Something changed in either the machine or the plastic because of the increased humidity. Apparently I need to run the plastic hotter than before. The filament got jammed in the print head so the hobbed gear was tearing up the filament that couldn’t go anywhere. It heated up to the point of melting. Here’s a pic of melted plastic along the Makerbot drive gear.
After disassembly for ease of cleaning, I popped the piece of plastic off and used a toothbrush to clean the drive gear. Hopefully that’s all it’ll take.
(later…) No, that wasn’t all it took. The print head was really jammed and had to be disassembled and cleaned. Now it runs well.