Large Transport; Low Wing; 2 Turbojet Eng
Flight Crew Air Transport Pilot (ATP)
Other / Unknown; Workload
Ground Event / Encounter Object; Inflight Event / Encounter Object
Equipment / Tooling; Weather
We were sitting at gate in JAX. Lightning had just closed the ramp and radar indicated a severe thunderstorm was approaching.  I had just made a maintenance write-up; so I decided that it was prudent to stop boarding and remove the passengers from the aircraft for their safety and comfort.  A few minutes after the last PAX deplaned; the storm hit with some violent winds.  I ensured the parking brake was set; as the aircraft began to be buffeted substantially from the winds; I was concerned about microburst/tornado dangers and ordered an evacuation of the aircraft....for the crew to leave the airplane and take refuge in the Terminal.  Being the 'last man out;' I walked up the aisle and a Flight Attendant called my attention to the exterior of the left side of the airplane.  To our horror; a wind-driven baggage cart was speeding across the ramp at a high rate of speed right towards us.  I saw the cart impact the fuselage behind me and apparently just forward of the left wing; I felt and heard the impact; too; so I knew it had to have damaged the hull.  We all moved into the Terminal and I called Maintenance Control to report the possible damage; and for them to inform the Maintenance Technician that was already on his way to troubleshoot the item I had written up; that this new issue had presented itself.Contract Maintenance arrived in the terminal at the gate; and we proceeded to the airplane to address the original write-up.  The Ramp was still closed due to lightning.  Eventfully; the storm passed and the ramp was re-opened; and we were able to go outside and inspect the damage.  The open-top baggage cart was from a Company X gate next to ours; and was able to be wind-driven because the brake had not been set.  The cart impacted the lower fuselage and put at least 3 dents in the airframe; along with several scrapes.  The Maintenance Technician documented the damage; I took pictures and forwarded to Maintenance Control; the local Airport Police observed the damage; and a Company X Operations Agent viewed the damage as well. Our flight was subsequently cancelled and my crew and I were released to rest.Apparently; the storm blew-in so fast; the Company X Ramp Personnel were unable to properly secure all of their ramp equipment.  As a result; it appears this cart's brake wasn't set; and the winds from the severe thunderstorm were able to accelerate the cart to a speed such that its impact was able to dent the fuselage of our aircraft.
The Captain reported that while still at the gate; a fast moving storm system with associated high winds caused unsecured ground equipment to be blown into the aircraft; causing damage to the fuselage.