SEVENTEEN BALLOONS LAUNCHED FROM VARIOUS LOCATIONS AROUND THE NAPA VALLEY FOR LOCAL SIGHTSEEING FLIGHTS. STATMENTS FROM PILOTS, PASSENGERS AND GROUND WITNEESES 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 INCLUDED 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 PILOT OF SEVERAL BALLOONS. REPORTED THAT THE SHAPES OF THE BALLOON ENVELOPES WERE SEVERELY DISTORTED BY THE SUDDEN MOVEMENT OF THE AIRMASS. EIGHT OF THE 17 BALLOONS ATTEMPTED FORCED LANDINGS.FOUR 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.