There’s an airline pilot forum linked off another FR thread on AF447 today...on there, pilots were saying that the particular type of thunderstorm spawned in the area where AF447 was flying is very hard to spot on radar for some reason, much harder than the storms we get in the US. Something about if you’re pointing the radar straight ahead, you might not get a return, but if you tilt it down toward the bottom of the clouds, they show up better.
}:-)4
According to this discussion the plane did not avoid the storms, but in fact flew through a storm cell. In his discussion on turbulence, the author states, "Young updrafts are particularly dangerous to flights because they contain significant rising motion yet precipitation fields have not yet fully developed and airborne radar signatures are weak, reducing the likelihood the crew will deviate around the cell."