Green’s Dictionary of Slang

(N.Z./US) an excl. to a woman that her slip or petticoat is showing.

1934		Daily News (NY) 10 Aug. 40/1: When her petticoat is hanging down, the proper phrase is, ‘It’s snowing down South’.
c.1935		D. Hurley Taranaki schools, postcard in DNZE (1998) 754/1: It’s snowing down south.
c.1940		J. Watson Mangapai School (Whangarei) Northland, postcard in DNZE (1998) 754/1: It’s snowing down south.
1955		Abilene-Reporter-News (TX) 22 Aug. 14/4: The supposedly clever ‘it’s snowing down south’ for a slip showing [...] is about as subtle as an auto crash.
1961		Miami News (FL) 2 Niv. 47/1: She wears [...] a rumpled yellow skirt and her slip is showing. (‘Snowing down South,’ we used to say).
1971		Sydney Morn. Herald 1 Sept. 12/6: OOOPPS! It’s snowing down below again this spring [...] any girl who feels liberated enough to wear her skirts on the thigh should have didspensed with [...] petticoats.
1983		Observer (London) 3 July 31/8: A girl whose slip was showing might well be charmed [...] to be told ‘it’s snowing down South’.
1995		Arizona Dly Star (Tucson, AZ) 4 Oct. 10/5: You said in as offhand a manner as you could muster, ‘It’s snowing down south’.
2005		Chicago Trib. 4 Aug. 5A-4/1: When was the last time you heard [...] the Catholic grade school special ‘It’s snowing down south’.