As the evening was winding down from a busy day at the 1 house very severe storms started rumbling through the area bringing torrential down pours and heavy winds and even reports of possible tornado's. Prince William County Communications had their hands full with multiple box alarms and several accidents with people trapped in both the Eastern and Western parts of Prince William County. At 1937hrs Box Alarm 15-31 was sounded for reports of a house on fire possibly caused by a lighting strike in the 4600 block of Allen Mills Blvd in Haymarket. Bringing Engine 501B, and Tower Ladder 501 from quarters. While units from Manassas were responding, Engine 515 went on scene with nothing evident and began to investigate. A short time later at 1953hrs while units from Manassas were still responding to the 4600 Block of Allen Mills Blvd, Prince William County Communications sounded Box Alarm 04-44 for reports of a house on fire in the 13000 Block of Fieldstone Way, in Gainesville. Being that Engine 501B was right around the corner from this address, the officer from Engine 501B asked Communications if they could divert to the house on fire in Gainesville. Communications advised them to respond to the incident. Engine 501B was added to the run and responded as the 2nd due engine on the run. Engine 501B encountered a locked security gate that was unpassable at one of the entrances to the development and was unable to gain access, due to the situation Engine 501B notified communications of the problem and advised them they will be delayed in arriving. Engine 504 went on scene of with fire through the roof a very short time later and began an initial exterior attack. Engine 501B arrived as the 5th due engine and laid a supply line to Tower Ladder 525 in preparation of ladder pipe operations. As the Engine driver was making the proper connections, the rest of the Crew from Engine 501B made its way to the command post where they were quickly given an assignment. Units were told to report to the second floor and relieve the Crew from Engine 504. Engine 501B proceeded to the 2nd floor where they were met with moderate fire conditions and heavy think smoke conditions still within the structure. Engine 501B officer noticed a slight glow coming from a room that had not yet been opened up so the crew repositioned the hand line and found heavy fire within a closet and extension into the wall. All fire was eventually extinguished by Engine 501B crew before going ready at 2129.