I agree completely with the above answer, and would only add a couple of other external factors to consider as far as tourism goes:
1) Currency Rates - This ties in to the above post in the reference to the strength of other countries' economies, but to name a specific example, the value of the Euro dropped significantly when a European Union bailout of Greece was announced. This affected American tourism because it immediately became more expensive for all Europeans to travel to the US, no matter what internal factors in the US were at play.
2) World Security - The airlines went into a tailspin (pardon the bad pun) right after the 9/11 attacks when people in general viewed airline travel as unsafe. No matter how irrational that may have been, it affected tourism revenue and the tourism industry itself had nothing to do with it. A similar thing happened after the subway attacks in Spain and London, and people avoided those countries for a while.
I assume that when you say "external" you are talking about factors that do not have to do with the destination itself -- they are external to the tourist destination.
If that is how you are using the term, there are many external factors. Perhaps the two most important ones would be:
- The economic situation in the home country of the tourists. For example, the tourism industry in Hawaii was hurt significantly when the Japanese economy declined. This meant that fewer Japanese could afford to go to Hawaii.
- The price of travel. With oil prices going so far up a few years ago, the tourism industry was hurt because fewer people could afford the prices charged by the airlines.
There are, of course, others, but these are the two I think are most important.