Panama gets a lot of rain. It is located in the tropics so there is a definite rainy season and a dry season in Panama. Dry season or locally referred to as Summer is December through April, which coincides with our peak season for tourism. Rainy season, which is locally referred to as Winter or the "green" season. The rainy season is May-November in Panama.
The average amount of yearly rainfall varies greatly depending on what part of Panama you are asking about. For example, in the Panama canal region on the Pacific side or Panama city side, the average rainfall is 110" per year. On the Caribbean or Colon side of the canal, only 50 miles or 80 kilometers away the average rainfall is double at 220" per year! Some areas of Panama, such as the Pacific beach areas of Gorgona, Chame, Coronado and also the Azuero peninsula is known for being consistently drier and even has a "desert"region. In the mountain areas, such as El Valle de Anton, Boquete or Volcan there is a higher rainfall as in most mountainous regions around the world.
Many visitors to Panama falsely believe that "rainy season" means that it will be raining hard all day long, every day of the season. Not True. I am originally from Seattle and I thought I knew what rain was. Also not true. The rain in the tropics is very different than the gray months of 40 F drizzle in Seattle. Here in Panama, rainy season is very pleasant. The rain comes in extremely heavy downpours. Think standing in the shower, where you are instantly soaked to the skin. The beauty of the rains here is that they are of very short duration, normally 10-20 minutes and are very localized. You can see the rain storms coming and going, and know that within 20 minutes, you will see the sun again. Normally, during rainy season you may have 1-4 of these rain showers each day, but you can count on also seeing the sun almost every day of the year. The other great thing about the rainy season here is that the rain is always warm. It is never cold here in Panama. In all lowland areas, never below 75 F.
In Seattle, we never let the rain stop us from going anywhere. If you waited for it to stop raining in Seattle, you might wait months! Here in Panama, everyone waits for the rain shower to stop and then they go about their day. I knew I was officially "Panamanian" when I found myself waiting out a rain shower when coming out of a store. 10 minutes later I was on my way, in the sunshine.
One great advantage to visiting Panama is there are less tourists here and many hotels have rainy season discounted prices. And for me, rainy season also means one of my favorite times of year, Humpback whale watching season, but that is another answer...