Weather Insight: A Look Into How Ensemble Models Can Help Forecast Monday Storm

The upcoming storm set to affect New England on Monday is a good opportunity to show how ensemble forecasting can help us clarify a fuzzy situation. Ensembles are a collection of different forecast models, running with different initialized conditions. The average or mean solution of the various ensemble members gives us a better understanding of how the big weather picture is trending.

This past week, the atmosphere over the United States has undergone some rather dramatic changes. The best evidence of this change comes in torrential rains California is receiving after over a year of receiving nearly no rain. Some familiar trends have crept back into the picture as well, namely extremely cold air over the eastern portion of the country.

With such large scale changes, operational models (the ones that run 2 or 4 times a day based in current observations) have had a difficult time handling two large scale features of the atmosphere.
Lets look at an upper level map from the operational Euro. I have circled the two features of importance.

The red circle is the forecasted position of the polar vortex on Monday evening. The yellow circle is the forecasted position and strength of the upper level storm that is the driving force behind the expected storm on Monday. All well and good, but lets take a closer look at the Euro ensemble.

Notice a difference? It's so small that I would be impressed if you did!. The subtle difference is in the placement of the polar vortex. The ensemble suggests a position located further south (seen in the black circle, causing the upper level feature to flatten out. A flatter upper level will result in a suppressed precipitation sheild, located further to the south over Long Island (as displayed by the arrows).

So, considering the ensemble is an average of several different model runs, it is safe to say at this point that the storm is trending further south. As a point of comparison, lets look at the snow storm that affected the area last January 21/22. 

This storm was memorable for the sharp cut off in precipitation amounts, with nothing falling north of the Mass Pike. This storm is not a perfect comparison given that temps were much colder and the orientation of the storm set up was more north to south. Mondays event will be more west to east. Still, given the trend, I would anticipate a precipitation shield being suppressed to the south and more narrow, like the image below.

I hope this wasn't too technical. Just a little insight into one of the methods we can use to forecast a storm!