I've got a M.Sci in Meteorology and have done 4 years as a weather forecaster. James V.A. has it right
*Climate* forecasting is still a rough area: by this I mean predicting a season's general trends (rainy/dry, cold/hot) ahead of time. Yet even here progress is seen
Such forecasts are highly successful and useful tools in the tropics - and has been for years. Although seasonal forecasts for mid-latitude areas have improved, there is still much left before they are reliable enough to be useful. Such as the absolute failure to predict the cold winter weather we've had this year in Northern Europe. (January was supposed to be slightly warmer than average - and wet. Instead we've got record breaking low temperatures.)
I guess my thesis here is that modern weather forecasts are reliable to the point of shaping preparations for activities. If forecasts show a front passing near a mountain maybe you shouldn't climb that day even if the airplane ticket is already paid, etc. Weather forecasts should be a basis for action and not merely passively collected as part of preparations.
Yes - and that works both ways: Finding a "window" where conditions are good for what you want to do.
Of course it makes sense to be prepared for foul weather anyway. Particular in the mountains local conditions can differ quite a bit for the general area.