Why have a 10-days forecast if the predictions change every day? Good question.
If you're planning something 7-10 days out, you can look 7-10 days out:
- if you see a large high pressure system, these are typically stable and you can be more certain you'll have good weather, i.e, invite people to the bbq party.
- if you see a low pressure system (storm or a band of rain), you know it might disappear, change it's path or arrive a day or so earlier or later. So you keep an eye on it. In this case, you'll tell your friend it might be bad weather and we'll confirm 3 days out.
I did exactly this for my wedding. I watched the weather 7-10 days out on a website called http://www.metvuw.com. It was a high system but windy. So we could plan our dry weather venue and not worry about our wet weather plan B.
After the wedding I thought this would be a good app and WeatherBomb was born six years later.