MD-11
Flight Crew Last 90 Days 75; Flight Crew Total 4000; Flight Crew Type 2200
Distraction; Human-Machine Interface
Aircraft Equipment Problem Critical; Inflight Event / Encounter Weather / Turbulence
Flight Crew Overcame Equipment Problem; General Declared Emergency
Aircraft; Weather
On arrival Approach descended us from 11;000 FT to 3;000 FT on a vector to the ILS XXR. We were mostly IMC in the descent; with ground contact starting at approximately 10;000 MSL. The aircraft was in a clean configuration with all engine and wing anti-ice on. Our vector took us around a single cell that was to the northeast or our position [during which] we encountered heavy rain and a possible lightning strike at approximately 9;000 MSL. Numerous EAD alerts including loss of all FMS data followed. We declared an emergency with ATC and after reporting souls on board and fuel remaining; requested extended vectors at 5;000 MSL to allow time to analyze all alerts/FMS issues. We asked a jumpseating pilot to come to the flight deck to assist as an extra observer and traffic monitor. Our weights were reinitialized; approach reloaded; and speeds cross checked with the QRH for last known landing weight. After running all applicable checklists; and checking the current ATIS; we requested vectors back to the ILS XXR where a normal; uneventful landing was made. Fire and rescue followed us to parking with no further action required.
[Narrative added no additional information].
An MD-11; being vectored around weather by Approach Control; encountered heavy rain and suffered a possible lightning strike which caused the loss of FMS data.  They declared an emergency; requested delay vectors while evaluating and re-establishing system functionality; and landed safely.