confidence on what the storm may end up producing across the area.

Track Forecast / Model Analysis: Latest observations show that the minimum pressure of the storm is approximately 998-1000mb, and has been steadily deepening, currently at a rate of 1-2 mb/hour. The low pressure is expected to continue tracking NNE from this point before turning north and significantly slowing down on Wednesday night into early Thursday. The model guidance varies with how far west or east the low will track, which is unusual for just 12 hours away from impact; the 18z RGEM and 0z NAM models are much more west with the low, with the NAM showing the nor’easter making landfall in New Jersey; the HPC, however, noted that the NAM had initialization errors and is too far west. On the other hand, the UKMET model remains further east, with the eastern set of models showing the low pressure staying east of New Jersey, about south of eastern Long Island or Rhode Island’s longitude; this still has a snowstorm and strong winds affecting the area, but less impact further inland.

The latest observed conditions are matching well with the initialization of the latest RAP short range runs, which take the low to a position close to the 18z NAM, or south of the eastern tip of Long Island on Wednesday into Wednesday night. Considering the current trends and observations, I am going with the eastern end of the model guidance, with a low south of eastern Long Island or Rhode Island on Wednesday afternoon into the overnight hours.

Impacts in the Region: Before I go into the details into the forecast, it is important to note that this is still not a very high confidence forecast, despite the storm being less than 1 day away. There is still a bust potential especially with the precipitation type and the intensity of the snow, and the storm track could also end up a bit west or east of the forecast.

Assuming the track takes the scenario mentioned above, to south of eastern LI or Rhode Island, precipitation will begin to fall across the area in the late morning, in the form of rain east of NYC, rain/snow in NYC, and likely snow for many places north and west of NYC. The worst of the storm will be limited to NJ and the NYC area, with a moderate to locally heavy wet snow in the late afternoon into early overnight hours. Most of New Jersey is likely to see snow, even coastal areas, with widespread light to moderate accumulations likely; the highest accumulations would be in a narrow corridor from eastern PA through northern NJ, SE NY and NW CT, with at least 3 to 6 inches of wet snow expected with locally higher accumulations, perhaps up to 6-8″ in some areas. Minor accumulations may be possible in NYC as well. The coastal areas will see rain and moderate to strong wind gusts; the strongest wind gusts are expected for Long Island, generally up to 40-55 mph with some areas perhaps reaching gusts above 55 mph.

To the left, I posted the snow potential map, with a large area of trace to 3 inches of wet snow, with a region of 3-4 inches focusing from SE PA and western NJ into northern NJ, SE NY and NW CT. As with the rest of the forecast, there is still no high confidence call yet on the snow totals, and the forecast may still be slightly revised on Tuesday; one possible change is to raise accumulation potentials, with the latest 0z ECM suggesting several inches of snow across all of New Jersey with the potential for some areas to receive up to 8-10 inches of snow. For now, I went with slightly more conservative snow accumulations due to the uncertainty.

Uncertainty in the forecast: The current solution assumes that the low tracks south of eastern Long Island. It is still important to emphasize that there is still uncertainty with the exact scenario with the model guidance still poorly handling this set up, and while the scenario mentioned above is increasingly likely, changes to this scenario cannot be ruled out as well. It is still possible that the storm ends up tracking more west, close to the 18z NAM but likely not the 0z NAM which was a western outlier; this scenario would result in stronger winds for the coast as well as the snow axis spreading further northwest; while the area would still begin with a burst of moderate to heavy snow north and west of NYC, the suburbs would change over to rain while interior areas see a longer and stronger snowstorm. Additionally, the exact snow totals are still uncertain, and may be slightly revised on Tuesday; the possibility is there that the next update may have to revise snow forecasts slightly upwards, as for tonight’s forecast I went a bit conservative with the snow outlook due to the higher than average uncertainty.

Due to the differences with the models and continuing uncertainty regarding the exact development of the storm, confidence is not as high as it typically would be for a storm 1 day out. An update will be posted on Tuesday morning for any potential changes in the currently expected scenario, and storm updates will be posted on Wednesday afternoon and evening.

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