6 hours into the Pocket Monsters Omega Ruby / Alpha Sapphire x NicoNico Live: Autumn Pokémon Special (ポケットモンスター オメガルビー・アルファサファイア × ニコニコ生放送 秋のポケモンスペシャル), Junichi Masuda was brought on to reveal new information on Omega Ruby and Alpha Sapphire. He explained that Rayquaza would get a new Mega Evolution, and showed footage of it in action. Afterwards, the hosts Kosuke and Ayana Tsubaki had a quick link battle with a demo version of ORAS, where Mega Rayquaza's powers were demonstrated.

Shortly after, the official Pokémon websites updated with information on Mega Rayquaza as well:

Mega Rayquaza Revealed:

Rayquaza is said to be somehow connected with the mysteries of Mega Evolution. Now its Mega-Evolved appearance can be revealed at last! The Legendary Pokémon Rayquaza is said to achieve Mega Evolution in a unique way—unlike that of other Pokémon—due to its unique biology. What is this unique method? And what is the mystery of Mega Evolution? All will become clear in Pokémon Omega Ruby and Pokémon Alpha Sapphire.

Rayquaza is a Pokémon distinguished by its high Attack and Sp. Atk. When it Mega Evolves, almost all of its stats get a boost, including these two, and it gains overwhelming power!

Delta Stream is an Ability that only Mega Rayquaza possesses: it fills the air with mysterious strong winds that affect the weather. These strong winds cannot be dispelled by changing the weather condition, except by a very limited number of Pokémon.

Category: ???
Type: Dragon/Flying
Ability: Delta Stream
Height: ???
Weight: ???

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The Dragon Ascent move, which Rayquaza alone can use, is a powerful one. It boasts one of the greatest power ratings of all Flying-type moves, putting it at the top of its class. Not only will you be surprised by its overwhelming power—you\'ll be mesmerized by its majestic appearance! After soaring up toward the heavens in a great burst of speed, it crashes down in attack, dealing massive damage.

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When Mega Rayquaza whips up strong winds with its Delta Stream Ability, this is what happens:

・The moves Rain Dance, Sunny Day, Sandstorm, and Hail will fail.
・The Abilities Drizzle, Drought, Sand Stream, and Snow Warning will fail to activate.
・Moves of a type that Flying-type Pokémon are usually weak to will only deal normal damage to any Flying-type Pokémon in the battle.

Also, with Delta Stream, weather effects last indefinitely as long as Mega Rayquaza remains in battle. When it is swapped out for another Pokémon on your team, the weather immediately returns to normal.

This is different than the Abilities Drizzle, Drought, Sand Stream, and Snow Warning, which can all affect the weather for five turns after being activated in battle, regardless of whether the Pokémon with these Abilities are still in battle or not.

The Delta Stream Ability and the Air Lock Ability are different in that Delta Stream fills the air with strong winds for as long as Mega Rayquaza remains in battle. Air Lock cancels out all weather-condition effects for all of the Pokémon in a battle as long as Rayquaza remains in the battle.

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Source: http://www.pokemonrubysapphire.com/en-us/pokemon/legendary-pokemon/mega-rayquaza