Date: 201208
Local time of day: 0001-0600
Location: ZDC.ARTCC, VA
Flight conditions: IMC
Weather elements / visibility: Thunderstorm
Aircraft: Commercial Fixed Wing
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Descent
Airspace: Class A ZDC
ATC / advisory: Center ZDC
Reporter organization: Air Carrier
Reporter function: Pilot Flying; First Officer
Human factors: Communication Breakdown; Distraction; Situational Awareness; Time Pressure; Workload
Anomaly: ATC Issue All Types; Conflict Airborne Conflict; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Detector: Person Air Traffic Control; Person Flight Crew
When detected: In-flight
Result: Air Traffic Control Issued New Clearance; Flight Crew Took Evasive Action; Flight Crew Requested ATC Assistance / Clarification; Flight Crew Returned To Clearance; General Declared Emergency
Contributing factors: Weather; Procedure; Airspace Structure; Chart Or Publication; Human Factors
Primary problem: Weather

Narrative: On the EMI arrival for BWI; there was a cell directly ahead. Other aircraft ahead of us were deviating around the cell. Approximately 30 miles from the cell; we informed the Controller that we needed a deviation to the left (north) around the cell. The Controller told us to standby; that he would have a deviation for us shortly. At approximately 12 miles from the cell; we informed the Controller that we could not accept any further delay in deviating. The Controller told us to descend to FL210; and reaching FL210; we were cleared to deviate. At this point; the cell was less than 10 miles ahead. We told the Controller that we would initiate a descent; but we couldn't wait until we reached FL210 before deviating and we needed to turn now. The Controller instructed us to make a 90 degree turn to the right (south). At that point; the geometry of the cell and our close proximity to it made it impossible to turn south and avoid the cell; and we simply were unable to argue with him any longer. We responded that we could not accept a turn to the south; that we were initiating a turn to the north at that time to avoid the cell. The Controller responded by asking us to squawk 7700 on our transponder; which we did. After leveling at FL210; we were shortly able to turn back toward the BUBBI Intersection; and we informed the Controller that we could accept a turn that direction. We were cleared back on course and told to return to our normal squawk code.Throughout this event; the Controller seemed to be completely overwhelmed by all the traffic in his airspace and the numerous deviations being requested. At no time did the traffic on our TCAS appear to be anywhere near us; and while I know TCAS doesn't provide the full ATC picture; the Controller also did not issue any traffic any heading or altitude changes to avoid us on our emergency deviation; so it remained unclear to us exactly why our requested deviation north was such a problem for the Controller or why an emergency squawk was necessary.




Synopsis: When ATC was unable to allow them to divert around a thunderstorm; the flight crew of an air carrier aircraft flying the EMI RNAV STAR to BWI was forced to declare an emergency in order to do so. The Controller's extremely high workload was felt by the reporter to have been a primary contributing factor.