Rory's answer is good, but I though i'd make it clear that there are really two different sorts of 'long range forecast'.
To meteorologists, a long range forecast is about 7 days. Because the weather systems are chaotic (in fact, Lorenz who founded much of chaos theory, did so as a result of his work on weather forecasting) the longer out you go the bigger the uncertainty gets and meteorologists wouldn't claim even their 7 day forecasts are incredibly accurate.
Then there are forecasts like the Farmer's Almanac who make predictions more than a year our, using in their own words
a top-secret mathematical and
astronomical formula that figures in
sunspot activity, tidal action, the
position of the planet in relation to
the Sun, and a number of other
That probably set off your skeptical-spidey-sense, and indeed wikipedia says cites an article (Walsh and Allen (1981) Weatherwise 212-215, not avaliable online as far as I can tell) that says they do no better than chance.
In New Zealand we have someone similar in Ken Ring, who claims to be able to predict the weather using astrology (the moon in particular). He's also been tested and found wanting [disclosure: this is from an article on my own blog]
His prediction was only on the right
side of the average 17 times in 48
attempts, about 35% accuracy and
significantly worse that you would
expect to get from tossing a coin (if
you’re one of those p-value fetishists
p, in this case, is equal to about
I haven't read the Farmer's Almanac predictions, or how they claim 85% accuracy, but Ken Ring's post hoc justifications will be familiar to skeptical types. Here's a page on his website in which he rates his ability - my favourite among them is this one
Prediction: There's 2 rain days around 19th-20th; possible rain in excess of 100mm each day
Result: Happened further west - Broken Hill got about 20mm on the 22nd
A hit! Just further West, a fifth as much rain and two days later