PLT OF SMA DEPARTING CPR DEVIATED FROM CLRNC AND HDG. DEP CTLR INTERVENED AND RERTED PLT AROUND HVY WX.

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ON THE GND AT CASPER ARPT AT CASPER; WY; I OBTAINED A WX BRIEFING AND FILED AN IFR FLT PLAN FROM CASPER ARPT (CPR) DIRECT TO CENTENNIAL ARPT; DENVER; CO (APA). NO SIGNIFICANT WX WAS EXPECTED ALONG MY PLANNED RTE EXCEPT SOME LEVEL 1 RAIN WELL S OF CASPER. I RECEIVED AN IFR CLRNC FROM CASPER TWR (CLRED TO APA VIA CASPER 1 DEP THEN AS FILED; CLB AND MAINTAIN 11000 FT; DEP FREQ 120.65; SQUAWK 5122). MINS AFTER BECOMING AIRBORNE I BEGAN TO MONITOR AN AREA OF BUILDING CUMULUS DIRECTLY IN MY FLT PATH THAT I ESTIMATED TO BE ABOUT 15 TO 20 MI AWAY. FROM LIVE RADAR THAT I SAW AT THE WX BRIEFING WITHIN THE LAST HR; I WAS SURPRISED TO SEE SUCH ACTIVITY SO NEAR CASPER. IN ADDITION; BECAUSE OF RISING TERRAIN S AND SW OF CASPER; I WAS ALSO MONITORING MY PROX TO THE GND. DURING THIS PERIOD WHEN I WAS MONITORING THE BUILDING CUMULUS; MY GND CLRNC; AND WATCHING FOR TFC I DEVIATED FROM THE RWY HDG OF 210 DEG (PART OF THE CASPER 1 DEP) BY PERHAPS AS MUCH AS 10 TO 15 DEG TO THE L. CASPER TWR MADE NO COMMENT ABOUT THIS DEV; HOWEVER; UPON SWITCHING THE CASPER DEP (APPROX 3 TO 4 MINS AFTER DEP); I WAS IMMEDIATELY AND CURTLY INFORMED THAT MY CLRNC WAS TO FLY RWY HDG AND I WAS GIVEN A HDG OF 260 DEG TO FLY. WITHIN ANOTHER 1 MIN I WAS ASKED IF I COULD VISUALLY MAINTAIN GND CLRNC; I RESPONDED YES AND WAS GIVEN A HDG OF 180 DEG TO FLY. I HAD BEEN IN CLR CONDITIONS SINCE TKOF AND WOULD REMAIN SO FOR ANOTHER SEVERAL MINS; HOWEVER; THE 180 DEG HDG WOULD TAKE ME DIRECTLY INTO A BUILDING CUMULUS WITH AN ESTIMATED TOP OF 25000 FT. AFTER ANOTHER FEW MINS; I LEVELED AT 11000 FT WITH CRUISE CONFIGN AND ASKED TO DEVIATE TO AVOID THE BUILDUP. AN EXASPERATED SOUNDING CTLR TOLD ME I COULD NOT DEVIATE W BUT DID ULTIMATELY GIVE ME A 100 DEG HDG. AFTER A FEW MINS THE CTLR TOLD ME THAT IF I COULD NOT FLY IN IMC HE WOULD HAVE TO RERTE ME TO VICTOR AIRWAYS MANY MI TO THE E OF MY CURRENT RTING BECAUSE HE SAID HE WOULD LOSE RADAR CONTACT OF ME. AT ABOUT THE SAME TIME I ACCEPTED THE NEW RTING; I WENT INTO IMC WITH LIGHT TO OCCASIONALLY MODERATE TURB. AFTER TRANSFERRING TO DENVER CENTER (135.6) A FEW MINS LATER; I WAS GIVEN A 110 DEG HDG AND 12000 FT TO JOIN VICTOR 19 INTO CHEYENNE (CYS). I ENCOUNTERED LITTLE TURB AND THE REMAINDER OF THE TRIP WAS UNEVENTFUL. I ATTRIBUTE THE ABOVE OCCURRENCE TO 1) UNEXPECTED CONVECTIVE ACTIVITY; 2) RISING TERRAIN; 3) GOOD VISUAL CONDITIONS REQUIRING OUTSIDE THE COCKPIT SCANNING; 4) A CTLR CONCERNED ABOUT TERRAIN CLRNC WHO OFFERED FEW OPTIONS TO AVOID CONVECTIVE ACTIVITY. BETTER MGMNT OF HDG WHILE VISUALLY SCANNING OUTSIDE THE ACFT AND PERHAPS BETTER COM TECHNIQUES SO AS TO AVOID AGGRAVATING THE CTLR; WOULD HAVE PREVENTED THIS OCCURRENCE.