Typhoon Dolphin was a powerful tropical cyclone that produced the first typhoon-force winds on Guam since Typhoon Pongsona in 2002. The seventh named storm of the 2015 Pacific typhoon season, Dolphin formed on May 6 in the vicinity of the Federated States of Micronesia (FSM). Moving eastward at first, the storm slowly organized before beginning a north and west-northwest trajectory. Dolphin intensified into a typhoon before passing between Guam and Rota on May 15, producing typhoon-force winds on both islands. It later rapidly intensified as it curved to the north. The American-based Joint Typhoon Warning Center (JTWC) designated Dolphin as a super typhoon, while the Japan Meteorological Agency (JMA) estimated 10 minute sustained winds of . Dolphin turned to the northeast and weakened, becoming extratropical on May 20 and exiting the western Pacific basin on May 24.
The storm first affected the FSM, notably Pohnpei where it dropped of rainfall over three days. The rains and gusty winds knocked down many trees on the island, one of which killed a person, and causing $1 million in damage (2015 USD). Dolphin passed between Guam and Rota, producing gusts of at Andersen Air Force Base on northern Guam. The winds left 40% of the island without power and left at least 3,300 people without water. The storm also dropped heavy rainfall, flooding Guam Memorial Hospital. Dolphin damaged 390 houses, including nine that were destroyed, leaving 1,055 people homeless. With damage estimated at $10 million, the island was declared a disaster area. The typhoon also brushed Rota, causing $2.5 million in damage there, as well as Saipan.
Meteorological history
The origins of Dolphin were related to a strong westerly wind burst that also led to the formation of previous Typhoon Noul. Early on May 5, the Joint Typhoon Warning Center (JTWC) began monitoring an area of deep convection approximately southwest of Pohnpei. It had a poorly-defined circulation and broad rainbands, while low to moderate wind shear and warm sea surface temperatures favored development. The convection quickly became better organized and more concentrated around the broad center, aided by increased outflow. The Japan Meteorological Agency (JMA) classified the system as a tropical depression at 06:00 UTC on May 6 about southwest of Pohnpei. At 21:00 UTC that day, the Joint Typhoon Warning Center (JTWC) also began issuing advisories on the system, classifying it as Tropical Depression 07W.
After its formation, the depression moved slowly with the low-level flow, in what the JTWC described as an "atypical eastward direction". Wind shear in the region exposed the convection from the circulation early on May 7, although the thunderstorms increased the next day, mostly in the storm's northwest quadrant. A building subtropical ridge turned the system more to the north. By May 8, the depression organized enough for the JTWC to upgrade it to a tropical storm. At 12:00 UTC on the next day, the JMA followed suit and upgraded the depression to Tropical Storm Dolphin. By that time, the system had developed rainbands spiraling around the circulation, although continued wind shear left the center exposed. Around that time, Dolphin passed about west of Kosrae in the Federated States of Micronesia (FSM), and over the next few days it passed near other small islands in the region. Despite the sheared structure, the storm developed an eye feature on May 11, indicative of further strengthening, while the storm was passing east of Pohnpei. That day, the shear began to decrease, allowing the outflow to improve and convection to blossom. Early on May 12, the JMA upgraded Dolphin to a severe tropical storm, estimating 10 minute winds of .
On May 12, Dolphin began moving steadily to the west-northwest. It developed a persistent central dense overcast over the center as conditions became increasingly favorable. At 00:00 UTC, the JMA and JTWC both upgraded Dolphin to typhoon status, based on a developing eye feature. The eye, initially only in diameter, became more defined in the center of the convection. The intensification trend was soon halted by a combination of moderate southerly wind shear and dry air from the west, causing the eye to become obscured on conventional satellite imagery. On May 15, the wind shear once again lessened. The compact core persisted during this time, although the center was slightly obscured. As the typhoon approached Guam, radar imagery tracked the eye underneath the deepest convection. Late on May 15, Dolphin passed between Guam and Rota, bringing its eyewall over both islands.