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Tropical Storm Ana (2015)
Tropical Storm Ana was the earliest recorded tropical cyclone to make landfall in the United States in any given year. A relatively rare pre-season system, Ana was the first tropical storm of the 2015 Atlantic hurricane season. It developed as a subtropical cyclone from a non-tropical low north of the Bahamas, and intensified to attain peak winds of 60 mph (95 km/h) before transitioning into a fully tropical cyclone early on May 9. Sustained in part by the warm waters of the Gulf Stream, Ana made landfall along the northeast coast of South Carolina early the next morning, becoming the earliest U.S. landfalling system on record. Overall, damage from the storm was minor. Heavy rainfall and gusty winds affected parts of the Carolinas, and some vulnerable beaches endured coastal flooding and several feet of erosion. Gusty winds damaged trees and powerlines, causing sporadic power outages, and a weak tornado was reported in association with Ana. In North Carolina, two deaths—one direct and one indirect—were attributed to the storm.
==Meteorological history==
In the first week of May 2015, a strong cold front became stationary to the north of the Greater Antilles, where warm ocean waters dissipated the temperature gradient across the front. The western section of the front drifted westward as a surface trough of low pressure. Arriving at the Bahamas by May 5,〔 the trough was attended by an expanse of showers and thunderstorms extending from the northwestern Caribbean Sea to southern Florida. A sharp mid- to upper-level trough interacted with the system and facilitated gradual cyclogenesis, and early on May 6, a distinct, non-tropical center of low pressure developed off southeastern Florida.〔 Tracking slowly toward the north over the next two days, the system showed signs of increased organization, and a Hurricane Hunter aircraft found tropical storm-force winds to the north and west of the center on May 7. With the development of deep convection, the system was classified Subtropical Storm Ana at 00:00 UTC on May 8, while situated about 170 mi (275 km) to the south-southeast of Myrtle Beach, South Carolina. Due to its involvement with the upper trough, as well as its large wind field, the system was considered subtropical, as opposed to fully tropical.〔
The newly designated storm drifted toward the northwest, caught in an upper-air blocking pattern that yielded weak steering currents. Dry air within its circulation initially kept Ana weak, with sparse thunderstorm activity. However, the warm waters of the Gulf Stream, coupled with atmospheric instability resulting from cold temperatures aloft, enabled the formation of curved banding features near the storm's core. At the same time, the wind field constricted, indicating an imminent transition to a fully tropical storm. Late on May 8, Ana attained its peak intensity with maximum sustained winds of 60 mph (95 km/h) and a minimum central pressure of 998 mbar. By the next morning, most of the convection on the storm's periphery dissipated; with persistent thunderstorms near the center and favorable anticyclonic outflow, Ana became a tropical storm about to the southeast of Myrtle Beach.
Dry air and northwesterly wind shear plagued the system, keeping the strongest thunderstorms confined to the eastern side of the circulation. Despite an improved presentation on satellite and radar imagery on May 9, wind speeds remained about the same. That night, the storm accelerated slightly toward the north-northwest and gradually weakened as it approached land. Diminished by the cooler waters of the continental shelf, Ana made landfall along the coast of South Carolina, between Myrtle Beach and North Myrtle Beach, at around 10:00 UTC on May 10. Over land, the storm turned toward the north in advance of an approaching trough. It weakened to a tropical depression on the afternoon of May 10. The system maintained well-defined rainbands as it curved northeastward,〔 passing through eastern North Carolina and into southeastern Virginia by the evening of May 11. After degenerating into a remnant low by 00:00 UTC on May 12, the system emerged off the coast of the Delmarva Peninsula back into the western Atlantic.〔 Accelerating northeastward, the low passed south of New England and merged with a frontal system near Nova Scotia late on May 12.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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