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 due also to the higher altitude.
On the coast, the climate is similar to that of the Italian Riviera: mild but rainy in autumn, winter and spring, warm and sunny in summer, but tempered by the breeze.
On the coast, the winter is mild, with average temperatures in January around 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 winds are 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, a cold wind coming from the interior, blows.
Here are the average temperatures of Ulcinj, on the southern part of the coast.
Average temperatures - Ulcinj
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 41 °F, while summer is hot and sunny, with torrid days; heavy rains are possible from October to March.
Average temperatures - Podgorica
Min (°F)343743485763686863544537
Max (°F)505259667582909081725952

Nikšic, the country's second city, is located 2,100 feet above sea level, and has a moderate continental climate: winters are cold, with snowfalls and frost during cold spells, while summers are generally pleasant, with highs around 79 °F, but with possible hot days, in which the temperature goes above 85 °F.
Average temperatures - Niksic
Min (°F)303036435055616154463732
Max (°F)414348546370797972615041

Continuing inland, the climate gets colder, so that in the far north, on the border with Serbia, at a certain altitude, for example 3,300 feet, the temperature is similar to that of the Alps.

Precipitation in Montenegro is plentiful, especially in the coastal and 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 throughout Europe. In addition, in inland areas and mountain ranges, afternoon thunderstorms occurring in summer (and more frequently in May and June) add rainfall to that of the other seasons. 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 65 inches per year.
Average precipitation - Podgorica
Prec. (in)

Kotor, Montenegro

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
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 vacation, especially in July and August. The weather is sunny and hot, but tempered by breezes. We cannot exclude some brief afternoon thunderstorm.
In Podgorica, summer can be uncomfortably hot, so that the temperature can reach 105 °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 afternoon.

What to pack

In winter: for 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: for 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.