I heard a meterologist on one of the Boston stations observe that this type of storm was unusual in its size, intensity and location for this time of year. Consequently, there were few computer models to analyze.
The global weather models used in weather forecasting can presumably make predictions based upon any quasi-realistic inputs (of the current weather conditions). However, models are developed using past data and may be less accurate when run on conditions that were not seen very often (or not seen at all) in the past data.
Let's not fall into the trap of dismissing computer models. There has to be a history & pattern of storms to create models, and with climate change, we don't have enough data points to create reliable predictions. My gut says unpredictability will increase as the year-to-year climate temps go higher. Higher temps mean the atmosphere can carry more moisture.
Computer models are the cornerstone of modern weather forecasting. They encapsulate much of our current knowledge of how weather works. However, many of the details still not known or poorly understood. The atmosphere is broken into gridboxes by location (5km and larger) and altitude and the enormous number of gridboxes required to approximate the earth requires an enormous amount of computation. In fact, the number of useable gridboxes is limited by the size of our largest supercomputers (more is better...). But still, there are many phenomena that are significantly smaller than the smallest gridbox and therefore cannot be modeled accurately. Thus while modern weather forecasting could not exist without computer models, the models are still just approximations. There are also multiple models--some are better at handling one situation and others are better at other situations. The forecasters generally look at the predictions from more than one model before making a prediction. (At least one of the Boston TV forecasters often shows the outputs of several models before stating his guess of which he believes is most accurate in the current situation.)
Weather itself is driven by the heat energy flow from the sun-warmed tropics to the polar regions where it radiates back out into space. (The heat is carried by air movements, water movement, moisture in the air, changes between ice, liquid water, and water vapor, and radiation.) Weather, as we experience it, is just turbulence in the atmosphere driven by the heat flow. Turbulence, by its very nature, has limited predictability and will always limit our ability to predict the weather. The current limit of prediction (average accuracy vs time) is about two weeks. (In certain situations, such as this just past storm, the predictions for only 24 hours out can contain large errors...).
As for whether the weather is becoming more or less predictable, I have no idea. The climate is changing rapidly while our prediction techniques are improving. (The climate change appears to be increasing the severity of certain storms but that is not the same as reducing the prediction accuracy.)
Doug
