Besides weather, altitude, experience of shooter and defender, cohesion, plane durability and armor and the guns themselves, are there other factors that effect the flak vs. aircraft resolution? The Catalina is so slow(and big) that it should be pretty easy target for the gunners, at least compared to, say, D3As or B5Ns. And the Japanese H6K/H8K even more so to Allies radar-guided AAA later in the war.
Have myself seen Catalinas attack naval units only twice, they sank an xAK (6 sorties combined) without a loss. Escorts were just PBs and a couple DDs though.