Dec 7-8, 2011 Storm Review

After a large ridge had been stuck over the eastern United States, bringing well above average temperatures across the area, a developing low pressure brought heavy rain across the area, with over 2 inches of rain observed. Although the impacts of this storm were not unusual, the forecasting of the storm was unusually difficult, as the model guidance kept showing a variety of solutions and then changing the position of the storm, failing to lock onto a single idea until only 24 hours away from the storm.

Storm Impact:

Occasional showers were observed across the area throughout Tuesday, 12/6, bringing light rain amounts across the area. On Wednesday afternoon, a more organized low pressure began to develop near North Carolina, with more widespread heavy rain developing across the region. Through Wednesday evening, most of the area ended up with at least 1/2 to 1 inch with higher amounts west of NYC. By Wednesday night, however, as the low pressure moved up the coast and severe thunderstorms in the southern Mid Atlantic moved into the NYC area, more widespread heavy rain was observed, as can be seen with the radar image to the left at 10 PM Wednesday.

The rain began to mix with snow in the interior parts of the area after at least 12 AM, with the mix spreading as far southeast as nearly the north/west suburbs of NYC, where some areas reported mixed precipitation. As the storm was in the process of phasing and rapidly intensifying, it also moved very quickly, moving from eastern Long Island to the eastern tip of Maine in just about 6 hours. As a result of the storm’s fast motion, the precipitation also moved out quickly, ending across the area by at least 3 AM.

Observed rain totals mostly ended up within the 2 to 2.5 inches range, locally up to 3 inches, across northern NJ, SE NY and southern Connecticut, near to slightly below 2 inches in NYC, and between 1.25 and 2 inches across most of Long Island. Snow accumulations in the area and across the region ended up lower than expected, with light accumulations in the interior parts of the area. Parts of West Passaic, Sussex and Orange counties observed 1 to 2 inches of snow. The wind aspect of the storm was more surprising, as there were indications that windy conditions would be observed but not as strong as they ended up; widespread wind gusts of 50 to 60 mph were observed in NYC and Long Island, with a peak gust of 69 mph in Westhampton, NY.

Forecasts For Storm: Poor Model Performance

There are several models frequently used for making forecasts; the GFS, NAM, ECMWF, GGEM and UKMET models. The models are not always correct, and are used as guidance for making forecasts instead of directly used as the forecasts. Typically, models begin to converge on a single solution for a storm at least 72-84 hours (3-4 days) away from the main event. In this case, however, the models had issues with handling the set up at 500 millibars until the last minute, resulting in poor model performance for the storm.

The coastal low developed as a result of a 500 mb low becoming negatively tilted while moving from the south central US into the Northeast and towards Canada while phasing and rapidly intensifying. The models were unable to figure out where the 500 mb low would end up until the short range, resulting in a variety of solutions with each model. Up until 66-72 hours away from the storm, the models did not even show the correct overall set up for the storm, resulting in a variety of solutions ranging from a large storm on Wednesday morning to no storm to a northern Mid Atlantic Thursday snowstorm. By that time, approximately 3 days away from the actual event, the models corrected the set up at 500 mb to represent a scenario closer to the actual one, but the new model guidance solutions were still incorrect as they showed a more separated 500 mb low well to the west of its actual position, which resulted in the models bringing in more cold air after a Tuesday cold front, placing initial cold air in place, and later on brought a snowstorm to the NYC area. This was the most evident with the NAM model, which brought heavy snow near NYC with a few of its runs.

Approximately 54 hours way from the storm, the models once again changed the position of the 500 mb low, now showing it more attached to the cold front and more amplified, which meant that the cold front on Tuesday would not be able to cross the area, preventing cold air from moving in ahead of the Wednesday storm, but that the cold air would gradually enter the storm as it develops and moves up the coast. This was closer to the actual scenario that happened with the storm, but the issue at that time was that the 500 mb low was still not in the correct position, and would not end up correct until nearly 24 hours away from the actual event. As a result, throughout the 1-2 day range prior to the storm, the models continued to adjust their solutions towards a further west storm, first showing a snowstorm near NYC, then showing rain to accumulating snow, followed by rain to a rain/snow mix. Finally, when a final solution has been reached, only a little over 1 day prior to the storm, it finally appeared that there would be plain rain from the storm in NYC with as much as 2-3 inches.

The winds with this storm were not expected to be as strong as they were, although there were plenty of indications to suggest that there would be a brief period of strong wind gusts as the storm came to an end. The models did not show very strong wind gusts, generally falling in the 30-40 mph range, although there was a tight pressure gradient in the back end of the storm on the models, which suggested that combined with the heavy rain falling at that time, the potential existed for brief yet strong wind gusts. With my final update during the storm, I went higher than the models, calling for gusts locally up to 45 mph, but even this call ended up too low, as actual gusts ended up in the 50-65 mph range across most of Long Island, and between 40-50 mph in NYC.

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