THE PLT OF A GRUMMAN AA-1 ENCOUNTERS SEVERE TURB 14 MI W OF VNY AND IS UNABLE TO MAINTAIN ALT.

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WHILE ON AN IFR FLT PLAN AND IN CRUISE CONFIGN LEVEL AT MY ASSIGNED ALT OF 5000 FT; I ENCOUNTERED MODERATE TO SEVERE TURB. THE ACFT BEGAN A DSCNT. I APPLIED FULL PWR AND PITCHED UP FOR BEST CLB. THE ACFT CONTINUED DSCNT AT ABOUT 500 FPM TO 4500 FT. AT THIS POINT ATC ASKED FOR VERIFICATION THAT I WAS LEVEL AT 5000 FT. I REPLIED THAT I WAS UNABLE TO MAINTAIN 5000 FT AND THAT I WAS IN MODERATE TO SEVERE TURB. ATC RESPONDED THAT THEY UNDERSTOOD. AT THIS POINT; I WAS IN A FULL PWR CLB SLOWLY CLBING BACK TO 5000 FT WITH TURB THAT WAS LIKELY TO GET WORSE BEFORE IT GOT BETTER; SO I ASKED ATC FOR CANCELLATION OF MY IFR FLT PLAN AND VFR FOLLOWING BACK TO MY DEP ARPT. ATC POLITELY APPROVED MY REQUEST. I FILED A PIREP WITH FSS TO ADVISE THEM OF THE TURB. I BELIEVE THAT THIS PROB WAS CAUSED BY UNUSUAL WX CONDITIONS. DURING MY WX BRIEFING; THE SPECIALIST NOTED THAT THERE WAS A NOTAM FOR TURB; BUT THAT IS FAIRLY COMMON. THERE WERE NO PIREPS TO SUPPORT THE NOTAM. THE TURB LASTED FOR A FEW HRS AND THEN SUBSIDED. TO HELP PREVENT SITUATIONS LIKE THIS; I BELIEVE IT WOULD BE HELPFUL IF MORE PLTS MADE PIREPS MORE FREQUENTLY. ATC WAS VERY HELPFUL AND UNDERSTANDING OF MY SITUATION.