Weather verbs in Catalan stand alone with nobody doing them: plou, neva. The rest use fer: fa sol, fa fred, fa bo. For a real possibility, si is followed by the plain present tense on both sides of the sentence.
Examples
- Plou des d'aquest matí.
- It has been raining since this morning.
- Si fa bo, anem a la platja.
- If the weather is nice, we will go to the beach.
- Avui està ennuvolat i fa fred.
- Today it is cloudy and cold.
The mistake to avoid
Never put the future after si: si plou, not "si plourà".
Where this fits in
Plans i cites, Catalan A2: Proposar alguna cosa, concertar i ajornar una cita, i parlar del temps que canvia els plans.