[alexus]

Code:

new Weathers[] =
{
	8,
	9,
	19
};

SetTimer("Weatherst", 300000, true); //OnGameModeInit


forward Weatherst(playerid);

public Weatherst(playerid)
{
    new rand = random(sizeof(Weathers));
    SetPlayerWeather(Weathers[rand]);
}

There was a mistake:
new RanWeath = random(sizeof(RanWeath));