Hi,
I have read several post over the last year that using an external antenna can be a problem during a thunderstorm as the lightning can foul up the inner workings of the radar unit. As an aside, I run a Boltek Lightning detection unit which can alert me to developing thunderstorms in my area. When it starts popping, I disconnect the antenna from the unit. It's not fool proof but it can help prevent the unthinkable.
Sherlock Holmes.