First of all, Obama already has pledged federal aid. National Guard is already operating in the affected areas, too.
Obama pledges federal help for Midwest flooding | FOX2now.com
Second, floods are pretty routine in the Midwest. While this flood is devastating for those families, the reality is that these floods are not extraordinary, and are
nowhere near the same level of devastation as Katrina. This is a flood affecting a relatively small section of mostly rural areas; Katrina was a hurricane that devastated several states and flooded several major metropolitan areas.
Unless St. Louis gets completely inundated, there is no reason for Obama to get personally involved.
Third, Obama already showed his abilities and attentiveness with Sandy. The federal government and Obama personally were right there, working with locals (including a Republican governor), giving them what they needed, and getting the job done. That's also what we are seeing here, even if we don't have the now-iconic flyover.
President Bush got pilloried, because the federal government completely and utterly screwed up with Katrina. They were caught flat-footed, and did not prepare sufficiently in advance; FEMA was run by an incompetent crony; they did not step in when the local government asked for aid. (We should note that local officials were hardly exculpated for their own failures and incompetence.)
Bush 43's flyover was not the fundamental issue. Rather, it was
emblematic of the profound ineptitude, and lack of concern, shown by his administration for the citizens affected by Katrina.
Or, to put it another way: Obama and his administration is more prepared and effective at disaster management while he's on vacation, than Bush 43 was when he was actively on the job.