That's why the National Weather Service makes specific weather forecasts — high and low temperature and probability of precipitation — only seven days in advance.
For extreme weather events such as hurricanes and cyclones, the agency sometimes makes longer-term predictions, based on such things as movements of the Madden-Julian Oscillation (a cycle of atmospheric weather in the Indian Ocean and the western Pacific) or the more famous El Nino weather pattern. The Weather Service also offer a 10-month forecast, but it's extremely vague, making such predictions as an above-average amount of precipitation over the course of a season.
Making specific predictions that far ahead isn't yet possible, and it may never be, according to Uccellini. He notes that three obstacles prevent scientists from making reliable forecasts even only 10 days in advance: observation systems, numerical models and computing power.
With weather satellites proliferating, there have been tremendous improvements in global data collection over the last decade or so. Computing power has also moved forward rapidly, although the ability to run computations that divide the world into small segments demands a staggering electronic infrastructure. The models are the real sticking point, but the National Weather Service is making progress by taking a sort of "poll of polls" strategy, to borrow a phrase from political scientists.
"We're now finding that if you run an ensemble of models, merging an envelope of solutions from second and third models, you can extract a more likely solution," Uccellini says.
Testing on this combined-model approach has suggested that the National Weather Service may be able to push its official forecast out to 10 days, but no decision to do that has been made. (The agency moved from a five-day forecast to a seven-day one only in 2000.)