This morning I noticed a bad vibration in my steering wheel once I got up to highway speeds. I just got the car about two months ago and it did a similar thing when braking, turned out to be warped rotors and I had them replaced. I checked my lug nuts and everything was tight, it seems to get a little better as I drove, maybe I was just getting used to it. The thing that worries me is it seemed to come on so suddenly. We had heavy rain here for about 6 hours yesterday, could that have caused it?

 

Sound like tire flat spots. No big deal when the tires cool down they tend to get a little area that is flattened that is against the ground. Its not too drastic but it will cause your car to drive like a 4X4 with super swamper tires. This along with your suspension needing to get warmed up causes some vibrations. Should ride alot better when everything is "warmed". If it didnt go away then it may be tire balance, but with your description it sounds like mild flat spotting of the tires due to cool down. The quirks of lower profile tires.

 

Sounds like you lost a weight on a tire. Unless you have your ABS off and have layed down major rubber under hard braking, I would bet it's an unbalanced tire.