In modern English, wintry is the preferred spelling of the adjective meaning of, like, or relating to winter. Wintery has a long history in English, but it has never been the preferred form, and it has no meanings of its own. In 21st-century books, it appears once for approximately every 20 instances of wintry. It is a little more common on the web and in newswriting, though still much rarer than wintry.
There is no evidence that wintery is becoming more common, but the spelling does accurately reflect how many English speakers pronounce the word—that is, with three syllables instead of two—so it may someday gain ground. For now, wintry prevails.
His nose was thick and flat and squared off at the bottom; it flamed a bright red in wintry wind. [Washington: A Life, Ron Chernow]
Wintry weather and snowfall has caused disruption to rail and road travel in Kent and Sussex. [BBC]
[A]ll is presented from the perspective of age, indirectuly adumbrated in the wintry description of the snow-capped mountain in the beginning. [The Oxford History of Classical Reception in English Literature, vol 3]
Thirty years and 300 leagues separate me from that wintry day and place, yet how well I remember. [New York Times]
He said criticism of the surface had been unfair after he battled wintry conditions to get it ready for early season shield cricket. [Sydney Morning Herald]