Late April trip weather

I suggest going to wunderground.com, entering the destination, choosing History > Monthly, and selecting April 2017 from the pull-down menu. Repeat for April 2016, April 2015, etc. That will give you actual day-by-day weather data for your proposed travel period. To me, that sort of information is more useful than monthly averages, but I see that for the 30-year period ending in 2010, it rained, on average, 14 days every April. What climate change has done to that stat in recent years, I have no clue.

My concern was the weather at the foot of the Julian Alps- are the mountain roads open in April?

Late April should be cool to warm and somewhat rainy, as is most of the year in Slovenia. The foot of the mountains will be in full spring, the road over Vrsic Pass might be open, provided there is not too much accumulated snowfall, the road over Predel Pass is open year-round, whereas the Mangart Road never opens before June. In any case, do expect the summits of the mountain roads to be quite wintery, so any hiking you might plan will have to be confined to the valleys. Also note that the Vintgar Gorge might not be open by the time you visit. Don't fret, though, the Mostnica Gorge near Lake Bohinj and the Tolmin Gorge in the Soca Valley are both alternatives that will be open.