Narrative:

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.

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Original NASA ASRS Text

Title: 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.

Narrative: 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.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.