A300
Flight Crew Air Transport Pilot (ATP)
Aircraft Equipment Problem Less Severe; Deviation - Altitude Excursion From Assigned Altitude; Deviation - Speed All Types; Inflight Event / Encounter Weather / Turbulence
General None Reported / Taken
Aircraft
Cruising at FL350  night/IMC the flight had encountered and deviated around several areas of convective activity.  The wx radar seemed to be working well.  The gain was in auto and I had adjusted the display brightness to a comfortable level; cockpit flood lights were off at this time  but had been on earlier in the flight.  Five to ten minutes prior to the turbulence we had encountered IMC with a great deal of St. Elmo's fire; but a mostly smooth ride.  As I worked the radar in range and elevation I did notice an area of precip off the nose which barely showed up as green with the radar tilt level.  I discussed this with the pilot monitoring and we did not see a need to alter our course.  At about 15 nm from the return I selected 20 nm scale on the radar and did see a stronger return with a little yellow precip in the middle.  I decided to deviate to the right of course.  The PM coordinated with center; I started the turn and we experienced moderate approaching severe turbulence; very heavy precip and St Elmo's fire.  It lasted for about 30 seconds.  The aircraft gained about 250 ft and airspeed plus and minus 10 kts.  This did not correlate at all what we had seen on the radar previously.  We reported the event to ATC.  There was no other traffic near and they did not seem to have an issue with the altitude.Cause: The radar return prior to the event did not match severity of the turbulence and precip.  This did not show on the radar as a strong return.  I interpreted it as a weak return.  I do not have a good explanation for this.  The radar was in auto gain during this encounter.  I question if the St Elmo's fire prior to this had an affect on the radar.  Next time night IMC I will deviate around any return.
A300 Captain reported flying into convective area with moderate to severe turbulence that was not painted by his weather radar.