You really can’t trust Boston weather reports to provide accurate forecasts for all the micro-climates between Route 28 and 6A, from the canal to Provincetown.
If you really want to go to the Cape, just go. There’ll be plenty to do even if it’s cloudy or rainy.
When in doubt, or when it really matters, call the radio station WQRC, 99.9 FM (508-771-5522), or consult capecodweather.net.
OR => Weather.com (The hyperlink defaults to the National Seashore, on the Outer Cape, so simply plug in any town or zip code for more localized information.)