The story
From ARkStorm, from atmospheric river (“AR”) + thousand (“1000, k”) (1-in-1000 years) + storm. From starting as a disaster planning scenario for a once in a thousand year storm event rainstorm fed by a Pineapple Express atmospheric river hitting California and deluging the state, resulting in a seemingly biblical flood, requiring an ark, as the Central Valley turns into an inland sea.
Etymology from Wiktionary, CC BY-SA 4.0.