A heavy jet on the SFO FMS Bridge Visual encountered severe turbulence; a 30 KT speed decrease and a 2;000 FT altitude loss passing the ARCHI waypoint at 8;000 FT.

Narrative

We were on the FMS bridge visual to 28R in SFO. We had just passed ARCHI at 8;000 when we were cleared for the approach. We were assigned speed 210. I had the aircraft configured flaps 5. As we were high; I elected to FLCH to try to catch the path. Approaching TRDOW I witnessed a lightning strike about a mile ahead. Weather conditions were VFR below a cloud base of about 10;000 FT. I could see the airport and the radar only indicated light rain. All of a sudden we lost 30 KTS of airspeed (180) and the VSI went 4;000 FPM down. I was hand flying so the aircraft became very heavy (out of trim) and the autothrottles responded with max power. Then the aircraft accelerated to 250 KTS while the throttles retarded to idle. My First Officer retracted the flaps quickly to prevent an over speed. We lost about 2;000 FT almost instantly (which put me back on profile) we reported the windshear to ATC and continued the approach to an uneventful landing. This was the worst shear I have ever experienced in 22 years (70 KTS) and in SFO to boot. Thank God for a light aircraft with surplus power.