Monday, November 21, 2016

Hurricane Otto (2016)

Storm Active: November 21-26

On November 12, a trough of low pressure making its way across the Caribbean stalled in the southwestern part of the sea north of Panama. The system organized very gradually without moving significantly over a period of many days. Thunderstorm activity increased on the 14th, leading to the formation of a low pressure the next day. The low drifted toward the coastline of Central America, halted, and reversed direction over the following couple of days, while conditions became less favorable and disorganized the system. However, it was still over the same region of warm waters on November 19 when conditions began to improve and the circulation became better defined. On November 20, the low deepened and was better defined still, but thunderstorm activity remained too weak for a tropical cyclone classification. Finally, early on November 21, Tropical Depression Sixteen formed in the southwestern Caribbean.

Located over warm ocean waters and surrounded by an environment of diminishing upper-level winds, the cyclone steadily strengthened. That afternoon, it was upgraded to Tropical Storm Otto. Otto was meanwhile trapped in an area of very weak steering currents and barely moved that day, managing only a slow southward drift. But while the system was nearly stationary, its structure improved considerably: strong thunderstorm activity increased and large curved banding features formed overnight. By midmorning of November 22, Otto was close to hurricane strength and its southern quadrant was bringing heavy rains to the northern coast of Panama. A little more intensification that afternoon resulted in Otto becoming a category 1 hurricane, breaking the record for latest hurricane formation in the Caribbean Sea set by Martha in 1969. Meanwhile, the system began to slowly move westward as a ridge established itself to the north.

The next day, wind shear and dry air weakened Otto slightly back to a tropical storm. This setback was temporary, however, as atmospheric moisture was increasing, and a large burst of intense convection that evening restored hurricane strength. Otto was also moving more steadily westward toward the Central American coast by this time as the ridge amplified. Tropical storm conditions began to affect southeastern Nicaragua and northeastern Costa Rica by the early morning hours of November 24. Around the same time, Otto quickly developed an eye feature and intensified into a Category 2 hurricane, reaching its peak intensity with 110 mph winds and a central pressure of 975 mb. With this intensification, Otto became the latest forming category 2 hurricane on record in the Atlantic basin. A few hours later, around 1 pm EST, Otto made landfall just north of the border of Nicaragua and Costa Rica. This was also the southernmost hurricane landfall in Central America on record. As a result, the storm affected some areas less prepared for landfalling hurricanes.

Soon, however, land interaction began to quickly weaken Otto. By the evening of the 24th it had weakened to a tropical storm. The weakened system, now moving more quickly westward, emerged into the East Pacific basin overnight. The Pacific waters were fairly warm, but Otto faced dry air and significant wind shear. It moved steadily just south of west and weakened until dissipation occurred on November 26.

The above image shows Hurricane Otto at category 2 intensity just before landfall.

Otto's track was unusual in many respects. In addition to becoming the southernmost landfalling Central America hurricane ever recorded, it was also the first cyclone to survive crossing from the Atlantic to the Pacific since 1996.

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