Considering that the United States has been saying the whole time that there's no place for Assad etc., I think it's safe to say that their strategic goals include him being gone. And there's more than enough circumstancial evidence to show that the U.S. is onboard with Qatar, Saudi Arabia and Turkey to get rid of Assad so they can get gas/oil to Europe via Syria which Assad refuses to agree to because he'd rather deal with Iran and Russia.
So while I don't dispute that the reduction due to weather is a reasonable explanation, that in no way invalidates the argument that the United States has been following a containment policy with regards to ISIS until after Assad is neutralised.