DSCNT AIRSPD IN A DC-8 EXCEEDED 250 KIAS BELOW 10000 FT. FLC WAS DEVIATING AROUND TSTMS.

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CHAIN OF EVENTS: FLT XXX WAS LAUNCHED OUT OF SDF AT XA18 LCL TO ORBIT NEAR MSP AS AN AIRBORNE SPARE TO COVER LAUNCH FAILURES IN THE N HALF OF COMPANY'S SVC AREA. AFTER APPROX 3 HRS AIRBORNE AND UPON RETURN TO LAND AT SDF; COMPANY TOLD US TO GO TO ORD. LANDED ORD; FUELED; AND RETURNED TOWARD SDF. DSNDING FROM FL270 AND PASSING FL200; SEVERE WX APPEARED ON THE RADAR SCREEN IN ALL QUADRANTS WITH NO WAY AROUND OR OUT OF THE WX. THE CAPT CHOSE THE PATH OF LEAST RESISTANCE AND WAS CLRED TO DEVIATE AS NECESSARY BY ATC. THE TURB WAS MODERATE TO SEVERE DOWN TO AROUND 5000 FT MSL. FLYING THE ACFT AT TURB PENETRATION SPD (280 KTS) AND STAYING ON HDG WAS A CHALLENGE TO SAY THE LEAST. DURING MOST OF THE DSCNT; I COULD ONLY SEE A BLUR FOR THE INSTS; SO I FIXATED ON ATTITUDE; AIRSPD; AND HDG. WHEN WE CAME OUT OF THE BOTTOM OF THE WX; THE CAPT SAID 'CHK YOUR AIRSPD.' I WAS STILL AT 280 KTS; PASSING 4000 FT MSL. I SLOWED TO 245 KTS BY AROUND 3600 FT MSL. THE REST OF THE FLT WAS UNEVENTFUL WITH A NORMAL LNDG AT SDF. CONTRIBUTING FACTORS: EVEN THOUGH MY DUTY TIME WAS APPROX 7 HRS AND FLT TIME WAS APPROX 5 HRS; I HAD BEEN AWAKE FOR 17 HRS AT THE TIME OF OCCURRENCE. I FEEL THAT FATIGUE PLAYED A LARGE PART IN THIS INCIDENT. SEVERE WX ALSO ADDED STRESS TO THE SIT. CORRECTIVE ACTION: SINCE ALL 3 CREW MEMBERS MISSED THE AIRSPD AT 10000 FT MSL AND THE WX DID NOT SHOW UP UNTIL WE WERE ON TOP OF OR ABOUT TO ENTER IT; I AM NOT REALLY SURE HOW IT COULD BE PREVENTED AGAIN EXCEPT FOR A BETTER SCAN BY ALL CREW MEMBERS AND MORE ATTN TO THE BIG PICTURE OF THE SIT BY THE PF -- ME.