piperboy84 A quick Gramet check before departure would have given him some ideas, but without a PIREP it would remain the computer's best guess.
Total speculation, but it could be he was slow in noticing ice accretion (at night with AP engaged he'd have fewer visual clues and not feel anything untoward), and couldn't get down quickly enough to warmer climes. Not much fun, but if he'd been able to maintain 1500' over the water any ice would melt and there are no mountains to hit... Not discussing the legalities, just the practicalities.