Roger Pielke Jr. has a post noting the scientific consensus that "it would be absurd to attribute the Katrina disaster to global warming."
As the comments note, it might be less absurd "to attribute a portion of Katrina's strength" to warming. The best way to handle this though, is to say that it's absurd for anyone to DENY global warming connection's to Katrina, or any specific storm event. In fact, it slightly more absurd to deny the connection than it is to affirm the connection, since the overall effect is more likely to make tropical storm events worse.
Possibly the "least spin" in a short statement is this:
"We don't know enough to make a definitive statement about climate change's effect on any single storm, but it is more likely than not that global warming made tropical storm ___ worse than it would otherwise be."
Or how about this:
"Global warming's effect on tropical storm ___ is like a weighting a pair of dice so that you're more likely to roll high numbers. If you roll high numbers with weighted dice, you don't know if the weighting is the reason, but the weighting made the high roll more likely than it would otherwise have been. Same thing with the severity of ___."
I'll defend either of these as more scientifically accurate than a statement denying that we have evidence connecting Katrina or other tropical storms to global warming.