Sunday, September 23, 2018

Hurricane Leslie (2018)

Storm Active: September 23-25, September 28-October 13

On September 18, an extratropical system associated with the remnants of Hurricane Florence moved away from the U.S. east coast over the tropical Atlantic. A new low formed along the frontal boundary around September 22 in the central subtropical Atlantic. Over the next day, the low developed spiral banding and lots its frontal nature. By the morning of September 23, it had transitioned into Subtropical Storm Leslie. This was the sixth subtropical storm of the 2018 season, setting a new record.

At the time of formation, Leslie was drifting westward, but steering currents were quite weak and it turned southward and ultimately eastward over the next day. The system had never had much in the way of deep convection, but what was there diminished further by September 25. Meanwhile, a new front was approaching from the west and interacting with Leslie, elongating its circulation. By late that morning, the system had become extratropical. Upon transition, it underwent a rapid burst of the strengthening and was producing hurricane force winds by the 26th. Since it was non-tropical, however, it was not designated a hurricane.

At the same time it continued to turn toward the north and then back west. Conditions were still fairly favorable for tropical cyclone development so it began to transition back the next day. On September 28, enough deep convection had reappeared near the center for Leslie to again be classified as subtropical. However, its maximum winds had subsided back to around 50 mph, so this was the initial intensity. The system moved slowly southwest over the next few days and gradually developed more banding features south and east of the circulation center. Leslie transitioned to a fully tropical storm for the first time on September 29. Sea surface temperatures increased and wind shear decreased along the storm's path, leading to some slow strengthening over the next few days as thunderstorms finally wrapped entirely around the center.

By October 2, Leslie was approaching hurricane strength and had dipped in latitude to below 30° N due to its unusual southwestward motion. A ragged eye formed that evening and the system was upgraded to a hurricane for the first time. Overnight, the cyclone became stationary around 500 miles east-southeast of Bermuda. It also peaked in intensity at maximum sustained winds of 80 mph and a central pressure of 975 mb. Due to the influence of an upper-level low pressure system to the north, Leslie began to move northward on October 3. This motion took it over cooler water, and convection waned again, with a shallow ring of convection around the center separated from the outer bands by a "moat" of dryer air. Leslie began to weaken as a result and soon was a tropical storm again.

Although the system was still quite distant from any landmasses, the large size of the circulation generated significant ocean swells that led to rough surf in Bermuda and even the east coast of North America. Leslie stalled again about 450 miles northeast of Bermuda on October 5, and began to feel the influence of the mid-latitude westerlies. The cyclone turned sharply eastward that day. Meanwhile, the structure of the storm had changed quite a bit; a central area of strong thunderstorms had replaced the large eye, and a large area of convection persisted to the north of the center. Leslie began to separate from a trough to its north and turned south of east on October 7. The storm accelerated southeastward over the next day, bringing it over warmer waters, and it began to restrengthen.

The cyclone developed a central dense overcast on October 8 and approached hurricane strength on the 9th, achieving category 1 status that evening a week after doing so the first time. Leslie turned due south for a little while on the 10th, reaching a southernmost latitude of 27.8 ° N. However, another trough moving to its north turned the system east-northeast and began to accelerate it toward the far eastern Atlantic. The inner core structure fluctuated a great deal in organization during the following day, but overall it became a bit better defined and Leslie strengthened somewhat. Late on October 11, Leslie reached its peak intensity as a top-end category 1 hurricane with 90 mph winds and a pressure of 969 mb.

The system picked up even more speed the next day and colder waters weakened the storm's convection. A tropical storm warning was issued for the island of Madeira, located southwest of Portugal. This was the first ever warning issued for the island and Leslie was the first known tropical cyclone ever to affect it in modern history. The center passed north of Madeira later on the 12th. Finally, on October 13, Leslie transitioned to an extratropical low just before making landfall in northern Portugal. This transition did not prevent the cyclone from bringing hurricane force winds gusts and heavy rain to the Iberian Peninsula. The low finally dissipated inland a few days later.

This image shows Leslie during its second and final stint as a hurricane, moving east-northeastward toward Europe.

Leslie's convoluted track included some highly unusual southward dips over the central Atlantic. Just after becoming extratropical, it moved over the Iberian Peninsula, though this is not shown above.

Saturday, September 22, 2018

Tropical Storm Kirk (2018)

Storm Active: September 22-24, 26-28

On September 21, a strong tropical wave entered the Atlantic. It began to travel westward at quite a low latitude, far to the south of the Cabo Verde Islands. It organized quickly and developed a closed circulation the next day. Satellite measurements indicated gale force winds at that time, so the system was named Tropical Storm Kirk on September 22. At the time of naming, Kirk's latitude was 8.3 °N, quite close to the equator for tropical cyclone genesis in the Atlantic. In fact, no north Atlantic system on record had reached tropical storm strength so far south since 1902.

Shortly after formation, a ridge to the north of Kirk began to accelerate it westward. Due to its fast forward motion and its proximity to the equator, the system had difficulty acquiring much spin, although some evidence of curved banding started to appear on September 23. After that time though, it struggled to maintain deep convection. In addition, its forward speed continued to increase to over 20 mph. On the 24th, this caused the circulation to fall apart and Kirk dissipated.

The remnants continued quickly westward and sea surface temperatures increased while upper-level winds remained fairly favorable. As a result, the system began to reorganize. On September 26, a well-defined center reformed and Kirk regained tropical storm status. It even underwent some strengthening that day to its peak intensity of 60 mph winds as measured by hurricane hunter aircraft. However, its window of favorable conditions was short-lived. By early on September 27, it had entered the area of wind shear that dominated the east Caribbean and its immediate vicinity for most of the season. This quickly exposed the center of circulation as convection was displaced eastward. Nevertheless, the center passed over the Lesser Antilles that day and deep convection continued to flare up in the eastern semicircle, bringing heavy rains and gale force winds to the islands, some of which were still recovering from the passage of Hurricane Maria the previous year.

After entering the Caribbean, Kirk began to lose organization more quickly. Late on September 28, the system dissipated in the eastern Caribbean.

The above image shows Kirk in its peak intensity during its second stint as a tropical cyclone.

Kirk dissipated in the eastern Caribbean, just as several of its predecessors.

Friday, September 21, 2018

Tropical Depression Eleven (2018)

Storm Active: September 21-23

On September 18, a disturbance developed in the tropical Atlantic well east of the Windward Islands. It moved generally west-northwestward over the following view days and began to exhibit a small but organized canopy of thunderstorm activity. A weak low pressure center appeared on September 20, but wind shear increased significantly around the same time and the atmosphere was quite dry as the system approached the Caribbean. Despite unfavorable conditions, sheared convection persisted near the surface circulation center long enough the next day for the system to be classified Tropical Depression Eleven.

Before long, upper level winds out of the west increased even further, and any bursts of deep convection from Eleven were swiftly blown away. The center moved erratically the next day and began to lose definition, and by the morning of September 23, the system was downgraded to a remnant low. This low dissipated soon after.

Tropical Depression Eleven never achieved much organization during its brief lifetime.

The area of high wind shear over the Caribbean was persistent during the 2018 season, and claimed Eleven as another victim.

Thursday, September 13, 2018

Tropical Storm Joyce (2018)

Storm Active: September 12-18

On September 11, a non-tropical low formed along a frontal boundary situated over the north central Atlantic, well west of the Azores. The low drifted generally southward or southwestward over the next day. By September 12, it was producing gale force winds and displayed some organized convective banding. However, the surface low was still colocated with an upper-level low, not the high found for tropical cyclones, so it was designated Subtropical Storm Joyce. Existing contemporaneously with Florence, Helene, and Isaac, the new system was one of four named storms simultaneously occupying the Atlantic basin. This was the first time this had occurred since 2008. Also, Joyce was the fifth subtropical cyclone of the season, the first time that had happened since 1969.

Shortly after formation, the cyclone felt the influence of the much larger Tropical Storm Helene to its southeast. Steered around the periphery of its circulation, Joyce moved west-southwest and then turned south around Helene's left side as it reached the same latitude. Meanwhile, on September 13, Joyce transitioned into a tropical storm. The next day, it strengthened slightly to a moderate tropical storm and turned eastward in the wake of Helene. This intensification was short-lived, however, as increasing wind shear out of the southwest stripped away the little convection that formed in bursts near the center of circulation. On September 16, Joyce weakened to a tropical depression.

The shallow system was left behind by now ex-Helene and instead followed the boundary of a mid-level high located in the subtropical Atlantic. This caused the depression to turn south of due east by September 17 and then south by the 18th. Late on September 18, Joyce had ceased to produce deep convection and was finally downgraded to a remnant low. The low moved slowly southwestward until dissipation.

The above image shows the small Tropical Storm Joyce as well as the edge of the larger Tropical Storm Helene to the east, which greatly influenced Joyce's motion.

Joyce did not affect any landmasses during its journey through the northeast Atlantic.

Friday, September 7, 2018

Hurricane Isaac (2018)

Storm Active: September 7-15

At the beginning of September, a tropical wave moved into the Atlantic. Though it was producing some showers and thunderstorms, but the disturbance remained disorganized for several days as it traveled westward. By September 5, the low had become better defined, but convection was quite limited near the center of circulation. On September 7, the disturbance was classified Tropical Depression Nine. That day, it also became nearly stationary as steering currents collapsed and it felt the pull of newly formed Tropical Depression Eight (which would become Helene) to its east. The center was nearly exposed at first due to shear, but the system's organization increased considerably on September 8. This prompted an upgrade to Tropical Storm Isaac.

The storm was very small, with tropical storm force winds extending only a few dozen miles from the center. Such storms are subject to rapid changes in intensity, and Isaac did gain strength quickly over the next day. Meanwhile, it finally picked up some forward speed toward the west. During the evening of September 9, it was upgraded to a hurricane. There was little change to the system over the next day, despite generally quite favorable conditions. The banding structure and core of Isaac struggled to improve, even with low wind shear. Soon, the satellite presentation became more ragged in appearance and the system was downgraded back to a tropical storm.

Early on September 12, an upper-level trough north of Isaac caused a sudden increase in shear on the system, quickly stripping convection away from the center. The cyclone began to rapidly weaken as a result. Nevertheless, it caused scattered heavy rains and tropical storm force winds as it passed among the Leeward Islands and entered the Caribbean during the morning of September 13. Thunderstorm activity started to make a comeback near the circulation center later that day, but the circulation itself was ill-defined and showed signs of becoming elongated. Isaac weakened to a tropical depression on September 14. Soon after, all traces of a closed circulation vanished and the storm dissipated. The remnants of Isaac brought scattered thunderstorms to Jamaica a few days later.

This image shows the small Tropical Storm Isaac moving over the open Atlantic.

Isaac dissipated shortly after entering the eastern Caribbean. This is a quite common event and this region is often referred to as a "tropical cyclone graveyard" by meteorologists.

Hurricane Helene (2018)

Storm Active: September 7-16

On September 6, an extremely large tropical wave entered the Atlantic basin from west Africa, to which it had brought torrential rains over the past few days. By the time it hit water, the system already possessed a clear spin, and only a lack of central convection lay between it and tropical cyclone designation. This was immediately remedied on September 7 as more concentrated thunderstorms developed near the center. That afternoon, it was designated Tropical Depression Eight, only a few hundred miles west of Senegal. The massive system consolidated fairly quickly for its size and strengthened into Tropical Storm Helene shortly after.

There was some Saharan dry air to the north of Helene, but convection still managed to wrap around the center on September 8 and the storm continued to intensify. That evening, the center passed well south of the Cabo Verde islands, bringing some rain due to its large size, but sparing them from worse impacts. The next day, Helene was upgraded to a hurricane as it moved west away from the islands and it closed off a very large eye that evening. The cyclone turned toward the west-northwest by September 10. The eye also became better defined and Helene intensified into a category 2 hurricane. The next day, Helene reached its peak intensity of 110 mph sustained winds and a minimum central pressure of 966 mb.

By this time, a weakness in the subtropical ridge to the north of Helene (caused by a low over the northeast Atlantic) allowed the system to turn northwest and then toward the north. As it gained latitude, the cyclone encountered lower water temperatures and a drier atmosphere. It weakened to a category 1 on September 12, and to a tropical storm the next day.

Helene began to appear less tropical as it accelerated northward since it became increasingly asymmetric. Nevertheless, it maintained its status as a strong tropical storm into September 14. Soon after, rain bands in its eastern semicircle began to sweep across the Azores Islands. The center of circulation passed just west of the islands on September 15 and veered northeastward. Helene picked up more forward speed and transitioned into an extratropical cyclone over the cold northeastern Atlantic on September 16. The system eventually brought some stormy conditions to Ireland and the UK a few days later.

Helene was a large system, and developed an unusually large eye when it intensified into a hurricane.

Though it did not affect any large landmasses, Helene did have impacts in both the Cabo Verde and Azores Islands.

Monday, September 3, 2018

Tropical Storm Gordon (2018)

Storm Active: September 3-8

During the final days of August, a tropical wave whose axis extended from the Caribbean south of Hispaniola northward to the adjacent Atlantic waters began to produce increased thunderstorm activity throughout the region. However, upper level winds were quite strong over the wave. This prevented further development for a few days as it moved toward the west-northwest. The system passed over the southern Bahamas on September 2 and its satellite presentation improved markedly, though it still lacked a surface circulation. Only on September 3 did Florida radar detect a well-defined surface low, prompting the naming of Tropical Storm Gordon. At the time of naming, Gordon's center was over southern Florida. It moved over the Gulf of Mexico shortly afterward but continued to bring rain to the southern half of the peninsula for the remainder of the day.

While the core intermittently showed signs of an eyewall forming on radar, the overall satellite presentation of Gordon was lackluster as it moved through the Gulf over the next day. The radius of tropical storm force winds remained very small and heavy rain did not extend much beyond it. Nevertheless, maximum winds at the center reached strong tropical storm intensity by September 4. The waters of the Gulf of Mexico were quite warm, but fortunately Gordon's forward speed remained fairly high due to consistent steering patterns, and it moved quickly northwest. Late in the day, the cyclone reached its peak intensity of 70 mph winds and a pressure of 997 mb just before making landfall along the Gulf coast near the border of Mississippi and Alabama. Mississippi was spared most of Gordon's flooding rain impacts since almost all rain was to the east of the center. Some hurricane force gusts were also reported during landfall.

The system quickly weakened inland and became a tropical depression on September 5. Its movement slowed considerably once over land and it gradually moved northward through the midwest over the following couple of days, bringing heavy rainfall to a wide swath of the central U.S. before being absorbed.

The above image shows Gordon just before landfall on the Gulf coast.

Gordon's slow movement after landfall contributed to flooding rains throughout the central U.S.