Map from Google - Paraguay

In Paraguay, the climate is subtropical, with a very mild winter from mid-May to mid-August (dry in the north-west and rainy in the south-east) and a long hot summer, from November to March, with possible afternoon showers and thunderstorms throughout the country. The absence of mountain ranges, however, makes the country exposed to different air masses, and thus to rapid changes in temperature.
Being in the Southern Hemisphere, the seasons in Paraguay are reversed compared with North America or Europe.
The northwestern part of the country, sparsely populated, is the region called Chaco Boreal, ie the northern part of the Gran Chaco, a plain occupied by the savannah which extends into neighbouring Brazil, Argentina and Bolivia, while the central- eastern region is more humid and often marshy. The rains increase proceeding from the north-west to the south-east: the Gran Chaco is the driest area, where precipitation drops below 1,000 millimetres per year, while the rainiest area is that of the Paraná River, covered by a forest, where the annual precipitation approaches 2,000 millimetres.
The main differences in rainfall are found in winter, which is dry in the Chaco, while in the south-east of the country it's rainy; so the south-east, being rainy even in winter, has no dry season. During winter, in fact, the eastern and southern part of the country are affected by weather fronts, which usually spare the north.
Although winter in Paraguay is mild, the country, especially in the south, may be affected by cold air outbreaks from May to September (the cold wind blowing from Argentina is called Pampero), which can lead to nighttime temperatures around freezing, but sometimes also a little below. In Paraguay, however, snow is virtually unknown, and the daytime temperatures are mild even in these circumstances.
Paraguay is exposed throughout the year to warm air masses, which can bring warm days even in winter, with peaks above 30 °C. During summer, however, the heat is permanent, although it is generally torrid in the north-west, with peaks above 40 °C, and more humid (and therefore more annoying) in the eastern and southern regions.
As we can see from the mean temperatures of Mariscal Estigarribia, the Gran Chaco is really hot in summer, and it's still warm in winter, at least during the day.
Average temperatures - M. Estigarribia
M. EstigarribiaJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)232222191614131417202122
Max (°C)353533302725262931343435

As mentioned, in this northern area the rainfall is lower than 1,000 millimetres per year, and the winter is dry. Here the average rainfall.
Average precipitation - M. Estigarribia
M. EstigarribiaJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)130951407540251015205595120825

In the far north, near the border with Bolivia, the average temperature rises above 20 °C even in the coldest months, so that the climate can be called tropical.
In the east, the city of Pedro Juan Caballero is less hot in summer and cooler in winter, because it's located at 600 metres above sea level.
In the capital Asunción the average daily temperature goes from 28 °C in January and February, to 18 °C in June and July.
Average temperatures - Asunción
Min (°C)232221191614131416192022
Max (°C)343332282523232526293132

Precipitation amounts to nearly 1,400 mm per year, including more than 100 mm per month from October to May, while there's a remarkable decrease in winter, when it drops to 40/50 mm per month. In fact, looking at the map above, we can see that the capital is located in the centre of the country, halfway between the north-west which has a dry winter, and the south-east which is rainy also in winter. Here is the average precipitation.
Average precipitation - Asunción
Prec. (mm)145130120165115804050901301651501380

In the far south-east, in Encarnación, the temperature is a few degrees lower throughout the year, and as mentioned in this area the rains are more frequent, without a dry season.
Average temperatures - Encarnación
Min (°C)212019151210101113151719
Max (°C)333231272422222425283032

In addition, in Encarnación the rains are more abundant, without a dry season, so that they exceed 100 mm per month even in winter. Here is the average rainfall.
Average precipitation - Encarnación
Prec. (mm)1551651351751501501051051502151601801850

Despite the thunderstorms, the amount of sunshine in Paraguay is good in summer, while in winter, nice days alternate with cloudy days. Even spring and autumn are rainy. However, the rains in countries having a tropical or sub-tropical climate are irregular: weeks may well pass without rain, and then maybe in a few days some heavy showers or downpours can occur.


When to go

The best time to visit Paraguay is from April to October. In fact, the summer from November to March is the sunniest season, but it's uncomfortably hot and has some chance of thunderstorms: it is probably the worst season.
The winter from mid-May to mid-August is typically mild or pleasantly warm, and is the driest season in much of the country, except in the south-east, but sometimes it can be a bit cold and windy for a few days in the south-east, and sometimes even in Asunción; weather disturbances in winter can bring cloudy days and rains.
You may therefore prefer spring and autumn (April to mid-May and September-October), at least in Asunción and in the south, bearing in mind that there is no shortage of showers and thunderstorms, while in the Chaco, because in winter there is practically no risk of rainfall, while even spring and autumn can be hot, if you want to avoid the heat you will have to focus on the period from April to August, or even better, June and July, which are the ones in which hot days are less frequent and intense.

What to pack

In winter (mid-May to mid-August): clothes for spring and autumn, light for the day, sweater and jacket for the evening. Raincoat or umbrella for the south.
In summer (November-March): lightweight clothes, of natural fibres, sun hat, light raincoat or umbrella, light sweater or sweatshirt for the evening in the south.