Will rapid ambient temperature drop kill yeast or just slow them down

It sounds like the kind of temperature change you're talking about shouldn't bother the yeast much unless it pushes them below or out of the proper range of fermentation temperatures for that specific yeast.

Rapid changes can shock or hurt the yeast, for example if you dump a small, warm liquid yeast or yeast starter into a cold wort/must then the yeast will undergo a rather quick change in temperature which could probably ruin them.

As far as the weather changing, and your brew being a considerable quantity or volume of liquid which means that its own temperature will change even more slowly than the weather, it could have some effect on them but I don't think it will kill or stop them unless the temperature moves to being outside of the range appropriate for that yeast.

I'd recommend that you refer to any documentation on the particular strain of yeast in question, if it's available, in order to discover any unknowns about acceptable temperature ranges.

Hopefully the weather is doing you a great favor, ultimately!

Partly it depends on the yeast. Westmalle (WLP 550, Wy 3787) is notorious for flocculating in the middle of a fermentation if it gets too cold, and thereafter being impossible to rouse. At that point, re-pitching is the only option. It can also very easily take off and get too hot--I've had 80F+ with it. Water bath is the best bet for that yeast. But it's one of my favorites.

A drop in temp isn't going to hurt your yeast, but they may chose to start flocculating a little early. Keep an eye of the ferment after you get it warmed back up and see if it starts again. You may need to rouse the yeast (get them back in suspension) if the beer doesn't get started again.

Also, don't make assumptions about what's going on via sight. When I say watch your fermentation, I mean take a hydrometer sample and see where the gravity is at and if its moving in the right direction.

If the beer was bubbling for a couple days it could be close to done anyway. A gravity check is the only way to know what to really do next.

I'm no expert, but I've had a few brews that had temp fluctuations and although it took longer the taste was not noticeably affected. I could have just been lucky.

I do think that the important aspect of fermentation is to get it going quickly. Getting a blanket of CO2 over the brew is really important in keeping out the nasties. So, as long as this is not disturbed and the temp can be raised back to optimum I would think you'll be okay.

That said, you won't want to take too long over the primary fermentation - can you get a belt heater or move it to a warmer place (assuming it's not more than 25 litres or so)?

I don't think this will kill the yeast as secondary fermentation is often performed at lower temps and the yeast will still re-activate when primed in a bottle or barrel - somebody else may want to comment on this.