FLC OF LGT CARGO FLT ENCOUNTERED TURB; LIGHTNING WIND SHEAR SHORTLY AFTER TKOF AT MEM.

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WX WAS MARGINAL WITH HIGH WINDS AND TSTMS IN THE AREA. HOWEVER; WHEN WE TOOK THE RWY (18L); A CHK OF OUR ONBOARD RADAR (30 MI SCALE) SHOWED A CLR SCREEN IN THE DIRECTION OF INTENDED TKOF. IN CLRING US FOR TKOF THE TWR MADE NO MENTION OF ANY SEVERE WX. IMMEDIATELY AFTER ROTATION THE ACFT BEGAN TO ENCOUNTER SEVERE BUFFETING AND WAS DIFFICULT TO CTL. WIND SHEAR WAS PRESENT AND WAS ALSO RPTED BY A PAX ACFT ON APCH TO 18L. AS WE ENTERED THE CLOUDS WE WERE GIVEN A L TURN TO 130 DEGS AND THE TURB INTENSIFIED. LIGHTNING FLASHED REPEATEDLY AND THE ACFT WAS BOUNCED ABOUT. AS WE WERE LEVELING AT 3000 FT MSL IT WAS DIFFICULT TO MAINTAIN ALT. A SEVERE POCKET OF TURB BOUNCED THE ACFT TO 3500 FT; AND OTHER JOLTS CAUSED ALT CHANGES OF +/-200-300 FT. OUR ONBOARD WX RADAR PAINTED NO WX UNTIL WE WERE CLRED TO CLB OUT OF 3000 FT. IT THEN; ALL OF A SUDDEN; SHOWED A BRIGHT YELLOW AREA SURROUNDING 3 BRIGHT RED AREAS. MEMPHIS TWR AND DEP CTL MADE NO MENTION OF THIS WX. AFTER WE WERE ALREADY IN IT AT 3000 FT; DEP ASKED US HOW WE WANTED TO NAV OUT OF THE AREA. WE ASKED FOR SUGGESTIONS; AND DEP PROVIDED US WITH SOME VECTORS. THE SITUATION WAS EXACERBATED BY THE INTERMITTENT NATURE OF OUR WX RADAR (WHICH WORKED FINE FOR THE REMAINDER OF THE TRIP). THIS SITUATION COULD HAVE RESULTED IN A DISASTER FOR 2 REASONS: 1) THE MEM TCA WAS VERY CROWDED AND ACCURATE ALT AND HDG CTL WERE IMPOSSIBLE; AND 2) THE WX MIGHT HAVE BEEN TOO SEVERE FOR A TKOF AT MAX GROSS WT (WE WERE VIRTUALLY EMPTY). THE ONLY WAY TO AVOID THIS TYPE OCCURRENCE IS TO PROVIDE BETTER DETECTION EQUIP TO CTLRS!