PWAT — Precipitable Water

PWAT is the integrated water vapour in a column, expressed as the equivalent depth of liquid water (mm or inches). UK climatological norms are roughly 15–25 mm in summer; >35 mm is anomalous and often shows up on the highest-impact flash-flood days.

PWAT doesn't drive storm intensity — CAPE and shear do that. It drives rainfall totals per storm, especially when combined with slow storm motion (weak mid-level flow) or training cells.