Special 132-h East Coast Analog Run: 19 December 2012

What a difference 24 hours makes. Yesterday morning's (20121212/1200) 12Z GFS 156-h forecast had a sub 984-mb surface low just southeast of New York City (top image below). This morning's (20121213/1200) 12Z GFS 132-h forecast (bottom image below) had a weaker 1004-mb surface low. For the most part, this was due to a slower and weaker midlevel trough. Although the surface low does deepen considerably in the next 12-24 h of forecasts as
the midlevel wave strengthens, the timing is such that the area of low pressure is too far southeast of New England to have a major impact.

One of the strengths of the analog system can be shown by comparing the analogs based on yesterday's GFS forecasts (blog post here) and this morning's forecasts. Yesterday's top 15 analogs had a mean 850-mb low down to 1290 m and a 996-mb surface low. In contrast, the top 15 analogs based on this morning's weaker forecasts only had a mean 850-mb trough (which in turn weakened the u- wind component standardized anomalies) and a 1008-mb surface low. Forecast input to the analog guidance algorithm where the upper-level dynamics had not phased selected similar historical events that had not phased and were weaker.

It's pretty obvious that a weaker low-level system will have weaker temperature and moisture advections and weaker low-level convergence which will have an impact on the amount of precipitation produced. The results of this are clear in the analog impact guidance. The mean COOP snowfall only has a 4-6" area of snowfall across interior New England and no probabilities of >12" of snowfall.

One of the strengths of the analog system is that when input from two completely different forecasts for the same verification time are used, the historical sensible-weather impacts also end up being different. Over the last four years we have been contacted by many meteorologists who have told us stories of how they used the analog guidance in operations and how it out performed sensible-weather forecasts from model output (e.g., precipitation amount, snowfall amount, wind gusts, etc.). This is due to the fact that the analog system has been optimized to excel when a type of sensible weather has an associated pattern. Over the past fifty years, operational meteorologists have used this approach (i.e., a composite analysis) when completing research to understand high-impact weather events. The analog guidance system is a real-time composite analysis of the upcoming forecast...a conditional climatology.