The alert covers a wide stretch of the South Plains, not just Lubbock proper. Riley's Tuesday evening update flagged active alerts already in place, including areal warnings tied to the flooding risk. Conditions could shift overnight and again into Wednesday, so the threat isn't limited to a single day.
Isolated severe storms can spin up fast with little warning, and flash flooding turns low-lying roads dangerous in a hurry, especially overnight when visibility is already bad. Anyone with plans that involve driving after dark this week should keep an eye on conditions before heading out.
Covering that much ground for two full days is unusual, and it says something about how unsettled the pattern is heading into the last days of September.