It is one thing to read the weather but another to be there. How is the weather? Shorts and t-shirts? Thinking of going next year.
Not many forum folks have been to Herceg Novi, and those who have visited the town have probably only been there briefly, which is not a sound basis for providing climate advice, and you haven't told us what time of year you hope to visit.
The website timeanddate.com is a good source of actual, historical, day-by-day weather statistics for most destinations of interest to travelers. I check the most recent five years for places I intend to visit since there can be considerable year-to-year variation.
Herceg Novi weather -- September 2023
Use the pull-down box at the right, above the graph, to change the month and year displayed.
I normally use the climate-summary chart in a city's Wikipedia entry to get an idea about precipitation. Herceg Novi's rainfall varies a lot from month to month. It's worst in the fall and winter. I myself got caught in a significant downpour in early October 2015. Fall can set in a bit earlier than that, I'd imagine, but it could also be later.
Wikipedia entry for Herceg Novi << Scroll down to the colorful climate chart.