Mar 4, 2013: Significant Nor’easter Likely Wed-Thu

Forecast Highlights:

Temperatures remained slightly below average today as the same pattern continued of breezy NW winds and partial cloud cover. Temperatures tomorrow will be slightly warmer with winds shifting NE ahead of the next storm, which will produce widespread impacts in the region with rain, wet snow, wind and coastal flooding.




Wednesday – Friday: Nor’easter To Produce Rain, Snow, Wind

The next storm to affect the region has its origins in the central US, with a snowstorm currently taking place in the north central US. A strong closed low shortwave at 500mb will continue to track southeast towards the Mid Atlantic, as a low pressure develops over Kentucky with a significant snowstorm extending towards Chicago and the rest of the Ohio Valley region. By Wednesday, a coastal low pressure will develop near the coast and strengthen, becoming a nor’easter producing significant impacts across the region including heavy rain, coastal flooding, strong winds and wet snow.


Model Overview: Today’s model guidance remained split between a northern and southern solution for the storm, although some better hints emerged today regarding the potential outcome of the storm. Over the next two days, a strong upper level low over the Midwest will continue to dive towards the Mid Atlantic region. The main area of uncertainty at this time is regarding the interaction of this ULL, an area of vorticity that splits from a weakening upper level low near New England, and the northern stream in south central Canada. As of yesterday, the 18z GFS was the only model to handle this in a way that resulted in a nor’easter tracking up the coast, while the rest of the models showed less interaction with the northern stream and a storm that continued to track east and offshore, with limited precipitation north of Maryland.

Throughout the day today, the GFS continued to trend even stronger and more west with this storm to the point where it results in a widespread significant precipitation event along the I-95 corridor from Washington DC to NYC, Boston and Maine. The GFS currently has the support of the GFS ensemble mean, which has been consistent in a significant nor’easter more than the operational run has, and the SREF mean which shows over 1 inch of liquid equivalent precipitation. The rest of the models, however, remain further south. The CMC held steady with a rain storm for most of the region mainly south of NYC, while the ECM and UKMET keep almost all of the precipitation south of the area, despite trending slightly north with their latest runs. As of this evening, the NAM’s 18z run was the first run of that model to side closer to the GFS.

There is no full model consensus on a major nor’easter at this time, and the models are only likely to start getting closer to a consensus with tomorrow’s runs. However, confidence continues to increase for more impact from the storm north of the Mid Atlantic, with the main support from the GFS. It is important to note, however, that the ECM and GFS are in significant contrast, as typically the ECM supports the more significant storm scenario, but in this case remains suppressed. Typically, both models do not have such a significant difference within the 60 hour range, and when this does happen, usually one of the models ends up incorrect, but the other is a bit too exaggerated with its scenario, and the end solution is closer to a compromise. In this case, the ECM is likely the southern model, and I am expecting it to trend north with future runs, although the GFS was also likely too far north with some of its latest runs, and I am siding at this time closer to a compromise of the two, leaning closer to the GFS regarding precipitation totals.


Storm Potential Impacts: At this time, significant impacts from the nor’easter are likely from the Mid Atlantic region into the area and parts of New England. One of the more notable impacts from this storm is the wind and storm surge, as tightly packed isobars are expected with a strong high pressure to the north of the storm, with widespread sustained winds of 25-35 mph along coastal regions with wind gusts up to or locally over 50 mph expected. This is especially a concern for the coast as winds initially start out from the ENE/NE, aiming at the coast towards locations that were devastated by hurricane Sandy in October. Regardless of the storm track, strong winds are expected with coastal flooding likely. More information on the coastal flooding aspect of this storm will be posted over the next few days.

Significant precipitation is also expected with this storm for parts of the region. There is uncertainty with exactly how far northwest precipitation spreads, ranging from the GEFS ensembles which are the most northwest out of all the guidance, and the UKMET which remains in the southern end of the guidance. The main idea at this time is for a significant wet snowstorm for interior parts of the Mid Atlantic, possibly the NYC area, and parts of New England. There is no new fresh source of cold air available for this storm, and most of the snow is expected as a result of dynamic cooling with moderate to heavy precipitation overhead. Temperatures are likely to be marginal for locations along and east of the I-95 corridor in terms of rain vs. snow, with the heaviest snow likely west of I-95. Locations that receive the heavier snow with this storm are likely to end up with a moderate to significant wet snowstorm with the possibility of downed trees and power lines, which is especially likely towards the interior Mid Atlantic region focusing over West Virginia.


NYC Preliminary Forecast: At this time, the forecast is for rain/snow showers to develop across the area on Wednesday afternoon, with the peak of the storm on Wednesday night. Winds will steadily increase throughout the day, with a NE wind at 15-30 mph, gusting to 40-45 mph. During the peak of the storm, moderate precipitation is likely, possibly heavy for some areas, with locations under the heavy banding likely to observe moderate to heavy snow rates with at least moderate snow accumulations. The strongest winds are also expected overnight, with a NNE wind of 20-35 mph with gusts up to 40-50 mph across the area, with the strongest winds over Long Island. Scattered rain/snow showers are likely to continue into Thursday and early Friday with breezy conditions expected to continue as well, with clearing cloud cover and lighter winds by the weekend. Tomorrow’s forecast will include more detailed snow and precipitation outlooks, although at this time the potential is there for moderate snow accumulations from NYC and further north/west.

There is still no solid confidence on the forecast, however, and additional changes are still possible in the forecast. At this time, I sided more conservative than the GFS model, which this season has had a history of exaggerating short range trends and slightly backing down as the storm approaches. Should the GFS scenario verify, more significant impacts would take place with heavier rain east of NYC and significant snow accumulations north and west of NYC. The rest of the models also remain more suppressed than the GFS, which support earlier thinking of light to moderate rain/snow showers with more significant impacts to the south of the area. Stay tuned for more information with tomorrow’s update.

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