BUF CTLR ALLOWS B738 TO ENTER RESTR AIRSPACE WITH COORD INCURRING A CONFLICT WITH ANOTHER ACFT OPERATING IN THE RESTR AIRSPACE.

Narrative

THERE WERE NUMEROUS SCATTERED TSTMS AND TWRING CUMULUS IN ALL QUADRANTS OF MY AIRSPACE. THE ASR-9; 6 CHANNEL WX WAS OPERATING. HOWEVER; PLTS WERE REQUESTING TO DEVIATE FROM THEIR ISSUED HDGS IN AREAS THAT DID NOT SHOW ANY POSSIBLE PRECIP ON THE ATC RADAR. THE CTLR WORKLOAD CAN INCREASE RAPIDLY WITH ONLY A MODERATE NUMBER OF ACFT IN THIS SIT. WHEN THE PLT OF THE ARRIVING B738 REQUESTED A COURSE DEV TO AVOID SEVERE WX; I ASSUMED THAT HE WOULD NOT TURN TOWARD AN AREA OF LEVEL 5 WX THAT WAS DISPLAYED ON MY RADAR SCOPE. I ISSUED A CLRNC TO MAINTAIN 3000 FT BASED UPON THAT ASSUMPTION. COORD FOR OTHER ACFT DEVIATING WITHIN MY AIRSPACE PREVENTED ME FROM NOTICING THAT THE B738 WAS ENTERING A RESTR AREA AND CAME IN CLOSE PROX TO A VFR ACFT IN THAT AREA. SINCE THE ASR-9 CAN ONLY DISPLAY 2 OF THE 6 LEVELS OF PRECIP AT ONE TIME AND ONLY SHOWS ACTUAL PRECIP; IT IS IMPOSSIBLE TO DETERMINE WHICH TWRING CUMULUS CLOUDS WILL CAUSE A PLT TO DEVIATE FROM COURSE. I LEARNED THAT CTLRS SHOULD ESTABLISH ALT SEPARATION FROM RESTR AREAS AND OTHER ACFT AND MAINTAIN THAT SEPARATION WHILE ALLOWING PLTS TO DEVIATE AROUND VISUALLY OBSERVED WX PHENOMENA.