In the overworld, you will find that the world is being scorched by sunlight in Omega Ruby, or flooded by torrential rainfall in Alpha Sapphire, both to the awe of the various people witnessing it.
According to the leader of the other team, Kyogre/Groudon will be heading for the Cave of Origin near Sootopolis where it will be able to then use Primal Reversion, unlock its true power which is many times worse than this.
Steven will then come down to you, apologizing for the delay before speeding along to Sootopolis; there, we will get some answers.
New Weather Effects
For all aboveground battles for a while, the weather will be significantly different. For some at least, this will have drastic consequences on the battles between here and Sootopolis and wild battles within the confines of the latter.
Omega Ruby : In this version, the weather will be Intensely Sunny. Fire’s damage is boosted by 50% and all Water moves are negated. There are several other effects, most of which just mimic plain Sunny weather.
Alpha Sapphire : In this version, the weather will be Oppressively Rainy. It is the inverse of the above: Fire is negated and Water is boosted, with several other effects mimicking those triggered by plain Rainy weather.
(In case you can’t tell, this weather is the same as brought about by Primal Groudon’s/Primal Kyogre’s abilities in battle.)
Also note that this weather cannot be overridden by any means; for example, the ability Drizzle will not affect the sunlight on Omega Ruby, nor will the move Rain Dance. Similarly, Sandstorm and Hail cannot be induced by any means.
We’re pretty sure even getting one of the Primal Pokémon out in battle - the one of the opposite version - won’t even affect it for plot-based reasons, but cannot be sure as, mechanically, it should. The same would possibly be true for the use of Mega Rayquaza and its Strong Winds.
Note that the ability Cloud Nine will let you ignore the negation effects of the weather.