UPDATE February 22, 2015: refound on Feb 21 and much better documentation secured - CLICK HERE to see the photos from this date.
This adult large gull was photographed at Brownsville Landfill, Cameron county, Texas on February 07, 2015, and has many features of Vega Gull (larus [vegae] argentatus). NOTE: these images were obtained by 'phone-scoping at a distance with some heat haze at play... The following description is based on field study via telescope:
The mantle tone was judged to be very slightly paler than that of nearby Laughing Gulls.
The legs looked fairly bright pinkish - as bright or brighter than nearby Herring Gulls - and the eye appeared to be dusky but not dark; the orbital ring color could not be discerned.
P8 was the longest primary on both wings, with an outer primary c.2/3rd grown; I think that this is P9 with P10 not yet visible, but it could be that this is P10 and P9 is not quite full and thus is sitting below P8.
White tertial crescent
fairly broad, but a couple of nearby adult Herring Gulls matched it. Scapular crescent thin. On brief glimpses of the secondaries, there seemed to be a broad white trailing edge.
Retained winter head markings fairly extensive, and slightly less cloudy (more defined) than on the few adult Herring Gulls in view.
Bill longish and evenly-shaped, being pale yellow with blackish subterminal smudge on maxilla and mandible.
My impression from the bill pattern, narrow scapular crescent, and apparent lack of an obvious mirror on P9 is that this may be a 4th/5th-cycle