A door seal that drops down just as the door closes and butts against the threshold.
Home Supply place, I'm pretty sure. They make a door seal that has a drop down center rubber. It rides in a track which forces it down against the threshold on angled inner surfaces. When you open the door this spring loaded strip moves back toward the jam and up into the track. When the door is open the very end stick out about a half inch toward the jamb on which the door is hinged. When you close the door, at the very last minute, the protruding end engages the jamb, pushing the strip with the rubber seal, in and down against the threshold. Pretty simple device really and not rocket science, ie. costly at all.