Thanks, Klaus. First off, I made a typo calling my 2004 XC-70 a 2014. But your analysis makes sense in that yes, it was raining hard, and soon after it first started some squealing I ran through a depression in the highway with an inch or so of water in it and it really howled (oops, not a howl but a shrill scream!). And it usually would gradually diminish in time, but sometimes I would slow down in fear, which indeed would have decreased the load of the belt on the pulley, as well as diminish the rain splash and entry through the grill.

And my attempts to stop and open the hood were futile since the squealing quit when I stopped and it was idling. I was so sure the noise was coming from the interior (the sound was so loud and so pure) that i did not even think to gun the engine to test for instant load on the belt (but it may have dried by then anyway).

My wife drove it 50 plus rain-free miles today and no squeal, so it probably is the serpentine. I do have stethoscope, which I will put to work so I can check out the bully bearings. But I think I'll order a new serpentine belt tomorrow regardless. Thanks again.