My 1997 850GLT occasionally flashes a warning light for a
fraction of a second, usually when I'm accelerating.  I think
it's the alternator light, and I'm assuming the belt is slipping
a bit.  But if your Check Engine light is coming on, I doubt
that it's a belt, since that light is usually emissions-related.
Of course, getting wet could encourage a belt to slip.

Last question first: should you be concerned? No. The "check engine" light
is there to announce something wrong that affects emission control (although
several mfr's have piggybacked things like automatic transmission controller
codes onto it). As long as the engine seems to run okay, it is safe to drive
normally even with the check engine light on solidly. A few failures can
cause the engine to run so rich that the catalytic converter can be damaged
in long high speed hill climbs, but that's about the limit of it.

We can only guess why the light is momentarily coming on, but whatever it is
only annoys you at this point. My guess is it will eventually get worse so
the light stays on more and more, and then you can get it checked out. It
will be easier (cheaper) to repair when the indications stay there.

Mike