Firstly, open the phone app, then scroll to the radar page, then back to the 'first' page. This will trigger a reset/resend of the token.
Next try restarting the phone, or closing Rain Parrot using the task switcher.
Also try opening the watch app before the app, and vice-versa. Note that you will need to fully close the app before it will try re-sync, which may mean restarting the phone.
It's a fiddly problem but it is resolved with a bit of playing around to get the phone to talk to the watch.