We were in the Grand Jan 2010 during what was a "moderate" (though the strongest since '98 ) El Nino pattern. Oscillated from warm temps (shorts after Marble) to 6 days of straight rain starting around Bass Camp; I believe that storm series was also responsible for the massive blow outs on the Salt River. 6 days of rain was challenging and even the classic camps like Ponchos became eroded from massive rivulets and were dripping bad. To be honest the worst part was actually the uncontrollable element of major rock fall, especially in the Mauv Gorge. Matkat Hotel proved to be an intense and frightening camp.
We had a great trip but appreciated multiple group tarps and even a wood stove. Tents need to be solid and dry suits work double duty in such conditions.
Best of luck; I will never forget not seeing another group for 20+ days.