For what it's worth. Climbing out from Fiji in a 737-200 for Wallis Island some one hour away. Afternoon trip with low cloud and rain forecast for Wallis. On the horizon on track was the greatest line of black cloud that I have ever seen. It went from horizon to horizon and my guess was from sea level to 40,000 ft plus.
We flew towards this lot and had a closer look at 80 miles on radar. We were in clear air at 27,000 and in continuous moderate turbulence. Radar showed a green mass with flecks of yellow and red and I guessed that severe rain attenuation was taking place.
I got that warning sixth sense feeling that this was not your usual Cb front. Something said don't go any further or someone is going to get hurt. We called ATC at Fiji and got a clearance to return to Fiji which we did. Passengers and crew stayed overnight and departed for Wallis next morning. At Wallis, the weather was fine and there was a lot of water on the airport. No problem with landing.
Turned out that for our ETA at Wallis the previous afternoon the actual winds were 75 knots, torrential tropical rain and runway flooded. The forecast was almost benign but the tropical depression had started with little warning.
One of the better decisions I have made in my career, thank goodness.
25 years on I still can see that cloud formation in my mind and figured I had looked at death. Sorry to appear to exaggerate but it really did scare me.