A B757 flight crew received a GPWS warning on descent into pty, first officer, while deviating WX.

Attributes
ACN	454875
Time
Date	199911
Day	Wed
Local Time Of Day	1201 To 1800
Place
State Reference	FO
Altitude	msl single value : 4000
Environment
Light	Daylight
Aircraft 1
Operator	common carrier : air carrier
Make Model Name	B757-200
Operating Under FAR Part	Part 121
Navigation In Use	other
Route In Use	arrival other
Flight Plan	IFR
Person 1
Affiliation	company : air carrier
Function	flight crew : captain oversight : pic
Qualification	pilot : atp
ASRS Report	454875
Person 2
Affiliation	company : air carrier
Function	flight crew : first officer
Qualification	pilot : commercial pilot : instrument pilot : multi engine
Events
Anomaly	other spatial deviation
Independent Detector	aircraft equipment : gpws other flight crewa other flight crewb
Resolutory Action	flight crew : took evasive action
Supplementary
Problem Areas	Flight Crew Human Performance
Primary Problem	Flight Crew Human Performance
Air Traffic Incident	Pilot Deviation

Narrative:

Sbound UA321 20N tbg. UA321 to pty cleared to descend to 5000 ft -- the MEA for this airway. Approximately 25 mi north tbg, was cleared to descend further to 3000 ft. Thought this was below MEA but familiar with terrain and knew it was left about 10 mi. IMC, numerous thunderstorms and heavy rain. Captain flying VOR mode and terrain GPWS selected on radar. Out of 4500 ft, decided to deviate left for a cell ahead. Bad decision. Got 'terrain, terrain' immediately at about 4000 ft. Used escape maneuver and climbed to 6000 ft. Lowest altitude observed was 700 ft on radar altimeter. Did not see any GPWS tint indicating terrain to our immediate left of course. Have reviewed area chart but can't determine why we got the warning. Best conclusion seems to be 'don't deviate off course for cells at low altitude in vicinity of terrain, maintain the airway.' also, 'don't descend prematurely even though cleared to do so.'

Original NASA ASRS Text

Title: A B757 FLC RECEIVED A GPWS WARNING ON DSCNT INTO PTY, FO, WHILE DEVIATING WX.

Narrative: SBOUND UA321 20N TBG. UA321 TO PTY CLRED TO DSND TO 5000 FT -- THE MEA FOR THIS AIRWAY. APPROX 25 MI N TBG, WAS CLRED TO DSND FURTHER TO 3000 FT. THOUGHT THIS WAS BELOW MEA BUT FAMILIAR WITH TERRAIN AND KNEW IT WAS L ABOUT 10 MI. IMC, NUMEROUS TSTMS AND HVY RAIN. CAPT FLYING VOR MODE AND TERRAIN GPWS SELECTED ON RADAR. OUT OF 4500 FT, DECIDED TO DEVIATE L FOR A CELL AHEAD. BAD DECISION. GOT 'TERRAIN, TERRAIN' IMMEDIATELY AT ABOUT 4000 FT. USED ESCAPE MANEUVER AND CLBED TO 6000 FT. LOWEST ALT OBSERVED WAS 700 FT ON RADAR ALTIMETER. DID NOT SEE ANY GPWS TINT INDICATING TERRAIN TO OUR IMMEDIATE L OF COURSE. HAVE REVIEWED AREA CHART BUT CAN'T DETERMINE WHY WE GOT THE WARNING. BEST CONCLUSION SEEMS TO BE 'DON'T DEVIATE OFF COURSE FOR CELLS AT LOW ALT IN VICINITY OF TERRAIN, MAINTAIN THE AIRWAY.' ALSO, 'DON'T DSND PREMATURELY EVEN THOUGH CLRED TO DO SO.'

Data retrieved from NASA's ASRS site as of July 2007 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.