Early EC Winter Storm: 7-8 November 2012

Models have been coming into agreement that an early-season winter storm may affect the East Coast from North Carolina to Maine in the next 4-5 days. The GFS 96-h forecast valid on Wednesday morning 7 November shows the cyclone off Cape May, New Jersey with a track north-northeast just off the East Coast.

Before using the analog impact guidance, it is important to assess the similarity between the mean of the top 15 analogs and the model forecast they are based on. If the two are similar, then more weight can be given to the impact guidance that is based on the sensible weather from these historical events. For example, in the following images the 500-mb height and sea-level pressure (PMSL) fields are very similar between the mean of the similar analogs and the GFS forecast. The 500-mb longwave troughs are both negatively tilted and the surface low centers are collocated.

One noticeable difference in the PMSL field is the strength of the mean surface low, which is between 1004 and 1008 mb and much higher than the GFS forecast. This is due to the range of surface low strengths of the top 15 analogs. The 850-mb wind fields are also important to analyze with EC storms due to the importance of the -U wind anomaly in transporting warm moist air off the Atlantic Ocean and into the mid-latitude cyclone's circulation. Shown below are the mean 850-mb U-wind standardized anomalies based on the top 15 analogs and they show a 2-3 below normal standardized anomaly of the 850-mb wind field. This and the 500-mb/PMSL fields suggest that the top 15 analogs are associated with organized cyclones.

One field that has less similarity between the GFS forecast and top 15 analog mean is the 850-mb temperature field (shown below). To the northwest of the surface cyclone, the mean temperatures of the top analogs are about 3C too warm compared to the GFS forecast. This will be important to remember when extracting the observed impacts from the similar historical events.

With early season winter storms, precipitation type to the northwest of the surface low track is more difficult to determine than it is during the heart of the winter...usually due to the limited amount of cold air this time of the year. Therefore, it will be important to analyze the impact guidance based on the top 15 analogs and extract impacts from some of the individual analogs that have a similar cyclone and thermal profile across interior New England. Shown below are the 72-h mean snowfall and the probability of 6" of snowfall based on the top 15 analogs. Low snowfall amounts are present in both the mean and probability fields except for the higher elevations along the spine of the Appalachians and across interior New England. This is common with early-season winter storms due to the lack of cold air at low-levels.

The top 15 analogs have a signal for >6" of snowfall in the higher elevations to the northwest of the surface cyclone track. However, it is also important to analyze the individual analogs for a more representative thermal profile. Shown below is the GFS 850-mb temperature forecast field along with thumbnail maps of the 850-mb temperatures and 72-h COOP snowfall fields.

It can be easily seen that the most similar analogs to the 850-mb temperature forecast are analogs 2 (October 2005), 15 (October 1988), and 7 (December 2002). The analogs from October 2005 and December 2002 have snowfall across the higher elevations of interior New England that extends south into the Appalachians while the analog from  October 1988 has limited snowfall. The main difference with the October 1988 analog is that the surface cyclone is very weak and takes a track further off the coast of New England than what is currently forecast.

Finally, if a surface cyclone occurs similar to the one that is currently forecast then strong winds along the East Coast are likely with this storm. This can be seen below in the higher probabilities for wind gusts > 40 mph from Boston, MA to Norfolk, VA.

   
It is important to remember that these historical events are the most similar to the 96-h GFS forecast from 3 November 2012 at 12Z. If the forecasts change significantly as the event gets closer, the analogs will change as well.

This analog run is located here.