On August 1, 1999, at 1440 eastern daylight time, a Piper J3C-65, N70894, was substantially damage during landing, when it departed the right side of runway 23 at Frederick Municipal Airport, Frederick, Maryland. The certificated commercial pilot was not injured. Visual meteorological conditions prevailed for the personal flight that originated from Haysfield Airport, Clarksville, Maryland. A visual flight rules flight plan was filed, and the flight was conducted under 14 CFR Part 91. According to the Pilot/Operator Aircraft Accident Report Form, the pilot obtained a standard weather briefing, and then filed a flight plan. He then received a weather update from flight service approximately 5 minutes before executing the flight. During the update, the briefer indicated that scattered thunderstorms in the Hagerstown, Maryland area were moving east towards Frederick. The pilot calculated that he would arrive at his destination prior to the thunderstorms, so he departed Haysfield. While approaching his destination, the pilot noticed the sky to the north of his position was "very dark." He also experienced an increase in turbulence. The pilot checked weather with Frederick UNICOM, and was advised the winds were 4 knots. With the approaching thunderstorms, the pilot expedited his turn from base to final for runway 23. He then executed a "normal" landing. As the airplane was rolling out it became airborne and started to drift to the right. The airplane touched down, exited the runway, and the landing gear collapsed when it struck a sign. The pilot shut off both magnetos, secured the throttle, and exited the airplane. The pilot added, "I think I encountered a gust and simply lost control."