The airline industry seems finally to be headed for positive passenger revenue per available seat mile.

"Positive RASM has been achieved," JPMorgan analyst Jamie Baker wrote Wednesday, in a report issued after Delta (DAL) - Get Report reported better-than-expected December RASM.

Baker said RASM figures provided by Airlines for America showed flat international RASM and a 0.1% domestic RASM increase. Including regional carriers, consolidated RASM rose 0.2%, "the first instance of positive RASM in 23 months," Baker said.

Baker acknowledged that a calendar shift from 2015 brought more Thanksgiving traffic into November and that December industry RASM is likely to decline about 2%.

"But given the apparent ease of monthly comps in 2017 (at least until Q4), positive 2017 RASM increasingly appears (to us) as a given, rather than a long shot," Baker said.

Favorable trends include moves to limit fare competition with ultra-low-cost carriers and "a consolidated capacity forecast inside of 3% for the first time in four years," Baker said. "We can envision upside risk to current consensus RASM forecasts, which we believe reside in the 1-2% range."

Delta said Wednesday that its December RASM was flat, "driven by strong demand trends and improving close-in domestic yields," and that fourth-quarter RASM will decline between 2.5% and 3%. Several analysts projected a similar fourth-quarter decline for the industry as a whole.

Cowen & Co. analyst Helane Becker concluded that Delta is likely to report positive first quarter RASM.

"Management's 1Q17 guidance of flat is conservative and {we} believe flat to up 1% is more likely," Becker wrote. "Previously, Delta cited a two-point headwind to December unit revenue from a Sunday Christmas."

Delta will report first-quarter results on Thursday, Jan. 12, when it is expected to provide guidance on first-quarter RASM.

Delta's traffic report triggered Wednesday gains in the shares of all airlines, led by Hawaiian, up 3%, and Delta and Southwest (LUV) - Get Report  , both up about 2.5%.

Despite the positive expectations for the industry, Becker on Wednesday downgraded six airlines to market perform, although she maintained outperforms for Delta and Southwest.

She said that after gains in the second half of 2016, airline stocks will likely "take a breather" before regaining momentum. "We believe investors will look to rising costs in 2Q17 once unit revenue is on a positive path," she wrote.

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.