SEVENTEEN BALLOONS LAUNCHED FROM VARIOUS LOCATIONS AROUND THE NAPA VALLEY FOR LOCAL SIGHTSEEING FLIGHTS. STATEMENTS FROM PILOTS, PASSENGERS AND GROUND WITNESSES SAID THAT WHILE THE BALLOONS WERE IN NORMAL DRIFTING FLIGHT A MICROBURST LIKE CONDITION SUDDENLY OCCURRED WHICH AFFECTED ALL OF THE BALLOONS. THE WEATHER PHENOMENA ENCOUNTERED INLCUDED STRONG GUSTING SURFACE AND ALOFT WINDS IN ADDITION TO SEVERE DOWN DRAFTS. THE BALLOONS WERE RAPIDLY FORCED TOWARD THE GROUND AS THEIR GROUND SPEED INCREASED RADICALLY. THE PILOTS OF SEVERAL BALLOONS REPORTED THAT THE SHAPES OF THEIR BALLOONS ENVELOPES WERE SEVERELY DISTORTED BY THE SUDDEN MOVEMENT OF THE AIRMASS. EIGHT OF THE 17 BALLOONS ATTEMPTED FORCED LANDINGS. FOUR OF THE BALLOONS COLLIDED WITH OBSTRUCTIONS AND THE GROUND DURING THE ATTEMPTED LANDINGS. AT THE TIME OF THE INCIDENT, A LARGE THUNDERSTORM WAS REPORTED 30 MILES TO THE NORTH.