SMA PVT PLT VEERS OFF RWY DURING XWIND LNDG AND WINDS UP ON ANOTHER RWY.

Narrative

I WAS CLRED TO LAND ON RWY 28R AT MYF; CA. I HAD A NORMAL APCH; THOUGH ON SHORT FINAL THERE WAS TURB; AND CROSSED THE THRESHOLD AT 70 KTS; THE RECOMMENDED SPD FOR THE C172SP GIVEN THE HEADWIND COMPONENT. WHEN I WAS ON SHORT FINAL THE TWR INFORMED ME THAT THE WIND WAS 250 DEGS OR 260 DEGS (I DO NOT RECALL WHICH) AT 18 KTS; GUSTING TO 25 KTS. I MENTALLY CALCULATED THAT THE DIRECT XWIND WAS NO MORE THAN 9 KTS; WITH GUSTS TO 12 KTS OR 13 KTS; WITH THE PLANE'S DEMONSTRATED XWIND COMPONENT OF 15 KTS. WHEN OVER THE RWY I TRACKED THE CTRLINE UNTIL THE L WHEEL TOUCHED DOWN. AFTER THE R WHEEL TOUCHED DOWN; THE PLANE SHIFTED SLOWLY TO THE R OF THE CTRLINE AND DECELERATED. ABOUT 100-150 FT FROM WHERE I EXPECTED TO STOP; THE L WING SUDDENLY LIFTED AND THE L WHEEL LEFT THE PAVEMENT. THE PLANE SWERVED TO THE R; TOWARD THE GRASS. BY THE TIME I GOT THE L WHEEL BACK DOWN; THE PLANE WAS AIMING FOR THE GRASS AT ABOUT A 30 DEG ANGLE TO THE RWY. I THOUGHT THAT IF I TRIED TO TURN IT ABRUPTLY TO THE L; THE PLANE COULD BE KEPT MAINLY ON THE RWY WITH ONLY A SHORT EXCURSION ONTO THE GRASS; POSSIBLY ONLY ON THE R WHEEL; BUT NOT WITHOUT ROLLING OVER ONE OR MORE RWY LIGHTS; SO I LET THE PLANE CONTINUE ONTO THE GRASS; EXITING BTWN LIGHTS. I LIGHTLY APPLIED THE BRAKES AND ALLOWED THE PLANE TO DECELERATE ON THE GRASS. IT CAME TO A STOP ON THE INTERSECTING RWY 23/5 (WHICH HAD BEEN NOTAMED AS OTS FOR THE DAY DUE TO AIRSHIP OPS AT ONE END). I ESTIMATE THAT THE PLANE CROSSED 50-100 FT OF GRASS BEFORE ENTERING RWY 23/5. THAT THE PLANE MOVED OFF THE CTRLINE AFTER TOUCHDOWN INDICATED I DID NOT MAINTAIN ENOUGH AILERON. THEN; WHEN THE L WING LIFTED; MY REACTION TO WHAT MAY HAVE BEEN A CHANGE IN WIND DIRECTION AND MAGNITUDE WAS TOO SLOW AND INSUFFICIENTLY VIGOROUS. THERE WERE NO DAMAGES TO THE PLANE; ITS OCCUPANT; THE RWY LIGHTS; OR ANYTHING ELSE.