Monday, September 23, 2019

Hurricane Lorenzo (2019)

Storm Active: September 22-October 2

On September 22, a vigorous tropical wave over Africa emerged into the far east Atlantic. Immediately, it showed signs of organization, and was classified Tropical Depression Thirteen that night while located southeast of the Cabo Verde Islands. The next day, it developed spiral bands and steadily strengthened, earning the name Tropical Storm Lorenzo that afternoon. Upper-level winds, sea surface temperatures, and humidity were all in Lorenzo's favor as it continued to intensify. The cyclone turned west-northwest on September 24 as it followed the boundary of a mid-level ridge. Later that day, Lorenzo achieved hurricane status. The cyclone was also quite a large hurricane, with tropical storm force winds extending over 200 miles from the center of circulation.

On September 25, an eye feature began to form on satellite imagery. This led to more rapid strengthening shortly thereafter. Lorenzo was able to overcome a small dry air intrusion and stabilize its eyewall overnight. By mid-morning on the 26th, it had rocketed to major hurricane strength, becoming the 3rd of the 2019 season. Nor did the rapid increase in strength stop there. By later in the day, Lorenzo was a category 4 hurricane. At that point, it was the second easternmost forming category 4 on record, behind only Hurricane Julia of 2010. In fact, by that evening, the system was brushing up against the maximum theoretical intensity for a hurricane forming in that region, given the sea temperatures and atmospheric conditions. Overnight, it peaked with 145 mph winds and a pressure of 937 mb. Meanwhile, the cyclone followed a very well-forecast curve toward the north into a weakness in the aforementioned ridge.

An eyewall replacement cycle commenced on September 27, weakening the hurricane back to a category 3 as the inner eyewall convection became ragged and asymmetric but expanding its windfield. The eye itself also clouded over on visible imagery. This weakening may have been exacerbated by southwesterly shear, but Lorenzo came back with a vengeance on the 28th, completing the replacement and developing very cold cloud tops in the new eyewall. With this organization came a truly extraordinary burst of strengthening in which the cyclone's winds increased 40 kt in just 12 hours. At its peak that night, Lorenzo was a category 5 hurricane with sustained winds of 160 mph and a minimum pressure of 925 mb, making it, by far, the easternmost category 5 ever observed in the Atlantic.

By September 29, the storm was curving northward and then north-northeastward into cooler waters and a more stable atmosphere, causing a steady decline of the maximum winds. The large storm continued to grow in size, however, as it gained latitude. On the 30th, Lorenzo, now a category 2 storm, began to accelerate northeastward toward the Azores islands. Some signs of extratropical transition were evident by October 1, but the storm still had a vigorous core of convection. That night, the center passed near the western Azores, by which time the wind radii were huge: hurricane force winds extended up to 150 miles from the center and tropical storm force winds up to 390 miles. Several of the islands experienced damaging winds and very high surf from the massive circulation. After passing to the northeast, Lorenzo became extratropical. Late the next night, the storm slammed into Ireland, bringing hurricane force wind gusts. The storm later passed over the UK before finally dissipating.

The above image shows Lorenzo as a category 4 hurricane over the eastern Atlantic.

Lorenzo did not affect any land for most of its life, but ultimately became one of the most powerful cyclones on record to affect the Azores when it passed over the western islands as a large category 2 hurricane.

Sunday, September 22, 2019

Tropical Storm Karen (2019)

Storm Active: September 22-27

A tropical wave that moved over the Atlantic on September 15 began to generate shower activity around the 18th while located a little less than halfway to the Lesser Antilles. This disturbance moved very quickly west over the next few days and its thunderstorms were popping up mainly south of the center. However, on the 21st it developed a well-defined circulation center. By the next morning, the associated convection had enough organization for the system to be classified Tropical Storm Karen. At the time of naming, the center was located just northeast of Tobago.

Karen was highly disorganized as it crossed the southern Windward Islands that day due to strong northeasterly wind shear. Nevertheless, it brought scattered downpours and gusty winds to nearby Caribbean Islands as it traveled west-northwestward. Thunderstorms regenerated near the center of circulation late on the 22nd, but the center lost some definition overnight. In fact, Karen came close to degenerating into an open wave early on the 23rd, when aircraft reconnaissance was unable to identify a well-defined center. In addition, the system weakened to a tropical depression. Despite all this, the broader circulation persisted and Karen came back to life later that day as towering bursts of thunderstorms erupted south of the center. Overnight, it restrengthened to a tropical storm. At the same time, the cyclone turned northward in the wake of Jerry and approached Puerto Rico and the Virgin Islands.

The center of Karen remained difficult to locate that day, in part due to the presence of several small mesovortices rotating about one another inside the broader circulation. This lack of a compact central core prevented significant strengthening, but thunderstorm activity was much more vigorous. Karen passed near eastern Puerto Rico, bringing 5 inches of rain to portions of the island and the nearby Virgin Islands. The cyclone continued steadily northward, though, and quickly emerged into the open western Atlantic. Its motion moved slightly toward the right that day. Despite at least somewhat favorable conditions, Karen did not strengthen, possibly due to an east-west elongation of its center of circulation. Convection continued to occur, but it did so in an amorphous blob, without organized banding structure. As a ridge built to its north, Karen slowed down and veered eastward. Simultaneously, upper-level atmospheric conditions became more hostile. On September 27, the cyclone weakened to a tropical depression as the center lost even more definition. Later that day, it degenerated into a remnant low. The remnants dissipated completely soon after.

The above image shows Tropical Storm Karen near Puerto Rico. Fortunately, the system was relatively weak and moving quickly, sparing the island significant damage.

Karen had organizational issues throughout its life and was ultimately unable to survive the high shear conditions of the western Atlantic.

Tuesday, September 17, 2019

Tropical Storm Imelda (2019)

Storm Active: September 17-19

Around September 13, an upper level low was located over the eastern Gulf of Mexico. This feature produced an area of showers and storms west of the Florida peninsula and also influenced the path of Tropical Storm Humberto to its east. Over the next few days, it moved westward, out of range of Humberto and into more favorable conditions for tropical cyclone genesis. By September 16, the disturbance was offshore from the Texas coastline, where it began to bring beneficial rainfall to southeastern Texas. Pressures began to fall in the area and a surface low formed early on September 17. Despite its proximity to land, the system was classified Tropical Depression Eleven early that afternoon. At the time of formation, its center was nearly over the coastline. Land observations indicated sustained winds near 40 mph, so the storm was named Tropical Storm Imelda at landfall around 1:00pm local time. Despite the fact that Tropical Depression Ten (which was to be named Jerry) formed slightly earlier the same day east of the Leeward Islands, Tropical Depression Eleven became a tropical storm sooner and therefore received the "I" name.

The center pushed slowly northward inland that evening and Imelda was downgraded to a tropical depression. Despite this, the circulation remained quite vigorous and it generated very heavy rain over portions of eastern Texas for the next several days. Finally, on September 19, it degenerated into an open wave, and by the 20th rains had mostly come to a close. However, this was not before Imelda dumped rain totals over 40 inches in a few locations over southeast Texas, with a large swath reporting 20 inches or more. Devastating flooding followed these rains, analogous to (though not as widespread or intense) the flooding following Hurricane Harvey in 2017.

The above image shows Tropical Storm Imelda shortly after landfall.

Imelda peaked as a minimal tropical storm, but its persistent circulation moved great swaths of Gulf moisture over southeast Texas and southwest Louisiana, inundating the region.

Hurricane Jerry (2019)

Storm Active: September 17-25

On September 13, a tropical wave located south of the Cape Verde Islands began to produce shower and thunderstorm activity. It moved generally west-northwestward over the following days and merged with another disturbance located to its west-southwest. The resulting system produced a vigorous but elongated area of convection and was slow to organize further. A small low pressure center appeared along the wave axis on September 15. Gradual improvement continued until the system was classified Tropical Depression Ten during the morning of September 17. During the evening, very cold cloud tops exploded throughout the circulation and winds increased. Tropical Depression Ten became Tropical Storm Jerry (though Tropical Depression Ten formed before Tropical Storm Imelda, the latter system strengthened to a tropical storm and stole the "I" name first).

Jerry faced little wind shear and had the advantage of warming ocean waters for strengthening. The only inhibiting factor was a fairly dry atmosphere, but in a low shear environment, cyclones are often able to "wall off" dry air from disrupting their circulations. The cyclone had impressive outflow and nascent banding features by the morning of September 18, with one notable feature arcing northeast from the circulation center. Hence, steady intensification occurred that day. A significant burst of convection occurred near the center that afternoon. This allowed a inner core to develop and winds increased some more overnight, bringing Jerry to hurricane strength on September 19 as it continued its journey west-northwestward. The cyclone peaked that evening as a category 2 hurricane with maximum sustained winds of 105 mph winds and a minimum pressure of 976 mb.

The next day, wind shear out of the northwest increased substantially and destabilized Jerry's core. Thunderstorm activity was pushed to the southeastern side of the circulation and the system quickly weakened. Nevertheless, bands of locally heavy rain swept across the Northern Leeward Islands through the afternoon and early evening as Jerry passed to the north. The cyclone remained on the smaller side so damage was minimal. The circulation fought back some against shear that evening with new deep convection blossoming about the core. However, maximum winds were still decreasing and the system weakened to a tropical storm by September 21. Following a weakness in the subtropical ridge in part created by Hurricane Humberto, Jerry turned toward the northwest that day and north-northwest on the 22nd. During that time, convection fluctuated but shear out of the west or northwest was a constant for the system. As a result, it remained a strong tropical storm.

On September 23, Jerry turned due north as its center became exposed to the west side of its thunderstorm activity. The atmosphere only became more hostile over the next day as drier air was entrained into the circulation from the southwest. Soon, the system's center was little more than a naked swirl of clouds, and weakening commenced by the 24th. The system began to move northeast and approach Bermuda. Fortunately for the island, which had just been hit by Hurricane Humberto last week, there was almost no rain associated with Jerry, and at worst some tropical storm force wind gusts. In fact, lacking convection for over 12 hours, the cyclone was classified post-tropical early on September 25. The center passed near Bermuda late that day, causing minimal impacts.

This image shows Hurricane Jerry approaching the northern Leeward Islands. Even near peak intensity, the effects of shear are evident in the relatively dry western semicircle.

Jerry brought some heavy rain to the northeasternmost Caribbean islands, but did not have major land impacts.

Monday, September 16, 2019

Hurricane Humberto (2019)

Storm Active: September 13-19

Starting the first week of September, a trough of low pressure tracked across the central Atlantic. It took a route a little north of the typical train of tropical waves and passed north of the Caribbean islands. The disturbance was moving over warm water, but wind shear was high in the area and little organization occurred. Matters improved slightly by the time the system reached the southeastern Bahamas on September 12 and it developed a circulation center. An upper-level low over the eastern Gulf of Mexico was still shearing the system, but it found a pocket of favorable conditions and was classified Tropical Depression Nine on September 13 while located over the northwestern Bahamas. Fortunately for those islands just devastated by Hurricane Dorian, most of the rainfall occurred north and east of the center.

The steering flow was rather complex and difficult to predict due to the influence and evolution of the aforementioned upper-level low. However, it soon became clear that a weakness was developing in the subtropical ridge to the north of the depression, allowing it to turn toward the north on September 14. Meanwhile, the system strengthened into Tropical Storm Humberto and became more symmetric as shear diminished. By the 15th, a central dense overcast had covered the center of circulation on satellite imagery and it gained strength more quickly. That evening, the storm was upgraded to a category 1 hurricane. A strong autumn-like trough over the U.S. east coast turned Humberto sharply to the northeast out to sea that night. A developing eye had difficulty closing off on satellite imagery the next day due to apparent dry air intrusions as Humberto consolidated its inner core. Its central pressure dropped steadily, but aircraft reconnaissance measurements indicated that winds lagged behind somewhat. This was in part due to the significant expansion of the cyclone's windfield on the 16th. Nevertheless, anomalously warm subtropical Atlantic waters supported Humberto's ascent to category 2 status by the morning of September 17.

That day, a large eye finally broke through and hurricane hunters found that the system had become a major hurricane. It accelerated some to the east-northeast overnight and Bermuda began to experience wind and rain from the system's outer edge early on September 18. Conditions worsened quickly on the island and it was experiencing hurricane force winds by late that afternoon. Humberto was already taking on a more extratropical appearance, losing its eye and becoming asymmetric. Nevertheless, it maintained its strength as it made its closest approach to Bermuda late on the 18th. Sustained winds of 100 mph and higher gusts were measured on the island despite the center of Humberto passing a few dozen miles to the north. Just after moving away from the island, the storm reached peak intensity of 125 mph winds, measured with a pressure of 952 mb (up from a previous minimum of 951 mb). It turned northeast, accelerated further, and lost most of its central convection to strong wind shear on the 19th. This also resulted in gradual weakening and Humberto was extratropical by the afternoon of September 19. The remnant low sped northeastward toward the frigid north Atlantic before merging with another system.

The above image shows Hurricane Humberto as a large and dangerous major hurricane in the western Atlantic.

Humberto's large wind radii resulted in extensive impacts in Bermuda even as the center of circulation passed to the north.

Tuesday, September 3, 2019

Tropical Storm Gabrielle (2019)

Storm Active: September 3-10

As August came to an end, another tropical wave moved over the Atlantic waters from its origin in west Africa. A circulation quickly developed over the next few days, but thunderstorm activity remained rather scattered. The system tracked west-northwest and more intense convection appeared on September 3. It was therefore designated Tropical Depression Eight. Due to a weakness in the Bermuda high about halfway Bermuda and the Azores, the cyclone was able to steadily gain latitude on a northwest heading over the next several days. The waters under the system were hovering right around the benchmark 80 ° F (26.6 ° C) typically needed for tropical development and the atmospheric conditions were only marginally favorable. Despite this, Seven strengthened steadily into Tropical Storm Gabrielle on September 4.

Southerly shear helped to give the storm a "comma" shape, with convection nearest the center on the northwest side, arcing off to the northeast. It lost some convection that day as shear increased a bit more, making conditions actually fairly hostile. Nevertheless, Gabrielle maintained its intensity into September 5. During the day though, shear eliminated any remaining cloud cover from near the system. Devoid of convection for 24 hours, the system was briefly declared extratropical during the morning of September 6. However, the storm redeveloped just 6 hours later as thunderstorm activity reappeared. Meanwhile, the storm continued northwest, but accelerated a little. Shear from the south decreased the next day, only to be replaced by strong shear out of the northeast! In fact, this drove a reformation of the center of the circulation farther west that morning.

Conditions finally improved a bit for Gabrielle later that day, and it strengthened some overnight. As it gained latitude, the system moved around the edge of the high pressure to its east and began to recurve northeastward. Meanwhile, it reached its peak intensity of 65 mph sustained winds and a pressure of 995 mb. By the morning of September 9, Gabrielle was speeding off toward the northeast. It weakened gradually in response to increasing wind shear and cooler ocean temperatures that day. Extratropical transition began that day and completed during the morning of September 10. The remnants of Gabrielle continued northeast until dissipation within a couple days.

Gabrielle was a relatively small cyclone, beset by shear most of its life.

Gabrielle did not affect any landmasses as a tropical cyclone.

Tropical Storm Fernand (2019)

Storm Active: September 3-4

During the last couple days of August, a broad low pressure system located near western Cuba produced scattered storms in the neighboring southeastern Gulf of Mexico and northwestern Caribbean. The disturbance moved generally west over the next few days, passing north of the Yucat√°n Peninsula. On September 2, thunderstorms increased near the low. Conditions in the western Gulf of Mexico were favorable for development and Tropical Depression Seven formed during the morning of September 3. Deep convection blowups appeared west of the circulation center that afternoon. These were measured to contain tropical storm force winds, so the system was upgraded to Tropical Storm Fernand. Even in the face of southeasterly wind shear, the system intensified a little more that evening, reaching its peak intensity of 50 mph sustained winds and a pressure of 1000 mb. Heavy rain bands in Fernand's western semicircle had already swept across northeastern Mexico by that point.

Since the heaviest thunderstorms were ahead of the center, they moved over land first, and Fernand's maximum winds began to drop ahead of its landfall along the Mexican coast during the afternoon of September 4. The center crossed the coast about 150 miles south of the Texas border. Fernand's circulation quickly lost definition after landfall and it weakened to a tropical depression. Late that evening, it dissipated. The cyclone caused some flash flooding in the mountainous terrain of northeastern Mexico, where over 10 inches of rain fell in some places.

The primary threat from Tropical Storm Fernand was heavy rainfall.

Fernand had only a day over water before its landfall in Mexico, but managed to strengthen into a moderate tropical storm.