Map from Google - Montenegro

The climate of Montenegro is Mediterranean on the coast, with heavy rains from September to April, while moving inland it becomes progressively more continental, with colder winters, also due to the higher altitude.
On the coast, the climate is mild but rainy in autumn and winter, while in summer it's hot and sunny, though tempered by the breeze. Winter is mild, with average temperatures in January around 7/8 °C (45/46 °F); cold air masses of Russian origin may reach the coast, bringing a dry, down-slope wind which descends from the mountains, and lowers the temperature but generally do not bring snow, except in limited areas where the wind is channeled through mountain passes instead of descending from the ridge. The Bay of Kotor is sheltered and mild, but in the cold season the temperature drops suddenly when the Bura blows, a cold wind coming from the interior.
Here are the average temperatures of Ulcinj, on the southern part of the coast.
Average temperatures - Ulcinj
UlcinjJanFebMarAprMayJunJulAugSepOctNovDec
UlcinjJFMAMJJASOND
Min (°C)4581015182121181496
Max (°C)101114162125282725201511
Min (°F)394146505964707064574843
Max (°F)505257617077828177685952

In the capital Podgorica, located in the plains but at some distance from the sea, the climate is still quite mild, but a little more continental than on the coast, so that winter is a bit cold, with an average in January is 5 °C (41 °F), while summer is hot and sunny, often with torrid days; heavy rains are possible from October to March.
Average temperatures - Podgorica
PodgoricaJanFebMarAprMayJunJulAugSepOctNovDec
PodgoricaJFMAMJJASOND
Min (°C)136914172020171273
Max (°C)101115192428323227221511
Min (°F)343743485763686863544537
Max (°F)505259667582909081725952

Nikšic, the country's second city, is located at 650 metres (2,100 ft) above sea level, and has a moderate continental climate: winters are cold, with snowfalls and frosts during cold spells, while summers are generally pleasant, with highs around 26 °C (79 °F), but with possible hot days, in which the temperature goes above 30 °C (86 °F).
Average temperatures - Niksic
NiksicJanFebMarAprMayJunJulAugSepOctNovDec
NiksicJFMAMJJASOND
Min (°C)-1-1261013161612830
Max (°C)56912172126262216105
Min (°F)303036435055616154463732
Max (°F)414348546370797972615041

Continuing inland, the climate becomes even colder, so that in the far north, on the border with Serbia, at a given altitude, the temperature is similar to that of the Alps.

Precipitation in Montenegro is plentiful, especially in the coastal strip and in hilly regions overlooking the sea. The rainy season is long and runs from September to April; sometimes the rains are torrential. Certain areas of Montenegro receive the highest amounts of rain in the entire Europe. In addition, in inland areas and mountain ranges, additional rainfall occurs in the form of afternoon thunderstorms, in late spring and summer (more frequently in May and June). On the contrary, in the coast and in Podgorica, at least in summer there is a break in the rainfall, and thunderstorms are rare. In Podgorica, precipitation amounts to 1,650 millimetres (65 inches) per year.
Average precipitation - Podgorica
PodgoricaJanFebMarAprMayJunJulAugSepOctNovDecYear
PodgoricaJFMAMJJASONDY
Prec. (mm)1951801451001056040501102302102251650
Prec.(in)7.77.15.73.94.12.41.624.39.18.38.965
Days101298954369111197

Kotor

The sea temperature is warm enough for swimming from July to September, while it is a bit cool in June and October.
Sea temperature - Budva
BudvaJanFebMarAprMayJunJulAugSepOctNovDec
BudvaJFMAMJJASOND
Sea (°C)141414151821242523201715
Sea (°F)575757596470757773686359

When to go

The best time to visit Montenegro goes from May to September.
On the coast, summer is a good season for a beach holiday, especially in July and August. The weather is sunny and hot, but tempered by breezes. We cannot exclude some brief afternoon thunderstorms (as we have said, more rarely than in inland areas).
In Podgorica, summer can be uncomfortably hot, so that the temperature can reach 40 °C (104 °F), especially in July and August, therefore it's better to choose May, the first half of June, and September.
In mountainous areas, summer is the best season. The sun often shines, even though there's some chance of thunderstorms in the afternoons.

What to pack

In winter: in the inland region, warm clothes, hat, down jacket, scarf, gloves, raincoat or umbrella. In Podgorica and the coast: warm clothes, sweater, jacket, raincoat or umbrella.
In summer: in the inland region, light clothes, t-shirt, but also long pants, light jacket and sweatshirt for the evening; umbrella. In Podgorica and the coast: light clothing, possibly a sweatshirt for the evening, especially in June.