We use the Global Forecast System (GFS) for savvy navvy's weather integration. We are also looking at complementing this with Weather Research and Forecasting (WRF) for local predictions in targeted areas.
At the moment savvy navvy uses the current weather, in real-time at the moment you plot your route. The next feature we're working on ( where you can plot your route by setting a, "Depart by" and "Arrive at," option) will also ensure the forecast along the route will be progressed. So if the journey takes a few days, the forecast will be updated throughout those few days for up to date weather information as you go.