CLE TRACON CTLR EXPERIENCED OPERROR AT 5000 FT WITH DEP AND OVERFLT TFC DURING BUSY AND COMPLEX TFC PERIOD.

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ON THE EVENING OF MAR/TUE/05; THINGS WERE A LITTLE HECTIC IN THE CLE TRACON ALL NIGHT. THROUGHOUT THE EVENING; THE WX WAS BAD. THIS INCLUDED SNOW SHOWERS THROUGHOUT THE NIGHT (HVY AT TIMES); ICING CONDITIONS; STRONG WINDS; AND POOR RWY CONDITIONS. WE HAD A REDUCED ARR RATE DUE TO THE WX; AND WERE HOLDING AT OUR ARR FIXES ALL NIGHT. CTR WAS ALWAYS CALLING TRYING TO GET US (OUR 'TMU') TO TAKE MORE AIRPLANES. JUST PRIOR TO THIS 'EVENT;' OUR RVR FOR RWY 24R DROPPED BELOW 2400 FT. BRAKING ON THAT RWY WAS POOR. ALL OTHER RWYS WERE CLOSED AT THE TIME (RWY 24L MIGHT HAVE OPENED AT SOME POINT -- NOT SURE) AND THEN RWY 24R WAS CLOSED). ARR HAD A LOT OF AIRPLANES THAT COULD NOT LAND AND THEY WERE HOLDING AT ALL ALTS IN THEIR AIRSPACE. I WAS WORKING DEPS COMBINED. JUST PRIOR TO THE EVENT; ARR COORDINATOR APPROVED 2 OVERFLTS AT 5000 FT (W OF THE ARPT) FLYING N-S. THIS WAS A VERY UNUSUAL SIT BECAUSE 1) OVERFLTS ARE NORMALLY (PER LOA) AT 6000 FT; 7000 FT; 11000 FT OR 12000 FT; AND 2) OUR INITIAL ALT TO CLE DEPS IS 5000 FT. THE FIRST OVERFLT WENT THROUGH WITHOUT INCIDENT. RIGHT AFTER THE SECOND ONE ENTERED MY AIRSPACE; I HAD BACK-TO-BACK GARS FROM THE TWR. TMU TOLD ME TO VECTOR THE FIRST ONE TO KEEP HIM OUT OF ARR'S AIRSPACE FOR AS LONG AS POSSIBLE. THE SECOND REQUESTED AN IFR TO ORD ON INITIAL CONTACT WITH ME. AT THIS TIME; I WAS ALSO WATCHING NUMEROUS (4-6) AIRPLANES HOLDING IN MY AIRSPACE AT 7000 FT AND 8000 FT (TALKING TO ARR); WORKING A 6000N FT OVERFLT FLYING S-N JUST W OF CLE AND WORKING DEPS. AS I WAS WORKING ON THE ACFT REQUESTING TO DIVERT TO ORD; AN E145 DEPARTED CLE ON A 280 DEG HDG. I 'RADAR IDENTED' HIM BUT COULD NOT CLB HIM DUE TO THE 6000 FT OVERFLT. I DIDN'T GIVE IT A SECOND THOUGHT BECAUSE 5000 FT IS ALWAYS SAFE (EXCEPT FOR TODAY) AND I WENT BACK TO THE OTHER TASKS AT HAND; FORGETTING ABOUT MY 5000 FT OVERFLT. I HAD A LOT OF TAGS ON THE SCOPE OVERLAPPING AND JUST DID NOT CATCH IT. THE CONFLICT ALERT WENT OFF WHEN THE E145 WAS ABOUT 10-15 MI W OF CLE. AT THE SAME TIME; TMU YELLED; 'DO YOU HAVE THOSE 2 AT 5?' I INSTRUCTED THE E145 (TWICE) TO 'TURN L HDG 180 DEGS IMMEDIATELY FOR TFC' AND THEN TRUSTED THE OVERFLT TO TURN R HDG 270 DEGS IMMEDIATELY FOR TFC. BOTH ACFT COMPLIED AND NEITHER PLT SAID ANYTHING. BOTH ACFT WERE TURNED BACK ON COURSE PRETTY QUICKLY AND I CONTINUED TO WORK MY OTHER TFC. ABOUT 45 MINS LATER; I WENT ON BREAK. BEFORE GOING BACK ON POSITION; I ADVISED THE SUPVR THAT I HAD TWO GET A LITTLE CLOSE BUT I REALLY DIDN'T KNOW HOW CLOSE THEY GOT. ANOTHER CTLR THOUGHT I HAD 3 MI OR DIVERGING COURSES AT ALL TIMES. I THOUGHT I HAD LESS. I WAS TOLD NOT TO WORRY UNLESS WE GOT A PHONE CALL (WHICH NEVER CAME). IN HINDSIGHT; PRIMARY CAUSE: I FORGOT ABOUT MY OVERFLT. SECONDARY CAUSES: 1) TMU TOOK TOO MANY AIRPLANES. 2) DEP SHOULD'VE BEEN SPLIT. 3) OVERLAPPING TAGS. 4) OVERFLTS SHOULD NEVER BE APPROVED AT 5000 FT.