Rain does affect how easy it is to start fires, and how easy they are to put out. Rain by itself is not going to extinguish that blaze, however.
Places gone to ash are probably more vulnerable to fires than normal forests. Its not like the fire burned up all the existing fuel, and now you've got even less moisture to prevent a fire from springing up.
Ever been in a forest fire? They spread rapidly as all hell.
I can think of several good things that forest fires do, not the least of which is make it more difficult for Sylvans to get the jump on you.
