RORY: But we want to be spontaneous. Jump a train to Paris, head off to Spain.
LORELAI: Oh no, it’s raining in Spain. But since the rain in Spain stays mainly in the plain . . .
Lorelai quotes from the song, “The Rain in Spain”, from the musical film, My Fair Lady, previously discussed.
The line is used in the film purely to teach elocution, and is not geographically accurate. The plains in Spain are parched and arid, and most of the country’s rainfall is in the north.
The Annotated Gilmore Girls 