Your "didn't go north" example, as it stands, to me implies that your trip was completely cancelled. You were perhaps considering going north on that day, but the bad weather meant you didn't go north after all. With more context though you can change this meaning; for instance, "The weather was very bad, so we didn't go north that day, but the next day the weather had improved, so we went then". The point of "didn't go north" is your focusing on the fact that the trip as a whole didn't happen, or at least didn't happen at the time you expected it to.
"We weren't going north", on the other hand, puts the emphasis more on the action of going north not happening at that particular time. For instance, you could say "the weather was very bad, so we weren't going north. This meant we could see the 5pm showing of the film after all". The point of this example is that since you weren't actively travelling at that time, you could instead watch the film (presumably in the south!). Whether or not you did eventually go north is kind of irrelevant to that sentence, and you can't make an inference either way.
An expression like this can ALSO be used, with a slightly more colloquial feel, to indicate that going north would be impossible for some period of time. For instance, "the weather was very bad, so it looked like we weren't going north any time soon" - this indicates that going north appeared impossible at least in the near future (as a British English speaker I would generally also take "any time soon" to be an understatement; that is, the speaker likely means that it's actually going to be impossible for quite a long time, but I'm not sure if this transfers to American English). You could also use a phrase like "wouldn't be going north any time soon" or other similar phrases in the same context; my ears at least can't detect any difference in meaning between the two.