At the moment, for our web app, you need a connection to the internet to plot routes. We are also working on native iOS and Android apps which will support offline mode.
Once the apps launch, you need a connection to plot your initial route. The app will then store all the required data (charts and weather forecasts) locally on your device, so that you can use it offline. This means that even if you go off-course, you can replot a new route without an internet connection.
As soon as you are within reach of a data connection, the app will automatically fetch the latest weather forecasts in the background, so that you stay up to date.