page_title: Typhoon Roke (2011)
section_title: Meteorological history
hierarchical_section_title: Typhoon Roke (2011) / Meteorological history
caption_attribution_description: English: Tropical Storm Roke on September 15, 2011
context_page_description: Typhoon Roke, known in the Philippines as Tropical Storm Onyok, was a powerful and persistent tropical cyclone that affected Japan, including some areas that had been damaged by another typhoon just a few weeks prior. It was the fifteenth named storm, the tenth severe tropical storm, the sixth typhoon of the 2011 Pacific typhoon season and overall, the 27th tropical cyclone to be monitored by the Japan Meteorological Agency during the year.
context_section_description: On September 8, the Joint Typhoon Warning Center started to monitor a tropical disturbance that had developed about 935 km (580 mi) to the northeast of Hagåtña, Guam. Unorganized deep convection was surrounding the disturbances developing low level circulation centre, in an area of low vertical windshear and improving outflow. During that day the disturbance gradually developed further while moving westwards under the influence of a subtropical ridge of high pressure. During September 9, as the system continued to move westwards, the Japan Meteorological Agency reported that the disturbance had developed into a tropical depression. Continuing to move westwards the system gradually developed further with the depressions low level circulation centre consolidating  before it was declared Tropical Depression 18W by the Joint Typhoon Warning Center during the next day.
  Over the next two days, the system gradually drifted west and intensified slightly, prompting the Joint Typhoon Warning Center (JTWC) to issue a Tropical Cyclone Formation Alert (TCFA) on it. Convection gradually consolidated the low-level circulation center (LLCC) and the JTWC initiated advisories on the system on September 11, designating it with 18W. The next day, the depression drifted into the Philippine Area of Responsibility (PAR) and the Philippine Atmospheric, Geophysical and Astronomical Services Administration (PAGASA) initiated advisories on the depression, naming it Onyok. However, just as similar to Kulap, Onyok also exited the PAR in 6 hours from entering the region. In an advisory, the JTWC reported that there were at least two more vortices associated with the system, that caused an abrupt, erratic movement. However, being located in an area of warm sea surface temperatures and low vertical wind shear, the depression continued to strengthen and on September 13, the JMA upgraded the depression to a tropical storm and named it Roke.
  On September 14, a Tropical upper tropospheric trough (TUTT) cell located approximately 05 degrees to the west of the system caused strong subsidence prompting the JTWC to downgrade Roke to a tropical depression. However, because of favorable atmospheric conditions, Roke strengthened again, and the JTWC re-upgraded Roke to a tropical storm. On September 16, the previously exposed LLCC was being re-consolidated with convection and Roke continued to strengthen again. Despite the upper low over the LLCC, Roke remained a warm-core tropical cyclone and the JTWC predicted that there were chances of Roke become a hybrid or a subtropical storm. On continuing its west-northwestward movement, Roke eventually slowed to under 2 knots (3.7 km/h; 2.3 mph) with a tongue of cloud-free air entering the LLCC from the west side. Due to a mid-level warm anomaly, the subtropical transition was taking place at an unusually slow rate. Later, that day, Roke turned to the southwest and slowed with convection consolidating and deepening near the LLCC. What began as a monsoonal depression with an elongated and weak LLCC was now more stronger and organized. Also, the TUTT cell above the system started weakening and data from an ASCAT scatterometer pass revealed that the storm started to strengthen and the JTWC no longer expected a subtropical transition.
  On September 17, the JTWC reported that Roke developed a small, deep convective eye with an unusually large convective band surrounding the southern periphery of the storm. At that time, Roke was approximately 170 nautical miles (310 km; 200 mi) east of Kadena Air Base, Japan with winds of over 60 knots (110 km/h; 69 mph). As a result, the JMA upgraded Roke to a severe tropical storm. However, Roke soon started become elongated around the northeast quadrant as a response to the strong westerlies ahead of a mid-latitude trough. But this was not for too long. Roke quickly became organized again and developed an excellent equatoward outflow and continued to strengthen. Then, being located in an area of low vertical wind shear and abnormally high sea surface temperatures, Roke underwent rapid deepen
caption: Tropical Storm Roke with an exposed low-level circulation on September 15
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