There is no way for meteorologists to know exactly where along a storm's path that the rain will fall. That storm's path as it moves from west to east could be hundreds of miles long and hundreds of miles wide to the north and south. That's why your local forecast will say something like 50% chance of rain on Thursday. Meteorologists can estimate where the rain will most likely fall, but they cannot pinpoint exactly when or where it will fall.

The rain from a storm over a region 50 or 100 miles across can generally run off to a variety of different rivers. Sure, all those rivers are likely to find their way to the same waterways somewhere down stream, but it will take the water from different areas different amounts of time to get there depending on the geometry of the rivers. Just like when you drive from home to work, it'll take you a different amount of time depending on which route you take, the runoff from a storm will take a different amount of time to reach a given point down stream. That means that the runoff from a large storm will not all generally reach the same down stream point at the same and those rivers will not be so likely to reach flood stage.

Yes, rain falling miles and miles away upstream will certainly make its way towards the camp. But if that rain falls miles and miles away downstream, then the flooding will be downriver from the camp. If the rain falls earlier than expected during the storm, then it will fall farther west and will runoff to the south of the camp as well. Same if the rain falls later and comes down to the east of the camp. But, again, meteorologists have no way to precisely predict either the path of a storm, nor where along that path that the precipitation will fall. They can give odds, but that's it.

This case was a worst case scenario. A very large amount of rain fell in a very short amount of time along a relatively narrow path. That means a huge amount of water fell over a small area that all drained into a pair of rivers that both ran into the Guadeloupe river at the same point. This raised the river almost 10 meters in 45 minutes. Forecasters could not possibly have predicted that this weather system would dump all that water in such a short time over such a small area.

They did accurately predict that there would likely be flooding and they issued alerts for large areas where it was likely to occur. As the event proceeded and the odds narrowed, increasing the chances of flooding in particular areas they issues further alerts warning communities of the increasing danger in those areas. Sadly, there were no sirens or other mechanisms in place to make sure that the people living in those areas would get those alerts in the middle of the night.