JPMorgan analyst Jamie Baker downgraded JetBlue Airways (JBLU) to Underweight from Neutral with a price target of $11, up from $9. The analyst does not believe the "current pace of equity ascent" in the U.S. Airlines sector can be maintained for much longer.

Transportation Security Administration recovery rates are likely to moderate this fall as corporate demand "inadequately backfills" pent-up summer leisure travel, Baker tells investors in a research note. Further, valuations are "beginning to stretch for some," adds the analyst.

In addition, the analyst believes Treasury may prove the "last easily identifiable near-term catalyst," potentially followed by dilutive equity raises. Baker, who sees "no potential immediacy for profit taking," shares of United Airlines (UAL) and JetBlue have surpassed prior price targets, leading to downgrades.