DrSyn
It may be the case that the ET foam is water-proof but it would only take a minor imperfection to allow water to infiltrate and affect the adhesive over 39 days of rainy freeze/thaw/re-freeze (mechanism for that described below). A large section of iced foam might detach not only because of degraded adhesive but because of the weight of water behind it. But there is a further factor - when you're talking about a cryogenic fuel-load.
"It was not a cold day and those who were around the stack prior to launch say that there was no visible ice present on the tank." Any poorly adherent foam section would look like any other but once the External Tank is fuelled with liquid hydrogen (at minus 250deg F), I understand that it is one large icicle. But you have to further consider that any water infiltrating behind the foam is not just ice, it is super-cooled and will contract quickly and mightily. That almighty contraction would suck in the surrounding foam - forming a circumferential crack (albeit one that may not be visible and may be initially only in the adhesive substrate - but that's where it counts). That crack then delineates the piece that will/could later detach. At max Q I would guess that there are areas of lower pressure around the ET (i.e. where the airflow sucks). Once thermodynamic heating reduces the adhesive quality of the ice itself behind that flawed section of increased weight foam (like hand-warming an ice-tray) that flawed section is free to detach (but it's still an icicle in stalactite form).
Even though the external tank's cladding may be tested waterproof where it's made, transportation, erection and attachment stresses on the empty vehicle may well compromise the water-proofing of the foam cladding on that flimsy, empty (and therefore flexible) tank. The solution may be to simply give it a good ScotchGuarding spray top-to-bottom once it is in the launch position.
But as further insurance, a sacrificial rubbery L.E. wedge (aka false leading-edge) on the Orbiter's wing would easily deflect any such stalactite and burn away early on re-entry. I will be surprised if they don't go for this as a fix.
OVERTALK