A response to a blunt statement like "it's all guesswork" doesn't really call for much embellishment. However, I will note that meteorologists use various tools to produce longer range forecasts, including extended-range models, global atmospheric drivers and teleconnections. At longer and longer lead times the outlook becomes more probabilistic; of course, nobody knows what's going to happen a fortnight to a month ahead, to address the second part of your statement, but with proper interpretation this aforementioned guidance can give decent ideas of trends in parameters like temperature, precipitation and wind, with various degrees of uncertainty depending on consistency and agreement.