Long term forecasts, actually called outlooks to emphasize that these predictions target average conditions over longer periods of time and a fairly wide areas, have come a long way since I was a young TV meteorologist.
Because we now know about El Niño/La Niña, a big sloshing of pressure systems spanning the tropical and subtropical Pacific Ocean and other oscillations of atmospheric pressure medium to long term forecasting has become more of a science and much less of a guess.
Other such sloshings are the Madden-Julian Oscillation, a tropical rain system pattern, Antarctic Oscillation, the Arctic oscillation, the North Atlantic Oscillation and the pacific-North America Pattern
along with a knowledge of blocking, patterns that persist for a long time have enabled the meteorological/climatological community to develop better long-term outlooks.
Remember that these outlooks talk about average conditions and not specific events so a period with an outlook for a dry, hot summer there can be a cool, wet week or two.