been riding winter for a long time and this never happened before.

snowed overnight and this morning the temp was hovering around freezing. as a result slush was everywhere.

halfway to work I noticed that I was unable to shift into certain gears - more accurately, they would skip continuously. stopped at an intersection and looked down to see the cogs caked with slush. tried to dig them out a bit with my gloved hands, and it helped a little bit. but before long it was back to mush.

any suggestions on how to prevent (other than riding singlespeed/IGH)? I guess I could just shift between gears occasionally to keep them from caking up