Friday, September 15, 2017

Hurricane Lee (2017)

Storm Active: September 14-18, 22-30

As is typical during mid-September, a strong tropical wave moved off of Africa and showed signs of organization by the morning of September 14. It was a fairly low-latitude system, passing well south of the Cape Verde Islands. The disturbance developed rather quickly, becoming Tropical Depression Fourteen that same night. After this, however, the system became a bit less organized, with the center becoming exposed to the north of the cloud canopy on September 15. However, as the system moved toward the west, it stayed south of the worst shear, and was able to slowly consolidate. Much more deep convection appeared during the morning of September 16, and the depression was upgraded to Tropical Storm Lee.

The system could not progress much further in its development, however, as upper-level winds renewed their assault from the north and west. The center once again became exposed later in the day, and the circulation was nearly devoid of thunderstorm activity by early on September 17. This caused Lee to weaken to a tropical depression. Pulses of convection intermittently covered the center over the next day but each was sheared away in turn. Thus,the storm maintained tropical depression status as it turned west-northwestward. Even this did not last, however. Late on September 18, Lee lost even more organization and degenerated into a remnant low, far away from land.

The circulation persisted over the next several days and turned northward, where atmospheric conditions eventually improved and there were still marginally warm waters. As a result, the remnants of Lee were able to regenerate into a tiny tropical depression in the middle of the subtropical Atlantic on September 22. Deemed the same system as before, Lee kept its name. Overnight, it once again became a tropical storm. At the time, the system was still drifting north. On September 23, however, a high pressure ridge was building north of the storm, and it quickly turned west and then began drifting south that night. Meanwhile, cool upper-atmospheric temperatures were helping Lee to strengthen (since they provided a large temperature differential with the marginally warm ocean waters). The small cyclone began a burst of rapid intensification and became a hurricane early on September 24 as an eye appeared on satellite imagery.

Although the satellite presentation was quite impressive for the small cyclone, the convection in the eyewall was not especially deep, and Lee leveled out as a strong category 1 later that day. The system also turned southeast before switching course yet again toward the southwest on the morning of September 25. Encountering cooler waters left it its own wake a few days previously, the storm weakened a bit that afternoon, but this trend was short-lived. The eye became quite well-defined that night and some deeper convection appeared. Turning westward and moving a bit faster, Lee became a category 2 hurricane on September 26. An eyewall replacement cycle then took place that evening, but the cyclone remained on the verge of major hurricane strength. By the morning of September 27, the eye had broadened and become more well-defined. As a result, Lee was upgraded to a major hurricane, the fifth of the season, and reached its peak intensity as a category 3 hurricane with 115 mph winds and a pressure of 962 mb that afternoon.

Shortly after, as Lee turned toward the north, the outflow from Maria to its west increased shear over the system, and a weakening trend began. Shortly after, the same trough that was pushing Maria out to sea picked up Lee as well, and it began to accelerate toward the northeast. The center became exposed to the northwest of the deep convection late on the 28th, and the system weakened to a tropical storm. As Lee rocketed into much colder waters on September 29, it quickly lost its remaining convection and tropical characteristics. The storm became post-tropical over the north Atlantic early the next morning as its forward speed exceeded 50 mph. It was absorbed by an extratropical low over the north Atlantic soon after. The combined system then brought rainy and windy conditions to the UK and Ireland a few days later.

This image shows Hurricane Lee at peak intensity over the open central Atlantic.

After a brief stint as a weak tropical storm, Lee redeveloped into a major hurricane.

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