Oak before ash

I spotted my first leaves on an ash tree this year. The oak leaves have been out for a couple of weeks now. There is a weather prediction that says that if oak comes before ash then the summer will be dry. Old lore often has some basis and I was hoping for an explanation. However https://www.bbc.co.uk/blogs/natureuk/2011/05/oak-before-ash-in-for-a-splash.shtml says that due to global warming, oak is out first almost every year now and that there is no link to rainfall.