A CESSNA 172-RG HAS AN ALT EXCURSION WHILE FLYING OVER THE MOUNTAINS S OF LAKE TAHOE.

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ON AUG/XX/96; PLT A (CFII) AND PLT B (STUDENT INST PLT) FILED AN IFR FLT PLAN FROM MODESTO; CA; TO S LAKE TAHOE; CA; AND WERE ASSIGNED THE RTE OF RADAR VECTORS LINDEN- V28-113-SPOOK-DIRECT S LAKE TAHOE AND TO MAINTAIN 13000 FT. AFTER PASSING SPOOK INTXN; WE EXPERIENCED A DOWN- DRAFT AND WERE UNABLE TO MAINTAIN 13000 FT MSL. WE MADE 2 ATTEMPTS TO RADIO ZOA TO INFORM THEM OF OUR LOSS IN ALT; BUT WE WERE BLOCKED BY OTHER RADIO CALLS TO ATC. UPON REACHING 12000 FT MSL; ZOA INQUIRED ABOUT OUR ALT AND WE INFORMED ATC THAT WE WERE UNABLE TO MAINTAIN OUR ASSIGNED ALT OF 13000 FT DUE TO SINK AND TURB. WE ALSO INFORMED ATC THAT WE TRIED TWICE TO RADIO THEM OF OUR LOSS OF ALT AND WERE BLOCKED. ZOA THEN CLRED US FOR A VISUAL DSCNT INTO TAHOE WHERE WE LANDED WITHOUT ANY FURTHER INCIDENT. UPON REVIEW OF THE SIT DESCRIBED; I HAVE FOUND THAT UNDER CERTAIN WX CONDITIONS (IE; IN TURB AND DOWN-DRAFTS) IT MAY BE IMPOSSIBLE TO MAINTAIN ALT EVEN WHEN FULL PWR IS APPLIED AS WAS THE CASE WITH OUR C172RG. CAREFUL CONSIDERATION WAS GIVEN FOR THE WX ALONG OUR RTE OF FLT. THE WX WAS FORECAST TO BE CLR ALONG OUR RTE OF FLT WITH A CHANCE OF TSTMS AFTER XA00 LCL TIME. NEAR OUR DEST; WE ENCOUNTERED UNFORECASTED TURB WHICH CAUSED OUR LOSS IN ALT. IF CURRENT PIREPS WERE AVAILABLE WE COULD HAVE AVOIDED THE UNFORECAST WX AND POSSIBLE AVOIDED OUR LOSS IN ALT BY CHANGING OUR RTE OF FLT.