The OED has “lightning” as a verb:
1903 Westm. Gaz. 16 Nov. 8/2 The two metal balls..thundered and lightninged as they delivered the message.
1926 H. CAINE in Strand Mag. Jan. 20/1 Mr. Gladstone leapt to his feet, whereupon the air of the House thundered and lightninged for a short ten minutes.
1935 in Z. N. Hurston Mules & Men (1970) I. i. 27 You know, when it lightnings, de angels is peepin’ in de lookin’ glass.
So the answer clearly is, “Look, it’s lightninging.”
Or just “Look, it’s lightening” – “lighten” meaning “to flash lightning”.