Everyone says that if you don't like the weather in Portland, wait 5 minutes.  The summers are hot, the winters are cold, but temperatures can vary by several degrees from one day to another, or sometimes even in one day.  Summer is definitely tourist season, and when it's warm and sunny, everyone will be outside walking with their children or shopping in the Old Port.  Sometimes it rains for days at a time, even in June, and temperatures can drop a lot when it rains.  Winters are typically quite cold and windy, making walking outside rather unpleasant, even with several layers on.  Definitely recommended is wearing leggings or long-johns under your pants in the winter time if you will be spending some time outside.  Usually there is a lot of snow, and watch for parking bans when your car will be towed off the street in order to ensure thorough snow removal.  If you like to ski or snowshoe, winter is a great time to visit Portland, but beware that this past winter, there were only a handful of good skiing days, because snow was light and temperatures were warmer than normal.  Fall is a beautiful season because the leaves change to brilliant colorful shades.  Whole tour buses come up to Portland and the surrounding area just to see the autumn foliage.  The past two springs have been very rainy and cool, making it seem that winter all of sudden changes into summer without a true spring in between.

Best Seasons to Visit

Spring and early summer are probably the best times to visit due to the volume of visitors that flood in after 4th of July weekend. Not only will all of Portland be very crowded in mid-late summer, but hotel rates will be higher and alot of the authentic local charm get replaced by tour buses and cruise ships. "Cruise ship season" officially starts in mid-August and continues through October. During this period, the Old Port area (where the cruise ships dock) will periodically become insanely crowded as cruise ship come and go. The fall season is also unexpectedly crowded in Portland, when nearby visitors from Boston and the rest of the east coast flock to the city to observe the autumn foliage. The changing leaves are indeed pretty, but you can expect a crowd spectating the beauty along with you in most places you go. 

Visiting in May/June will yield comfortably warm temperatures (although maybe not beach weather just yet) and beautiful blooming flowers and trees. The downtown area is not yet in full swing in terms of crowds, but every restaurant/store will start operating at their summer hours and you'll have less trouble finding a table at a restaurant or reserving a hotel room.