Map from Google - Guatemala

In Guatemala the climate is tropical, hot all year round in the lowlands, and cooler with increasing altitude in mountainous areas.
Geographically the country is divided into three zones: in the north, an area almost entirely occupied by plains (departments of Petén, north of Alta Verapaz and Izabal), with a short stretch of coastline on the Gulf of Honduras (Caribbean Sea); in the center a large mountainous area, where the capital is located; and finally in the south, a thin flat strip south of the mountains, overlooking the Pacific coast.
Here are the average temperatures of Puerto Barrios, on the coast of the Caribbean Sea.
Average temperatures - Puerto Barrios
Puerto BarriosJanFebMarAprMayJunJulAugSepOctNovDec
Puerto BarriosJFMAMJJASOND
Min (°F)707073757777777777757372
Max (°F)828490919191909191888482

In almost all of Guatemala, there is a dry season from late November to mid-April, when the northeast trade winds blow, and a rainy season, from May to October, when the predominant currents come from the south-west.
However, there is also an area, north of the mountains and near the Gulf of Honduras, where it rains all year round, because in the dry season the trade winds reach the coast after picking up moisture from the sea, or they are forced to climb when they encounter the first mountain slopes: here the best period goes from February to April, when however, about 4 inches of rain per month fall (but it's still better than in the rest of the year). Due to the abundance of rainfall, the climate here can be defined equatorial, and in fact the rainforest grows there.
Here is the average precipitation of Puerto Barrios.
Average precipitation - Puerto Barrios
Puerto BarriosJanFebMarAprMayJunJulAugSepOctNovDecYear
Puerto BarriosJFMAMJJASONDY
Prec. (in)7.74.53.74.75.710.415.712.811.411.6118.9108.3
Days

The area we are talking about, which does not have a dry season, is shown in the following map.

Guatemala, humid zone

Let's now look at the areas with both a dry and a rainy season.
To get an idea of the rainfall pattern on the south side, we can look at the precipitation of Mazatenango, located at 1,200 feet above sea level, about 30 miles away from the southern coast.
Average precipitation - Mazatenango
MazatenangoJanFebMarAprMayJunJulAugSepOctNovDecYear
MazatenangoJFMAMJJASONDY
Prec. (in)0.40.21.43.910.217.514.613.818.516.520.899.8
Days113717222122232152145

Along the south coast, the heat is intense throughout the year, because in winter the trade winds heat up as they descend on the leeward side. So here winter is hot, with highs around 86/90 degrees Fahrenheit, but sometimes even above; on the positive side, it's sunny and not too humid. In March and April, before the rains, the hottest days occur, with peaks above 95 °F.
Here are the average temperatures of Puerto San José, on the south coast.
Average temperatures - San José
San JoséJanFebMarAprMayJunJulAugSepOctNovDec
San JoséJFMAMJJASOND
Min (°F)686870737373737373737268
Max (°F)888888908886888886868886

For swimming, the sea is warm enough all year round: the water temperature varies between 81 °F in winter and 86 °F in summer, and this applies to both coasts.
Sea temperature - San José
San JoséJanFebMarAprMayJunJulAugSepOctNovDec
San JoséJFMAMJJASOND
Sea (°F)818182848686868684848482

In the north, the Department of Petén is a bit cooler in the winter months: high are around 79/82 °F. The rains are a bit more frequent than in the southern coast. Here the best period is from February to April, because in December, and in part even in January, it still rains quite a lot. Starting from March, the temperatures rise, leading to pretty high values. From May to October, the temperature drops a bit, but the heat becomes definitely sultry, as happens in all lowland areas of Guatemala in this raiiny period.
In this area we find the Mayan site of Tikal, surrounded by the forest (the National Park of the same name, UNESCO World Heritage Site).

Tikal

In the central area, occupied by mountains and plateaus, the rains are scarce from December to April, as in the south, and the temperatures vary according to altitude. During the rainy season, the rains are usually plentiful, but there are also some sheltered slopes which experience only moderate rains.
The capital Guatemala City is located in the tierras templadas, at 4,900 feet above sea level, and has a spring-like climate all year round: the daytime temperatures range from 74 °F on average in November and December, to 82 °F in April. The weather is rarely too hot, even though the temperature may exceed 86 °F from February to May. At night the temperature is cool, sometimes a bit cold, actually it can drop below 50 °F. The city is quite windy. Here are the average temperatures.
Average temperatures - Guatemala City
Guatemala CityJanFebMarAprMayJunJulAugSepOctNovDec
Guatemala CityJFMAMJJASOND
Min (°F)545557596161616161595755
Max (°F)757781828177777775757373

The rainy season runs from May to October, with frequent and abundant rains from June to September, while from December to mid-April, it rains very rarely. Here is the average precipitation.
Average precipitation - Guatemala City
Guatemala CityJanFebMarAprMayJunJulAugSepOctNovDecYear
Guatemala CityJFMAMJJASONDY
Prec. (in)0.30.10.51.25.910.88.17.19.14.90.80.349
Days313413211918201553125

In Quetzaltenango, located at 7,500 feet above sea level, therefore in the tierras frìas, the climate gets cooler: the average temperature in January is only 54 °F, with high temperature range between night and day, so that at night it often drops to around freezing. From April to August, the daily average is 59/60 °F. Here are the average temperatures.
Average temperatures - Quetzaltenango
QuetzaltenangoJanFebMarAprMayJunJulAugSepOctNovDec
QuetzaltenangoJFMAMJJASOND
Min (°F)363739434848464848464337
Max (°F)727579757372727270727072

Of course, even here there is a pronounced rainy season, but the total annual rainfall amounts to just 31 inches, compared with 80/120 inches of the rest of the country, because the chain of mountains and volcanoes stretching in the southwest part, partly block the moist air masses coming from the sea. However, the rains in excess of 4 inches per month between May and October. Here is the average precipitation.
Average precipitation - Quetzaltenango
QuetzaltenangoJanFebMarAprMayJunJulAugSepOctNovDecYear
QuetzaltenangoJFMAMJJASONDY
Prec. (in)0.10.20.61.65.15.93.94.15.33.70.80.331.7
Days112617221818231462130

The relatively arid area, in which Quetzaltenango is located and which receives less abundant precipitation, is shown in the following map.

Guatemala, semi-arid zone

In Guatemala there are 37 volcanoes, the highest of which is Volcán Tajumulco, that stands out with its 13,845 feet, and it's also the highest peak in Central America. Above 11,500 feet, at night it can freeze throughout the year.
From December to March, Guatemala can be reached by cool air masses from the United States, and in these cases at night the temperature can drop below 50 °F in Guatemala City, and around 55/59 °F in the plains.
Although the tropical rains occur mainly in the form of heavy showers, the amount of sunshine in the rainy months is not good, because clouds usually begin to develop in the warmest hours, and linger on for many hours.
Guatemala is located in the path of tropical storms and hurricanes, which may hit the country in the period from June to November, coming from the east, ie from the Caribbean Sea. The period in which they are most likely is from August to October.

When to go

The best time to visit Guatemala runs from December to mid-April in the central plateau and on the southern side, and corresponds only to the period from February to April along the Caribbean coast and in the Department of Petén.

What to pack

In Guatemala City and tierras templadas: all year round, light clothing, a sweatshirt or sweater and a jacket for the evening; raincoat or umbrella from May to October.
In plains and coasts: lightweight clothes throughout the year; in the rainy season, a light sweatshirt and a light raincoat for thunderstorms.
For the reef, equipment for snorkeling, water shoes or rubber soled shoes.
On high mountains: warm clothes, jacket, hat, gloves, shoes.