Monday, May 5, 2008


In the last week of April, an area of deep convection and concern persisted near a low-level circulation in the Bay of Bengal about 1150 km (715 mi) east-southeast of Chennai, India.[1] With good outflow and low wind shear, the system slowly organized as its circulation consolidated.[2] At 0300 UTC on April 27, the India Meteorological Department (IMD) classified the system as a depression,[3] and nine hours later the system intensified into a deep depression.[4] At the same time, the Joint Typhoon Warning Center classified it as Tropical Cyclone 01B. With a ridge to its north, the system tracked slowly north-northwestward as banding features improved.[5][6] At 0000 UTC on April 28, the IMD upgraded the system to Cyclonic Storm Nargis while it was located about 550 km (340 mi) east of Chennai, India.[7]
On April 28, the motion of Nargis became nearly stationary while located between ridges to its northwest and southeast. That day, the JTWC upgraded the storm to cyclone status, or the equivalence of a minimal hurricane on the Saffir-Simpson hurricane scale.[8] At around the same time, the IMD upgraded Nargis to a severe cyclonic storm.[9] The cyclone developed a concentric eye feature, which is an eyewall outside the inner dominant eyewall,[10] with warm waters aiding in further intensification.[11] Early on April 29, the JTWC estimated Nargis reached winds of 160 km/h (100 mph),[12] and at the same time the IMD classified the system as a very severe cyclonic storm.[13] Initially, the cyclone was forecasted to strike Bangladesh or southeastern India.[14][15] Subsequently, the cyclone became disorganized and weakened due to subsidence and drier air; as a result, deep convection near the center markedly decreased. At the same time, the storm began a motion to the northeast around the periphery of a ridge to its southeast.[16] The circulation remained strong despite the diminishing convection, though satellite intensity estimates using the Dvorak technique indicated the cyclone could have weakened to tropical storm status.[17] By late on April 29, convection had begun to rebuild,[18] though immediate restrengthening was prevented by increased wind shear.[19]
On May 1, after turning nearly due eastward, Cyclone Nargis began rapidly intensifying, due to greatly improved outflow in association with an approaching upper-level trough.[20] Strengthening continued as it developed a well-defined eye with a diameter of 19 km (12 mi), and early on May 2 the JTWC estimated the cyclone reached peak winds of 215 km/h (135 mph) as it approached the coast of Myanmar.[21] At the same time, the IMD assessed Nargis as attaining peak winds of 165 km/h (105 mph).[22] Around 1200 UTC on May 2, Cyclone Nargis made landfall in the Ayeyarwady Division of Myanmar.[23] The storm gradually weakened over land, with its proximity to the Andaman Sea preventing rapid weakening. Its track turned to the northeast due to the approach of a mid-latitude trough to its northwest, passing just north of Yangon with winds of 130 km/h (80 mph).[24] Early on May 3 the IMD issued its final advisory on the storm.[25] It quickly weakened after turning to the northeast toward the rugged terrain near the Myanmar-Thailand border, and after deteriorating to minimal tropical storm status, the JTWC issued its last advisory on Nargis.[26]

No comments: