We will be traveling from Zagreb down the coast to Dubrovnik and Montenegro.
The weather forecast sites ate all different 60’s to 80’s Fahrenheit. Cooler in north, warmer in south.
But when I Google one city, like Zagreb, and several different weather sites, the temperature comes up different on each site.

What is the most reliable site for weather in Croatia?

https://meteo.hr/index_en.php

The weather does vary between north and south. I don’t even look at the weather until two days before.

Thanks. I’ll wait until 2 days before to make final packing decisions

We were there in May/June this year and based on this limited data can offer (a) the forecasts change frequently, (b) it usually wasn't quite as bad as the forecast and (c) the weather varied a lot in just a few miles or a couple hours. It was sunny and warm and two hours later it was raining. There was also a huge difference between the coast and inland. It would be sunny and warm on the coast yet you could see dark, heavy gray clouds/rain and even lightning over the inland areas not that far away, especially where the mountains rose up steeply in a short area. We were at Plitvice and our phone said it would rain at 3 p.m. so we started out at 11:00 to beat the rain. Well, by noon it was pouring, and I mean pouring (with thunder and lightning). We waited it out under a covered area, for a bit more than an hour when it seemed like it was stopping. Ten minutes later the sun was out. And it was nice, until 3:30 or 4:00 when it started to rain again....so just be prepared for rapid changes.

I look at the weather two days before the day! Not the trip.

Don’t pack anything heavy, and bring an umbrella. Or buy one when you get there.

It would be nice if it cools down just a bit as it looks very hot right now.

We are going up in the Zagorje hills for dinner.
Does it tend to be cooler up there?