B737 Captain reports lightning strike on downwind after go around. Aircraft was dispatched with radar deferred. Emergency is declared and remainder of approach is uneventful.

Narrative

While operating a B737; with a radar on MEL; to ZZZ; We received a possible lightning strike to the left side of the aircraft. After a go-around from runway due to an aircraft on the runway; we were again being vectored for left downwind at 7;000 FT for Runway 25R when the strike occurred. The forecast and ATIS for ZZZ showed no convective activity in the ZZZ area. We also requested and received a report from Dispatch prior to approach to ZZZ that stated an area of rain shower activity mainly north of ZZZ with maximum tops of FL200. The last ATIS received showed: 'ZZZ XA56Z; 170 DEGREES AT 18 KTS GUSTING TO 29 KTS; 8 SM -RA FEW030 BKN065 OVC080 12/05 A2938 RMK VIS LWR SOUTHWEST-NORTHWEST MOUNTAINS OBSCURED SOUTHWEST-NORTHWEST.' Visual approaches in use. Landing Runways Y1; X2.' Our release stated that no significant convective activity expected. All reported and forecast conditions complied with our MEL restrictions. The first indication of any convective activity was the lightning strike to our aircraft which was a flash and a noticeable boom on the left side of the aircraft. We had just passed through the area a few minutes prior during our first approach and were in line with other aircraft on the arrival. After the strike; we declared an emergency with ATC because we were operating in IMC and were not sure if we had any damage. The aircraft operated normally throughout the remainder of the flight and taxi in. Had we had an operable radar or ATC informed us of the building activity; we could have avoided the area. If we had known that convective activity could have been in the area; we would have diverted to another airport because of the MEL.