Narrative:

Convective activity in the area of ZZZ. ATC was directing outbound traffic through a very small opening in the WX. Our assigned altitude of 15000 ft was going to put us in the WX. ATC would not give us a climb. I declared an emergency and climbed to approximately 15600 ft. We had traffic 1000 ft above and I monitored the TCAS and made sure we were clear. A B757 75 mi behind us reported that hole in the WX had closed and told the controller not to send anyone through. The controller concurred.

Google
 

Original NASA ASRS Text

Title: B757 DEP FROM ZZZ ENCOUNTERED WX; REQUESTED CLIMB; ATC WAS UNABLE DUE TO TFC; FLT CREW DECLARED EMER AND INITIATED CLIMB.

Narrative: CONVECTIVE ACTIVITY IN THE AREA OF ZZZ. ATC WAS DIRECTING OUTBOUND TFC THROUGH A VERY SMALL OPENING IN THE WX. OUR ASSIGNED ALT OF 15000 FT WAS GOING TO PUT US IN THE WX. ATC WOULD NOT GIVE US A CLB. I DECLARED AN EMER AND CLBED TO APPROX 15600 FT. WE HAD TFC 1000 FT ABOVE AND I MONITORED THE TCAS AND MADE SURE WE WERE CLR. A B757 75 MI BEHIND US RPTED THAT HOLE IN THE WX HAD CLOSED AND TOLD THE CTLR NOT TO SEND ANYONE THROUGH. THE CTLR CONCURRED.

Data retrieved from NASA's ASRS site as of May 2009 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.