Rain can vary a lot this time of year. It could be anything. That said, "disposable" umbrellas can be found at any convenience store, supermarket, etc. etc. for usually 300~500 yen, so don't need to worry about that. I would pack extra socks just in case though

Also, here's a page that shows the current radar over Tokyo, which is updated every 5 minutes:
http://tokyo-ame.jwa.or.jp/en/index.html

EDIT: Looks like there are no looming typhoons other than the current one:
http://www.jma.go.jp/en/typh