The sources pointed out that the MacBook Air for 2013 will feature a new processor platform, but its industrial design will not see any major changes. The sources noted that pressure on ultrabook vendors mainly comes from the Air's price since Apple is likely to reduce the prices for its existing MacBook Airs before the launch of the new models which could take away demand for ultrabooks.
It's estimated that notebook shipments will be 17 million in 2013; however, sales may be less.
MacRumors notes that this aligns with predictions from KGI Securities analyst Ming-Chi Kuo who correctly predicted that Apple would introduce two new notebooks alongside its current lineup.
As you can see in the chart Kuo expects the MacBook Airs to go Retina when all notebooks move to the Haswell platform.