After months of hot and humid weather, the rains of May come as a bit of a relief. However, the rainy season in Guatemala is not like the monsoon—the rains are usually lighter, and mostly in the afternoon. It's very rare that it rains all day, but it does happen on occasion, so be prepared.
The rainy season runs from May to October, but May doesn't usually receive as much rainfall as the rest of the months, so it's still a good time to go exploring—although it is still very humid. The temperature is also warm and pleasant, with highs of around 85-90°F in the Central Highlands and lowlands, and 73°F in Quetzaltenango, although it can still get down to 48°F at night so you'll want to pack a light jacket.
Guatemala very occasionally experiences hurricanes and tropical storms that blow in from the ocean and can wreak havoc on the coastal communities. Although these normally occur from August to October, on the Pacific coast they have been known to happen as early as May, so although it's highly unlikely that this will happen, make sure you keep an eye on the weather and avoid the Pacific coast if you are worried.
Crowds and Costs
May is pretty quiet on the tourist front, with the hoards of visitors who descend on Guatemala for Semana Santa long having departed, and the majority of people being scared off by the threat of the rain. For those who brave the odd shower, though, the rewards are great, with hot, sunny mornings to explore, fewer crowds, and cheaper off-season prices. The warmth means that even if you do end up stuck in the odd downpour you'll dry out pretty quickly, and the ponchos that they sell in all the corner shops and market stalls will come in very useful for covering you and your backpack if you do end up caught in a storm.
Where to Go
Deciding where to go in May might be the trickiest part of all. The rainiest place in the country in May is the Pacific coast, although the idea of sunny mornings swimming and lazy afternoons watching the storms roll in off the ocean on the black sand beaches may not be too terrible for you either, so don't rule it off entirely.
Lake Atitlán is still super pleasant, and again, a great place to watch the lightning storms, especially if you have a view of the volcanoes. Most volcanoes have their own microclimates and often have the best lightning storms, so, with three volcanoes surrounding the city, Antigua is also a great spot for watching the storms roll in and enjoying a coffee or a cocktail or three.
Tikal is hot and humid, but if you head there first thing in the morning for a sunrise tour then you should avoid the heat of the day and miss the afternoon rain. However, if you happen to get caught in a downpour, just bring an umbrella and wait it out, chances are everyone else will disappear and you'll have the entire place to yourself.
What to Do
As long as you get up and out and explore early enough, pack an umbrella or a poncho, and are happy to roll with the punches, May is a great time to explore Guatemala. The only thing that it's probably not best for is climbing Acatenango, with hazy days, cold nights, and rainy afternoons, you could end up wet, cold, and not even able to see Fuego erupting or the incredible sunrise vistas.
Still, if you want the experience of climbing a volcano, perhaps you could try Pacaya instead. It's only a half-day trip from Guatemala City or Antigua, and you can leave early in the morning to get the best weather. At least that way even if the views aren't the best, you'll still be able to see the steam coming from the ground and toast marshmallows over active volcanic vents.
Events in May
Día del Trabajo (Worker's Day) - May 1st: Celebrated across the world, International Worker's Day in Guatemala is a national holiday, with parades and parties across the country. The biggest celebrations take place in Guatemala City.
Traveling to Guatemala in May? Check out this great itinerary.
Best of Guatemala - 8 Days. This week-long trip starts in Antigua, includes a bike trip around the surrounding villages and farms, and a hike up Pacaya volcano. After that, spend a few days relaxing on Lake Atitlán before flying to Tikal to explore the ancient ruins. Although the tour ends in Guatemala City, if you're not ready to say goodbye to Guatemala just yet, you could always extend it and add a few days in Rio Dulce on the way back down to the capital.