Most cities in the US have nice trails not far away. I'm most familiar with New York, and it has some of the best trails for running in the US within an hour by train (in the City you don't need/want a car). Our temps are rarely too hot and humid or too cold. We might get a short stretch of each each year. But there's something to be said for having four seasons, and upstate New York is as beautiful a place as you're likely to find anywhere. Don't know about ultras, but there are races in the City on nearly every week-end year round, plus plenty just outside and plenty of competitive running clubs.