Workload; Situational Awareness; Time Pressure; Communication Breakdown
ATC Issue All Types; Deviation - Track / Heading All Types; Deviation / Discrepancy - Procedural Clearance; Inflight Event / Encounter Weather / Turbulence
Air Traffic Control Issued New Clearance; Air Traffic Control Issued Advisory / Alert; Flight Crew Took Evasive Action; Flight Crew Requested ATC Assistance / Clarification; General Declared Emergency
Airspace Structure; Weather; Procedure
Taxiing out for Cokem 7 RNAV departure Runway 26L in ATL.  First officer's leg.  On first contact with Ground Control informed of runway change to  8R.  We assumed the change was due to rapidly developing thunderstorms that we could see off the departure end of rwy 26L.  Plenty of time to get everything set up for new departure runway.  We were the third airplane to depart from 8R.  Climbing out weather radar depicted a  well  defined large cell (with a nice patch of red in the center) over the first RNAV fix 'RONII'.  I asked the Tower for a deviation left to avoid weather and he responded that he couldn't authorize that and we would have to wait until switched to Departure Control at 2 miles.  First Officer started a left deviation as we were rapidly approaching what the radar showed as a dangerous storm.  That put us on a parallel course to the RNAV course.  When we switched to Departure Control I asked for a left deviation for weather.  He replied 'unable due to departing traffic'.  I responded that we were deviating left.  He then said that they had stopped departures and gave us a left turn heading of 360 degrees which took us well away from the thunderstorm although we did catch the edge of it in the turn.I was very surprised that Departure would not give us an immediate turn as the RNAV route would have taken us into the middle of what our radar depicted as a strong thunderstorm.  I fully back up the First Officer's turn with my emergency authority for the safety of the airplane. I did not take our deviation lightly but that it was  necessary to avoid a dangerous storm. The event was the result of weather and the lack of timely clearance to avoid it.
Despite the increased workload and threat that this presented; we had ample time to preform all associated duties and I don't believe this was a contributing factor. As we taxied to 8R we could see a developing thunderstorm developing off the departure end of our original Runway (26L). We were the third aircraft to depart in the new direction and as far as I could tell; the first to depart via the COKEM 7 RNAV DEP with a RONII BEDRK routing. As we lined up with the runway I looked in the sky ahead and at the Captain's radar display. While I could see weather to the east; it looked to me that we would be turning north well before we encountered it. I didn't see any weather of concern on the radar. After rotation; I called for gear up and established the proper pitch attitude then looked ahead at the weather and again at the Captain's radar display. By then; the situation had dramatically changed; there was a large developing cell centered over our first fix of RONII. The cell was painting sharp; well defined edges going quickly from green to amber to a sizable portion of red surrounding RONII and extending four to five miles from it. At about 500 feet AGL I told the Captain that I would need an early turn to the north and he then informed Tower. Tower replied that he'd be handing us to departure in another two miles. This comment and the fact that we couldn't imaging a Controller vectoring us through the cell ahead left both of us with the impression that a turn would be approved. Once we were given the hand off I started a slow turn to the north while the captain said; 'We'd like an early turn due to weather.' Departure replied; 'Unable due to traffic.' This initiated some conversation between the Captain and the Controller about the urgency of our need to avoid the cell. On my part; 23 years of experience; 11 of which based in the Midwest and Florida; was telling me not to fly into that storm. I turned back to the East but did not completely join the RNAV track but instead paralleled it. Shortly there after Departure approved a turn to the north which allowed me to avoid the worst of the cell. Sometime during this conversation another air carrier flight radioed to tell Tower that; 'Departures to the East aren't going to work either.' Departure called us and said asked if we'd started a turn prior to the handoff to departure. The Captain replied; yes that we had told Tower that we needed to turn. My memory is that I waited until the hand off; fully expecting Departure to approve a weather deviation. Causes leading to this event were: a dynamic and rapidly changing weather environment. Perhaps; confusion on the part of ATC of the urgency of our need to deviate. I don't know why the cell was not painted on the radar while we were on the runway. Had it been; I never would have taken off.
An Air Carrier departed ATL on the COKEM 7 RNAV and immediately detected a large cell over RONII. ALT Tower would not approve a deviation; so upon contact with the TRACON; the First Officer turned even though it was not approved using the Captain's emergency authority.