Translation:A thunderstorm consists of thunder and lightning.
Fulmotondro comes from the verb Tondri (to thunder). It is then converted into a noun --> Tondro and then Fulmo (lightning) is prepended -- thus: Fulmotondro (thunderstorm). [Source: La Esperanta Vortaro and some various notes I've taken.]
However the phrase of this thread is essentially telling us how the word is built ^.^ (something of which I had briefly forgotten while composing this response -- and then became reminded once more after accidentally scrolling to the top of the page :))
Onweer is an interesting word. You probably know this, or could decipher this on your own, but onweer could be interpreted literally as un-weather. I've read some about its possible etymologies (primarily the prefix on-).
I've noticed a pattern amongst many Germanic roots regarding the prefix un- (or on- or a similar variation). One common apparent meaning of the prefix is as a pejorative or negation prefix. Other etymologies note an oppositional role -- similar to the Esperanto prefix mal-. If I recall correctly there is an Esperanto affix -aĉ-, which is used to add a pejorative connotation to a word.