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Well, I guess the hydrologists in Houston are a lot dumber than the ones around here (/s), because NWS / NOAA gives us a pretty good idea of what areas are at risk when heavy precip. systems are on the way. No, it’s not perfect when intense rain happens in very localized areas (which was not really the case in the Houston area*), but that’s not the point. There’s never a perfect solution: What CAN be done is try to get, say, half the population in moderate risk areas, out, and hopefully more out of high risk areas, using the best estimates of areas at various risk levels, as well as the best estimates of how many people will leave / not leave. Maybe shoot for a movement of 25% of the total population - that number different for any given county and forecast. Relocate as many people locally, as possible, as routes well away are limited. Start early, with highest risk areas.

The looming problem can only be reduced. OTOH, and this is key, decision makers do not need “certainty” of severe flooding to urge residents to leave. “Is there a 40% chance this neighborhood will have 5-15 ft. flooding?” That question can be answered reasonably well, with systems like this (Harvey), and IMO, if the answer is “yes”, the above evac steps should be taken. One must keep in mind Houston’s severe weather / flooding history, AND that the forecast might be “off” in the low direction, too.

*I’d point out that the Houston TV forecasters I’ve heard the last couple days have commented on how consistent the rainfall distribution was. And, on how wrong they were on not trusting forecast guidance that (contrary to your assertions) was mostly in fairly close agreement. The exceptions are “cherry picking”.

There’s never a perfect solution: What CAN be done is try to get, say, half the population in moderate risk areas, out, and hopefully more out of high risk areas, using the best estimates of areas at various risk levels, as well as the best estimates of how many people will leave / not leave. Maybe shoot for a movement of 25% of the total population - that number different for any given county and forecast. Relocate as many people locally, as possible, as routes well away are limited. Start early, with highest risk areas.

I agree.

There is no excuse for not evacuating the most flood-prone areas in advance of this storm.

"What CAN be done is try to get, say, half the population in moderate risk areas, out, and hopefully more out of high risk areas, using the best estimates of areas at various risk levels, as well as the best estimates of how many people will leave / not leave" Some unknowns there within your reasonable suggestion. But here's where the unknowns come in: if you have a cat-4 on the way and goes to the right, then you have a Rita. You will need to take into account the Rita-effect in your estimate of how many people will leave. That's going to be hard to quantify.

One thing you left out is the gradations of evacuation. Voluntary and mandatory seem to be the two grades. I think a third one in between, "urgent" would have been appropriate in Harvey, based only on models which can be wrong even with what they showed on Thursday.