These long-term forecasts almost always turn out to be wrong.
The truth is that—with very few exceptions—there is simply too much chaos in the atmosphere to reliably forecast beyond two weeks or so (or less in certain situations, such as hurricane season, where a few degrees angular difference in a cyclone's track one way or the other can alter weather patterns fundamentally). Technology has improved rapidly in the past few years; it used to only be a week, and before that only a few days, but now the NWS is starting work on a 3-4 week general outlook. Even so, natural variability and the limits of measurement will be a hindrance to any long-range outlook.
The farmers' almanacs, by the way, I suspect incorporate astrology—in other words, pure pseudoscience, perhaps corrected with modern astronomical measurements.
