A CGA LR60 CANCELS IFR ON APCH TO SYI; THEN ENCOUNTERS LIGHTNING AND HVY RAIN ON FINAL. THE PLT MADE A 180 DEG TURN AND DIVERTED TO FYM ARPT. LATER; AFTER DEPARTING FYM IFR; ATC COULD NOT APPROVE HIGHER ALT DUE TO OTHER TFC ABOVE THEM. THE RPTR IS CRITICAL OF ATC FOR NOT PROVIDING WX RPTS.

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ENRTE TEB-SYI; APCHING SYI FROM THE E; BNA 'TAF' HAD CALLED FOR POSSIBLE TSTM. (CHANCE 30%; AS I RECALL; WHICH I INTERP AS 'PROBABLY SOMEWHERE IN THE AREA; BUT PROBABLY SCATTERED.') A CLUSTER OF TSTMS HAD FORMED DURING OUR NEARLY 3 HRS ENRTE. IT EXTENDED FROM SYI; E AND NE; WITH ANOTHER GROUP ORIENTED NW-SE IN THE BNA AREA. AS WE DEVIATED S OF THESE CELLS; WE COULD SEE THAT THEY APPEARED (ON RADAR) TO BE OVERHEAD -- OR AT CLOSE RANGE IN ALL QUADRANTS -- FROM SYI; MOVING E; VERY SLOWLY. SYI AWOS WAS INOP; SAYING ONLY THAT ALL PARAMETERS (SKY CONDITION; VISIBILITY; ETC) WERE NOT AVAILABLE. WHEN WE (FINALLY) GOT AN ANSWER FROM SYI UNICOM; HE SAID THAT A LIGHT WIND FAVORED THE N RWY AND THAT IT WAS NOT RAINING AT SYI. THE CELLS ENDED; ABOUT 4 NM N AND W OF SYI; AND WE WERE OPTIMISTIC ENOUGH TO HOPE THAT THEY HAD MOVED E OF SYI (IN THE LAST TWILIGHT; IT LOOKED LIKE THAT MIGHT BE THE CASE; BUT THEY WERE STILL VERY CLOSE TO SYI). APCH GAVE US RADAR VECTORS TO A POINT ABOUT 15 NM NNW OF SYI; AND GAVE US CLRNC FOR A VISUAL APCH; WHICH WE BACKED UP BY INTERCEPTING AND FOLLOWING THE VOR DME FINAL APCH COURSE AND; ONCE ESTABLISHED INBOUND; THE PUBLISHED STEPDOWN ALTS. AT ABOUT 6-7 NM (SYI DME); IT LOOKED LIKE WE WOULD BE ABLE TO MAINTAIN GND CONTACT AND VFR VISIBILITY; WHILE DSNDING TO A VFR PATTERN ALT (ABOUT 2300 FT MSL); AND I CANCELED OUR IFR TO GIVE MY FULL ATTN TO THE UNICOM (CTAF) FREQ AND MONITOR THE PATTERN AND RUN CHKLISTS. (MY FO WAS THE PF.) ALMOST IMMEDIATELY; AT ABOUT 5 NM; I SAW LIGHTNING AND HVY RAIN AHEAD AND ORDERED A 180 DEG TURN. I CALLED APCH BACK; WAS CLRED TO 6000 FT AND BRIEFLY HIT AN UPDRAFT THAT CAUSED ME TO ASK FOR A BLOCK ALT. (WE HIT ABOUT 6700 FT; THEN DSNDED ASAP AND MAINTAINED 6000 FT -- NO TFC CONFLICT.) WE DIVERTED TO FYM; WITHOUT FURTHER INCIDENT. BEFORE DEPARTING FYM; I CALLED FSS; FILED (IFR; FYM DIRECT ABS; FL430); AND GOT A WX BRIEFING. SUPPOSEDLY; RADAR SHOWED CELLS N AND NW OF FYM. ONCE AIRBORNE; WE FOUND OURSELVES IN A LARGE 'HORSESHOE' WITH CELLS IN ALL QUADRANTS EXCEPT; ROUGHLY; 090-180 DEGS. WE COULD'VE USED THE LEAR 60'S LEGENDARY CLB RATE (6000 FPM OR MORE AT SEA LEVEL; TO OVER 1000 FPM AT FL400; ASSUMING 'ISA' AND A MID WT AIRPLANE) TO GET ABOVE THESE RELATIVELY LOW CELLS; BUT ATC COULDN'T/WOULDN'T MAKE SPACE FOR US AMONG THE (OVERHEAD) MEM ARRS. DEP/CTR ALSO COULDN'T/WOULDN'T TELL US WHERE THE 'BREAK' (SE OF US) WAS. FINALLY; AFTER BEGGING FOR AND FINALLY RECEIVING CLRNC FOR NEARLY 360 DEGS WORTH OF TURNS; WE IDENTED THE CLR WAY OUT OF THE 'HORSESHOE' WITH OUR OWN RADAR. I'VE READ THAT THE NEWER APCH CTL AND CTR RADARS HAVE 'WX-Z' AND 'WX-3' KEYS; TO ENABLE THE CTLR TO SPOT AREAS OF MODERATE-TO-HVY PRECIP. MAYBE THOSE RADARS AREN'T INSTALLED AT CHA OR MEM -- I DON'T KNOW. HOWEVER; I FLY 800 HRS PER YR ALL OVER THE CONTIGUOUS 48 STATES; AND GETTING ANY CTLRS TO USE THAT 'WX-KEY' CAPABILITY IS LIKE PULLING MOLARS! (THIS; I ASSUME; IS ON THE ADVICE OF THE CTLR'S UNION LAWYERS.) TFC SEPARATION IS THEIR PRIMARY JOB; NOT THEIR ONLY JOB; AND ALL OF THE SAME OLD STUFF ABOUT 'YOUR RADAR SEES IT BETTER THAN MINE DOES' IS CRAP. (MY RADAR SEES A 60 DEG ARC; FROM 11:00 - 1:00; WHILE THEIR RADAR COVERAGE EXTENDS ALL AROUND ME.) AS A TAXPAYER; I'M NOT GETTING MY MONEY'S WORTH FROM THE NEW RADARS.