This answer is somewhat personal and not based on any scientific facts. However we find the forecast useful for approx. 3 days. The weather in Iceland is known to change and there is a saying: If the weather is bad, wait 15 minutes.
Perhaps this is the reason why our product Camper Iceland is so popular. When your vacation start you can choose which direction you go. Stay where the weather is nice and remember this:
"We have had customers that started their vacation at the same time. One listened to the forecast and drove away while the other had his trip planned the other way around Iceland. When they returned one had experienced nice weather all the time and could not believe when the other old him it rained all the time. Be the winner and travel with the weather. Plans are good but nature does not adjust to you but you should adjust to nature.