This workaround to get Forecast working on iOS9 was posted a while ago but I just found it today so I thought I'd post it here for anyone else wanting to get Forecast back working again.
Credit to mramirez23
It's so good to have this back! Let me know if you have any issues doing this and I'll do my best to help.
Turn off location for the stock weather app, instead search for the location you want on the stock weather app.
Install the latest Forecast and turn everything off in the settings. Except for the weather page.
if you're having safemode issues you need to enter the settings page right after installing instead of respringing.
Credit to mramirez23
It's so good to have this back! Let me know if you have any issues doing this and I'll do my best to help.