In what month and year did Welch's IEEE article first describe LZW?

Welch's IEEE article first described LZW in June 1984.

The article was Terry Welch's “A Technique for High-Performance Data Compression,” published in the IEEE magazine Computer. It presented a fast, practical variation of the LZ78 compression approach developed by Abraham Lempel and Jacob Ziv.

Welch had filed a patent application for the method in 1983, but the 1984 article was the first public description of the technique. The name LZW combines the surnames of Lempel, Ziv, and Welch.

LZW later became especially famous because CompuServe selected it for GIF, introduced in 1987. A common mix-up is confusing the article's publication date with the 1983 patent filing or the 1985 U.S. patent grant. LZW's later association with GIF also led to a major software-patent controversy during the 1990s.