if you are looking for an accurate picture of when the rain/storms are gonna be moving in go here:
www.weatheroffice.gc.ca
and use the radar feed and zoom in on the area you are looking. you can activate it to see how fast the system is moving. its a great tool to get an idea of when it will actually hit. although on hot, sunny summer days, daytime heating will cause pop-up thunderstorms to happen fast and unpredictably but typically don't last long (but they can be nasty). if something has been in the long range forecast for some time before hand it is usually a fairly stable system that you can track accurately as it gets close.
hope this helps,
AA.