Weekend Storm? Maybe!

Last night's GFS computer model came in with a fun looking Sunday forecast. A blizzard for Southern New England! Will it happen? Maybe, but probably not. 

In the 5 or 6 day range it's better to use computer model ensembles as opposed to just one specific computer model run. As you go out in time a computer model's forecast becomes less and less accurate and its errors grow exponentially. The ensemble approach allows us to make small changes to a model's initial conditions and see the different outcomes that those changes produce. Sometimes solutions are tightly clustered and there's high confidence in a certain outcome. Other solutions can be wildly disparate which would imply a low confidence or uncertain forecast.

GFS Ensemble plume diagram showing possible outcomes with Sunday's storm. 9 of the 20 members have no snow, 5 of 20 have over 6" of snow, and the remainder have a more modest impact.

The GFS ensembles this afternoon show a wide spread. A bunch of possibilities none of which we can write off. Of course we have other computer models besides the GFS. The European model is even less bullish on the Sunday storm. Here's a different way to view ensemble data - by map. This is a look at the probability of >3" of snow at any given location based on the 50 European Ensemble members. For instance, if 25 of the 50 were dropping 3"+ of snow in Hartford the plotted probability would be 50%. 

Percentage of the 50 European Ensemble members that have over 3" of snow on Sunday. This shows between 10 and 20 percent of them producing over 3" of snow in Connecticut.

So what to make of this? While the overall weather pattern turns colder it's not particularly favorable for a big snowstorm. That said, there is still the possibility that this storm could "thread the needle" and wind up developing in spite of a relatively hostile pattern for snow. It's happened before! The odds of seeing a snowstorm (over 6") remain low for now. We'll see what happens over the coming days. 

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