Narrative:

Took position on runway. The WX radar was showing a large cluster of red radar returns several mi past the departure end of the runway. We were given a heading of 185 degrees for WX and cleared for takeoff. Checked in with departure control and advised them we will need a turn shortly to the west for WX. They said unable at the present time due to traffic. By now we were getting seriously close to a wall of solid red thunderstorm returns in our immediate path; painting from the ssw through to the east and extending well above us. I informed departure control that I needed a turn to the west in a mi and if needed; will declare an emergency and then turn west. ATC queried: 'are you declaring an emergency?' I said yes; I refuse to fly through a thunderstorm. ATC then said advise passenger onboard. I gave them that information as I directed the copilot to turn west to avoid the large convective cells. Proceeded to destination without further issues.

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Original NASA ASRS Text

Title: B757 CAPTAIN DECLARES AN EMERGENCY TO AVOID WEATHER ON DEPARTURE AS ATC WILL NOT APPROVE A TURN WITHOUT THIS DECLARATION.

Narrative: TOOK POS ON RWY. THE WX RADAR WAS SHOWING A LARGE CLUSTER OF RED RADAR RETURNS SEVERAL MI PAST THE DEP END OF THE RWY. WE WERE GIVEN A HDG OF 185 DEGS FOR WX AND CLRED FOR TKOF. CHKED IN WITH DEP CTL AND ADVISED THEM WE WILL NEED A TURN SHORTLY TO THE W FOR WX. THEY SAID UNABLE AT THE PRESENT TIME DUE TO TFC. BY NOW WE WERE GETTING SERIOUSLY CLOSE TO A WALL OF SOLID RED TSTM RETURNS IN OUR IMMEDIATE PATH; PAINTING FROM THE SSW THROUGH TO THE E AND EXTENDING WELL ABOVE US. I INFORMED DEP CTL THAT I NEEDED A TURN TO THE W IN A MI AND IF NEEDED; WILL DECLARE AN EMER AND THEN TURN W. ATC QUERIED: 'ARE YOU DECLARING AN EMER?' I SAID YES; I REFUSE TO FLY THROUGH A TSTM. ATC THEN SAID ADVISE PAX ONBOARD. I GAVE THEM THAT INFO AS I DIRECTED THE COPLT TO TURN W TO AVOID THE LARGE CONVECTIVE CELLS. PROCEEDED TO DEST WITHOUT FURTHER ISSUES.

Data retrieved from NASA's ASRS site as of January 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.