Map from Google - Croatia

In Croatia, there are three types of climate:
- the climate of Istria, the Adriatic coast and the islands, which is mild (especially in the southern part), and rainy;
- the climate of the Dinaric Alps, which is cold and snowy in winter, and cool in summer, with thunderstorms in the afternoon;
- the continental climate of the interior plains.

Croatia is a rainy country: it's moderately rainy in plains and inland valleys, and definitely rainy along the coast and the western slopes of the mountain ranges. The rainfall pattern is Mediterranean in the coastal strip, with a minimum in summer and a maximum in autumn and winter, while in the interior plains, precipitation is frequent throughout the year, and in winter it often occurs in the form of snow, but is more abundant in summer, when it often occurs in the form of thunderstorm.

In the Adriatic coast the climate is Mediterranean. The northern coast, in Istria and in cities such as Pula (Pola in Italian) and Rijeka (Fiume), has fairly mild winters, with an average in January around 5 °C (41 °F), but with sudden drops in temperature when the Bora blows, a violent wind coming from the freezing Russian plains. The Bora brings rare and not abundant snowfalls, because the cold continental air is dry. Here summers are warm and sunny, sometimes hot, though the breeze tempers the heat; sometimes there can be cool days, with rains and even a bit of Bora, even in summer.
Here are the average temperatures of Rijeka.
Average temperatures - Rijeka
RijekaJanFebMarAprMayJunJulAugSepOctNovDec
RijekaJFMAMJJASOND
Min (°C)336913161919151174
Max (°C)91012162125282824191310
Min (°F)373743485561666659524539
Max (°F)485054617077828275665550

The temperature of the sea is warm enough for swimming in the summer, especially in July and August, but also in September.
Sea temperature - Rijeka
RijekaJanFebMarAprMayJunJulAugSepOctNovDec
RijekaJFMAMJJASOND
Sea (°C)121111131722242523191614
Sea (°F)545252556372757773666157

Further to the south, the climate becomes progressively milder, so that in Dalmatia, where Split (Spalato) is located, and in the large islands of the Dubrovnik (Ragusa) region, the average in January rises to around 7/8 °C (45/46 °F) and even more in the most sheltered areas. In winter, however, outbreaks of cold air are possible here as well, with a wind similar to the Bora (but somewhat milder), especially at the mouth of the valleys, where the air is channeled between the mountains.
Here are the average temperatures in Dubrovnik, located on the southernmost part of the coast, where winter is particularly mild.
Average temperatures - Dubrovnik
DubrovnikJanFebMarAprMayJunJulAugSepOctNovDec
DubrovnikJFMAMJJASOND
Min (°C)76911151922221915118
Max (°C)121214172125282925211713
Min (°F)454348525966727266595246
Max (°F)545457637077828477706355

In the southern part of the coast, the sea temperature is a bit warmer in winter, while in summer it is quite the same.

Precipitation is abundant in the entire Adriatic coast, where between 1,000 and 1,500 millimetres (40 and 60 inches) of rain per year fall, especially in autumn and winter. There are, however, some relatively sheltered areas, in the bays and along the parts of the coast that are protected by the islands, such as the one where Zadar (Zara) is located, which receives about 800 mm (31.5 in) of rain per year, because it is protected by three islands arranged in a line, over which a part of the rainfall is discharged. Summer is dry and sunny throughout the coastal strip, with occasional rains or thunderstorms, most likely in the north, in Istria; only in the northernmost zone, where Rijeka is located, the rains are quite frequent and abundant even in July and August.
Here is the average precipitation in Rijeka, where 1,500 mm (60 in) of rain per year fall.
Average precipitation - Rijeka
RijekaJanFebMarAprMayJunJulAugSepOctNovDecYear
RijekaJFMAMJJASONDY
Prec. (mm)135115105110100110801001651751851551540
Prec.(in)5.34.54.14.33.94.33.13.96.56.97.36.160.6
Days119101212129910111212129

In Dubrovnik, in the southern part of the coast, rainfall amounts to about 1,000 mm (40 in) per year. As you can see, the rains in summer become quite rare.
Here is the average precipitation.
Average precipitation - Dubrovnik
DubrovnikJanFebMarAprMayJunJulAugSepOctNovDecYear
DubrovnikJFMAMJJASONDY
Prec. (mm)95901009075502560801101401251035
Prec.(in)3.73.53.93.53212.43.14.35.54.940.7
Days11111211107456101113111

Dubrovnik

In the area of the Dinaric Alps, the climate becomes more continental, but it remains rainy due to the proximity to the sea, especially in the slopes exposed to the west and south.
For example, in Gospic, located less than 20 kilometres (12 miles) away from the coast, and 550 metres (1,800 ft) above sea level, the average temperature in January is -1 °C (30 °F), while in July and August it's 18 °C (64 °F).
In this area, the rains are abundant all year round (therefore also the winter snowfalls), with a relative minimum in summer. During winter, cold waves from the north or east can occur. In spring, pleasant days alternate with the return of cold weather, with possible snowfalls even in April. During summer, some afternoon thunderstorms are possible, and nights are still cool, but sometimes the days can be hot.
The Plitvice Lakes National Park is located in this area, about 550/600 metres (1,800/2,000 feet) above sea level.

In the northern and eastern plains, the climate is continental, and the rainfall pattern is very different from that of the Mediterranean: summer is the rainiest season, mainly because of thunderstorms occurring in the afternoon, while in winter, although precipitation is frequent (and occurs often in the form of snow), it is not abundant.
In Zagreb, the capital, the average temperature of January is -0.5 °C (31 °F), that of July is 20.5 °C (69 °F).
Average temperatures - Zagreb
ZagrebJanFebMarAprMayJunJulAugSepOctNovDec
ZagrebJFMAMJJASOND
Min (°C)-4-31591314141062-2
Max (°C)36111621252726221694
Min (°F)252734414855575750433628
Max (°F)374352617077817972614839

Precipitation is still quite abundant, 860 mm (34 in) per year, with a minimum in February of 42 mm (1.7 in), and a peak in June, of about 100 mm (3.9 in). The rains remain quite abundant even in autumn.
Here is the average precipitation in Zagreb.
Average precipitation - Zagreb
ZagrebJanFebMarAprMayJunJulAugSepOctNovDecYear
ZagrebJFMAMJJASONDY
Prec. (mm)5040506080100809085708565855
Prec.(in)21.622.43.13.93.13.53.32.83.32.633.7
Days111011131314111010101212137

The situation is similar across the northern plains, except in the extreme east, on the border with Serbia, in cities like Osijek and Vukovar, where the autumn rains are much poorer, therefore the annual precipitation drops below 700 mm (27.5 in).
For the rest, the climate in the northern plains is similar. Winter is cold, often a cold drizzle or snow falls, days can be dull and foggy, but sometimes cold air masses from Russia can lower the temperature to about -20 °C (-4 °F). Summer is warm or even hot, with pleasant days alternating with hot and sultry days, when the maximum temperature can reach 35 °C (95 °F), and with some chance of thunderstorms in the afternoon.

Zagreb

When to go

For swimming and sunbathing, the best months are July and August. It is worth while to recall that the northernmost part of the coast (Istria, Rijeka), can sometimes experience thunderstorms, or rainy days even in summer, which are rarer along the coast of Dalmatia.
In general, autumn rains (which are heavy in the coastal area) begin in the middle of September. In autumn, the moist southerly winds prevail.
The rest of Croatia (inland regions, including the capital) can be visited from May to September. The sun often shines, but during the afternoon, thunderstorms are possible (on average one day every 3 or 4). In July and August it can be hot at times, especially in the plains and in inland valleys, so those who don't like the heat may prefer May, June and September.

What to pack

In winter. In inland areas: warm clothes, hat, down jacket, scarf, gloves, raincoat or umbrella. On the coast: warm clothes, coat, raincoat or umbrella; hat, scarf and gloves for the days with the Bora.
In summer: light clothes, T-shirts, but also long pants, light jacket and a sweatshirt for the evening and for cooler days, especially in inland areas and in Istria; umbrella or raincoat, especially in inland areas and Istria.