As in all computer modeling, the old adage applies: garbage in, garbage out. So it is extremely important to get the initial conditions right, Sullivan added.
"If you don't have those initial conditions right the error increases with time," he said.
Then there is the matter of tailoring the forecast to a specific area. That's even more of an art, explained Dan Satterfield, chief meteorologist for the CBS affiliate WBOC TV in Salisbury, Md.
"A model that is going back and forth or showing unrealistic temperature forecasts or has been verifying badly is not likely be given a lot of credibility," said Satterfield. "A particular fave of mine is the WSI Rapid Precision 4-km resolution model. This model has great physics and has done very well here on Delmarva for snow."
The same ensemble process is used whether it's summer or winter, said Sullivan, but there are some advantages to certain kinds of models in the winter, according to Satterfield.
"The models with the higher resolution will do better with snow than the global models because they have better physics and also usually a better vertical resolution," said Satterfield. "A good forecaster develops a list of rules in his or her mind as well, and one for me is that if the upper level trough is negatively tilted then the storm will be as strong or stronger than the models show. This was the case in this snow event as well."