Actually that's a misconception. Weather forecasts have a very high accuracy rate.
Often, stating the forecasts in terms of probability gives a certain wiggle room, and therefore leads to a certain amount of criticism, but in the end , probability is what its all about.
With due regard for length of time in the future one is attempting to predict, weather forecasts are Extremely accurate in the short term, Very Very accurate in the mid term and reasonably accurate in the far term. (For some values of near, mid and far).
AS for your Vietnam analogy, I disagree that conflicts are taking that direction. If anything, we are making efforts never to allow an enemy supply routes from the safety of a untouchable third country. Which is why Syria and Iran are so quick to deny any such involvement.
In fact I can't think of an engagement like Vietnam, since, well, Vietnam.
Nice insight on adjacent countries and supply. Hadn't considered that.