There isn't any way (that I'm aware of) to generate an alert based on rate of change. The best way I've found to handle this is to have a "warning" alert and then a "critical" alert at a higher level, although this still wouldn't achieve your timescale requirement
This would certainly make a great feature request.
might something like this work?
custom pollers for the statistic as a rate
transform poller to convert it into a percentage;
advanced alert to work off the transformed value
The lowest poll rate is every 10 hours, so at polling every 6 hours this is (very approx) 2.4% (think compound interest)