Just heard from the AC repair guy - looks like the water in the ground floor ducts blocked the airflow, and the dirty filter (that I cleaned yesterday in an attempt to help the situation) contributed to a hard freeze of the entire AC system, from the inside coils to the outside compressor unit. Have to leave the fan running and the AC compressor OFF for 3-4 hours to let it melt out. We did discover that the water level dropped enough to get half the first floor air moving again, but I need to do some intensive shop-vac work tonight.