Sunday, October 29, 2017

Tropical Storm Philippe (2017)

Storm Active: October 28-29

On October 23, a broad area of low pressure formed in the southwestern Caribbean Sea. Since this is a favorable area for late-season development, it was monitored closely. The broadness of the low made organization quite slow, despite plenty of moist air and fairly favorable atmospheric conditions. In addition, the circulation spent the next few days in close proximity with the coast of Nicaragua, where it dropped heavy rains. As a result, it was not until October 28 that the disturbance became Tropical Depression Eighteen. By the time it formed, the cyclone was already accelerating toward the north and northeast under the influence of a trough over the United States. Conditions were still favorable though, and the system strengthened into Tropical Storm Philippe as it passed over western Cuba. The rain bands of the Philippe extended well to the north and east of the center, so Cuba and Florida has already been experiencing heavy rains. Early on October 29, the storm crossed south Florida and emerged into the Atlantic.

As Philippe approached the cold front to its north, upper-level winds increased to enormous values. The system quickly became elongated from north to south and dissipated during that afternoon before its remnants merged with a developing extratropical system off the coast of the Carolinas. Tropical moisture from Philippe contributed to an already powerful developing nor'easter, enhancing rainfall over many of the northeast and mid-Atlantic states. The storm ultimately brought heavy snowfall to parts of eastern Canada before dissipating.

Philippe was a disorganized but large tropical storm that brought heavy rainfall to Cuba and Florida.

While Philippe's time as a tropical cyclone was short-lived, it contributed to a large storm that affected the northeast U.S.

Monday, October 9, 2017

Hurricane Ophelia (2017)

Storm Active: October 9-15

Around October 6, an area of low pressure began to form along a stationary frontal boundary located over the eastern Atlantic. The next day, the system began to separate from the remainder of the frontal boundary, although it still displayed a long, curved, front-like band of convection emanating from the center. Slowly, it developed some subtropical characteristics as it drifted in the northeast Atlantic, moving little. By October 8, the low was on the verge of tropical or subtropical cyclone status and satellite data indicated gale-force winds near the center. Overnight, a region of shower activity persisted just east of the center. Available information pointed to winds just below tropical storm strength, so the system was designated Tropical Depression Seventeen. Banding features started to appear during the morning of the 9th, and the depression was upgraded to Tropical Storm Ophelia.

Initially, Ophelia was moving slowly to the north and northeast in a region of weak steering currents. Over the next day, however, a mid-level ridge built in northwest of the cyclone and it turned toward the southeast. Meanwhile, shear was diminishing and convection was able to wrap around the center, which allowed for some strengthening. The largest inhibitor to development was some dry air inside the circulation. Interaction with this dry air caused intensity fluctuations on October 10, but deeper convection completely enclosed the center that night. At the same time, an eye feature formed, and Ophelia strengthened more rapidly. During the afternoon of October 11, Ophelia reached hurricane strength, becoming the tenth consecutive tropical cyclone of the 2017 season to develop into a hurricane.

Steering currents collapsed later that day as well, leaving the system to drift slowly eastward overnight. The appearance of deeper convection near the center suggested that additional strengthening had occurred. Sea surface temperatures remained just lukewarm, but unusually cool upper atmospheric temperatures created a steep enough gradient to support intensification. On October 12, Ophelia reached category 2 status, an unprecedented achievement for a hurricane so far northeast that late in the hurricane season. The cyclone began to gradually accelerate east-northeast overnight, reaching an intensity of 105 mph winds and a pressure of 970 mb. The eye clouded over briefly the morning of the 13th, but this was a short-lived trend. Later that day the eye cleared out and became even better defined, with deep convection completely surrounding the center. As a result, Ophelia maintained its remarkable category 2 status even farther north and east.

The system was not finished, however. A final burst of intensification on October 14 brought Ophelia to major hurricane strength, and it reached a peak intensity of 115 mph winds and a pressure of 960 mb. In doing so, it became the easternmost major hurricane ever recorded. The gap between it and its predecessors was even more impressive in its latitude range, where it was 900 miles farther east than any previous major hurricane. Finally, early on October 15, much colder waters and higher shear began to weaken Ophelia and induce extratropical transition. Later that day, the storm became extratropical as it sped toward Ireland. The center made landfall in southwest Ireland during the morning of October 17, bringing damaging hurricane-force winds. Since the system was moving at over 40 mph, it quickly passed over Ireland and the UK. The post-tropical cyclone brought gale force winds all the way to Scandinavia before finally dissipating.

Hurricane Ophelia is shown above as a major hurricane near the Azores, an unprecedented event in Atlantic hurricane records dating back to 1851.

Ophelia was no longer a tropical cyclone when it reached Europe (triangle points) but it still brought hurricane-force winds to many parts of Ireland.

Wednesday, October 4, 2017

Hurricane Nate (2017)

Storm Active: October 4-9

During the last week of September, a tropical wave tracked across the Atlantic Ocean. As the season was progressing to its later stages, conditions were less favorable over the open Atlantic, but the wave continued across into the Caribbean. On October 3, its southern end began interacting with a vorticity called a monsoonal gyre in the southwestern Caribbean, just north of Panama. This interaction led to increased spin, and a combination of low wind shear and warm waters supported further development. During the morning of October 4, Tropical Depression Sixteen formed east of Nicaragua. Over the next day, it moved slowly northwest. Lacking an inner core, the system was not able to develop quickly, but it did become Tropical Storm Nate early on October 5.

Shortly after, the center made landfall in Nicaragua, bringing heavy rainfall to the country as well as neighboring Honduras. Late that night, it reentered the Caribbean, prompting some intensification as convection increased. Meanwhile, Nate was accelerating toward the north-northwest. Its rapid speed hampered strengthening somewhat, but parts of an eyewall appeared on the 6th in the southern and western quadrants, and the system became a strong tropical storm. That evening, it passed just east of the Yucatan Peninsula. Fortunately, the part of the circulation over land was the weaker western side, minimizing damage. However, the lack of significant land interaction allowed Nate to continue to intensify steadily and become a hurricane around midnight. Conditions were still favorable in the Gulf of Mexico, but the fast-moving system had difficulty assembling a complete eyewall. Winds still were increasing to the north and east of the center, though, and the central pressure continued to fall. At midmorning, Nate reached its peak intensity of 90 mph winds and a pressure of 981 mb.

Slight weakening occurred over the next few hours, but the system still made landfall as a hurricane that evening near the Mississippi river delta. Once inland, it moved quickly north-northeast and weakened. Nate was tropical depression strength by midday on the 8th and became post-tropical early the next morning as it sped toward the mid-Atlantic states. The system brought precipitation to a large swath of the eastern U.S., but its rapid motion mitigated flooding. The remnants of Nate dissipated soon after.

The above image shows Hurricane Nate near peak intensity shortly before its Gulf coast landfall. The system's rapid motion prevented further organization.

Though Nate reached hurricane strength only in the Gulf of Mexico, its worst impacts occurred in central America, where prolonged heavy rains caused extensive flooding.