Flight Crew Multiengine; Flight Crew Flight Instructor; Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 105; Flight Crew Total 7105; Flight Crew Type 1600
ATC Issue All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued Advisory / Alert; Air Traffic Control Issued New Clearance; General Declared Emergency
Airspace Structure; Weather; Procedure
Enroute line of frontal storms across track; approximately ZZZ-ZZZ1. Cleared to deviate right of track up to HDG 020 deg M. Was holding HDG 020 to fly through only available gap. Turbulence continuous moderate; but wasn't sure if there would be worse. In IMC and hand flying to minimize possible turbulence vs. autopilot induced loads on airframe ('g' and IAS outside limits). Was right between cells with a large storm to the left. MIA Center gave an instruction to turn left. Don't remember the new heading other than it would have put me into a TS cell. Instructed accompanying pilot to reply with 'Unable;' then I queried the instruction in case I had misunderstood. Had understood correctly; but a left turn was not possible due TS. Advised Center that I was still unable to turn as directed. Center advised that I would need to declare an emergency to disregard the instruction - so I declared. Unable to select TXDR 7700 at the time due to a need to adjust power settings to control speed excursions. By the time I could change the code I was clear of the immediate storms with a clear path for the rest of the flight; so I cancelled the emergency and turned to a newly assigned heading of (I think) 340 deg M.
PA31 Captain reports being unable to comply with an ATC heading instruction due to a thunderstorm and is instructed to declare an emergency; which is accomplished.