I have to admit, I think there was an interval one could specify in the program but I couldn't find it. I even went back to RTN2012 and it was different but just as limited as RVPNG2013.
However, when displaying the map and the weather is being displayed, click on "User", then slide down to "North America weather" and there are some options that will show up. Try clicking on these and see what happens. Sorry I can't help any more than that, but the most I have done is "Refresh" the display and then the display could change if it's been a while since the last update. Do keep in mind this will only work on a map area that shows some level of precipitation, but I believe I'm being obvious.
Try sending an email to
support@uncertowsoftware.com and ask them for this feature. They might have a reason for not having it or removing it if they did ever have it. Keep us posted.