"One" seems like an odd pronoun to use here...I'm more accustomed to seeing it used in proverbs and gentle admonitions ("one should not scorn charity"), or suppositions ("What would you do if one were to get past your defenses"), not in basic declarative statements--I'd expect either he, she, they, or 'the meteorologist'.
I changed it to passive, to avoid the pronoun altogether: "Thunderstorms ARE predicted." This was accepted as correct, and feels much more natural to me, a native English speaker, than the mysterious "one", or having to introduce a fictitious "they", or even: "the forecasters". I don't doubt that in reality, the "one" in question must be the forecast or the forecaster. But it goes beyond translation to infer it and add it in.
I translated it the same way you did, and was pleased to see it accepted. That said, I think it's perfectly within the realms of translation to make this sort of inference - not much different from translating idioms, really. The purpose of translation is to communicate meaning, and that's what both you and I did here.:)
We mostly use 'ze' for that purpose in everyday speech. "Ze hebben onweer voorspeld." "Ze zeiden dat het warm zou worden."
If you don't want to use a pronoun, you can just as easily make it passive by saying: "Er is onweer voorspeld." We use this just as much as "Ze hebben onweer voorspeld."
Exactly like that. Onweer means bad weather. Storms, thunder, dark clouds, rain... When the prefix on- is added to a word, it makes it negative or makes it the opposite. (This doesn't mean you can add it to any word you want!) So you have 'vriendelijk' is friendly, and 'onvriendelijk' is unfriendly. Same with weather. Add on- and it's negative. Bad weather. (Also, this doesn't mean that 'weer' is only used for good weather!)
If you spelt it as you have here ("lightening"), it is wrong in any case, but even with the correct spelling ("lightning"), it is strange to my ears (native UK speaker). I don't really recognize "lightning storm" - although I would know what was meant, of course. It's just that "thunderstorm" is almost always preferred. I suppose it's a little odd that we usually name it after the sound it makes, rather than how it looks, with vision being the dominant sense, for most people. But that's the way it is.