A storm which grounds all flights on Monday allows you to reschedule at no charge. Bigger storms might clobber multiple days of flights, and with all those people rebooking, the next available flight (unused seat) might be Friday.

Return ticket on a different airline is Friday. Because the storm did not interfere with Friday's schedule, airline is not offering a no-charge change but will gladly (gladdly) keep $100 to rebook (which is one of the reasons the airline industry has made record profits last year. That and charging you for everything except the seat belt (and won't let you bring your own seat belt.)

If both trips are on the same airline, you can still get a 5 day vacation and not pay a dime extra.

My solution is to have travel insurance on the 2nd airline, with the logic that if I'm not in Jamaica, I couldn't possibly make the flight.
But there is a discussion on the yellow board about an Insurance company which denied coverage because of a passengers pre-existing medical condition and the guy is now stuck with an $80K bill.

Thanks Lola. Sorry I missed your question. I would have emailed you the above but your profile has no address.