Friday, November 1, 2019

Subtropical Storm Rebekah (2019)

Storm Active: October 30-November 1

During the last week of October, another nontropical low over the northern Atlantic slowed to a stop west of the Azores islands. The powerful low weakened some as it drifted generally southeastward over the next couple of days, but it moved over slightly warmer water. By October 30, the low had enough thunderstorm activity to be classified Subtropical Storm Rebekah. Though convection was concentrated in a band wrapping halfway around the center, the center was located under an upper-level low, an indicator of subtropical characteristics. The cyclone moved eastward at a good clip over the next day and changed little in intensity. On October 31, conditions near Rebekah degraded as it encountered cooler waters and strong upper-level winds. Deep convection vanished by that evening, and the storm became post-tropical early on November 1. By this time, the remnants were nearing the Azores, but impacts on the islands were minimal.

Lacking deep convection and colocated with an upper-level low, Rebekah was classified as a subtropical storm.
The above image shows the meandering track of Rebekah over the northern Atlantic. Square points represent times at which Rebekah was subtropical.

Saturday, October 26, 2019

Tropical Storm Pablo (2019)

Storm Active: October 25-28

On October 23, a non-tropical low centered several hundred miles west of the Azores began to deepen and produced strong winds and scattered rains over a large area of the northeast Atlantic. The low moved generally east-southeastward over the following days. On October 25, a small pocket of deep convection formed about the center of circulation. That afternoon, the system became Tropical Storm Pablo, a very small tropical storm in the middle of what appeared on a larger scale as an extratropical cyclone. Pablo was named at the same time as Tropical Storm Olga in the Gulf of Mexico, a new record for the latest in a season that two storms were named simultaneously. Though ocean temperatures were below the ordinary threshold for tropical cyclone development, a cool upper atmosphere gave the system enough instability to support tropical development.

Upon formation, the tiny Pablo had an even tinier eye feature on satellite imagery. Gale force winds associated with the tropical cyclone extended only a few dozen miles from the center, even though the parent extratropical system still was generating comparable winds in its much bigger northwest quadrant. On October 26, Pablo turned east and then northeast, strengthening a bit as it did so. Early that evening, the center passed close to the easternmost Azores islands, bringing additional rain, strong winds, and high waves. Defying expectations, the cyclone intensified further as the eye became better defined. During the morning on October 27, Pablo achieved hurricane status. At 42.8° N, 18.3° W, this was the furthest northeast any hurricane had ever formed on record. Pablo reached its peak intensity of 80 mph winds and a central pressure of 977 mb a few hours later. Meanwhile, the storm turned back toward the north and slowed down somewhat.

Soon after, even colder waters along Pablo's track at last caused convection to degrade. The small system suffered a rapid demise overnight, weakening to a low-end tropical storm and then becoming post-tropical early on October 28 when it degenerated into a swirl of low clouds. The remnant low drifted slowly northwest before it was absorbed by another low pressure system.

The above image shows Pablo near hurricane strength on October 27.

Pablo became a hurricane farther to the northeast than any previous cyclone on record.

Friday, October 25, 2019

Tropical Storm Olga (2019)

Storm Active: October 25

During the latter part of October, a tropical wave moved across the Caribbean and toward central America. As it passed near Belize around October 22, it began to exhibit disorganized thunderstorm activity. The system moved west-northwestward over the following days but did not develop further until emerging into the Bay of Campeche. On October 24, a circulation began to spin up in earnest over water west of the Yucatan Peninsula and convection increased markedly. Already, a autumn cold front approaching from the northwest was beginning to interact with the developing low. Nevertheless, it managed to become Tropical Depression Seventeen during the morning of October 25.

A few hours later it strengthened into Tropical Storm Olga. The front accelerated the newly formed Olga north-northeast that afternoon and its circulation elongated. Aircraft data collected during the evening suggested that the center had become embedded within the frontal boundary, and that a clear temperature gradient existed across the two sides of the circulation separated by the front axis. Thus, Olga was deemed post-tropical, only 12 hours after initially becoming a tropical cyclone. That night, ex-Olga made landfall in Louisiana. The combined system brought very heavy rain and widespread wind gusts to gale force, with some reaching 70 mph. Olga's tropical moisture drove up rain totals with the storm for the next few days as it pushed eastward.

The above image shows Tropical Storm Olga on October 25 interacting with the front.

Just like Nestor before it, Olga was a short-lived tropical cyclone in the Gulf of Mexico whose primary impacts occurred after merging with a front.

Saturday, October 19, 2019

Tropical Storm Nestor (2019)

Storm Active: October 18-19

Around October 12, a large low pressure area formed over the southwestern Caribbean sea. The system moved slowly northwest over the following days but land interaction with central America stifled any chance at development initially. It emerged into the Bay of Campeche on October 16 and began a gradual turn toward the northeast. The disturbance deepened over water, but it also began to interact with a trough of low pressure to its northeast across the northern Gulf of Mexico. This interaction spawned a cyclone with some tropical characteristics, but which was also highly asymmetrical. The pull of the front also caused the system to accelerate northeastward. Finally, during the afternoon of October 18, the cyclone began sufficiently tropical to be classified Tropical Storm Nestor.

Though rather disorganized and not resembling a classical tropical cyclone, Nestor gained a boost in strength from the nearby trough, pushing it to its peak intensity of 60 mph winds and a pressure of 996 mb late that evening. By this time, the storm was approaching the panhandle of Florida, already bringing rain and gale force wind gusts. Nestor lost its tropical characteristics the morning of October 19 as convection retreated well to the east of the circulation and became post-tropical. The post-tropical storm made landfall a few hours later. Soon after, it crossed the U.S. southeast and exited toward the open Atlantic waters.

The above shows the disorganized Nestor shortly after classification as a tropical cyclone.

Nestor's remnants spawned a few tornadoes over the U.S. southeast as the system passed through.

Wednesday, October 16, 2019

Tropical Depression Fifteen (2019)

Storm Active: October 14-16

On October 13, a large tropical wave emerged into the Atlantic, exiting western Africa. Ordinarily, tropical waves do not organize so far east by mid-October, but unusually warm waters and low wind shear allowed the disturbance to consolidate. During the afternoon of October 14, the wave developed into Tropical Depression Fifteen. The formation took place southeast of the Cabo Verde islands, making the depression one of the easternmost forming tropical cyclone ever observed so late in the year. It tracked northwest over the following day but changed little in organization. Having a very broad circulation, Fifteen struggled to develop deep convection. However, this did not prevent the cyclone from bringing locally heavy rains and gusty winds to the Cabo Verde islands on the 15th. The system's circulation became elongated soon after as atmospheric conditions began to deteriorate. Fifteen dissipated during the morning of October 16.

Friday, October 11, 2019

Tropical Storm Melissa (2019)

Storm Active: October 11-

On October 8, a non-tropical low pressure center formed along the western edge of a dissipating frontal boundary situated west-southwest to east-northeast across the western Atlantic ocean. When the low formed, it was located a few hundred miles off the North Carolina coastline. It moved north-northeastward over the next few days, deepened some, and absorbed another disturbance approaching from the south. Steering currents collapsed on October 10 and the system became almost stationary east of the mid-Atlantic coastline. Even as a non-tropical system, it brought dangerous ocean conditions and strong winds to the coastline, especially southern New England. There was not much in the way of rainfall associated with the low at first, but convection increased in a curved band north of the center early on October 11. Shortly afterward, the disturbance was classified Subtropical Storm Melissa, already with maximum winds of 65 mph and a central pressure of 995 mb.

That day, the cyclone drifted slowly southward, but soon westerly flow steered Melissa east and caused it to accelerate some away from the U.S. coastline. The structure changed some by October 12, with convection moving closer to the center of circulation. The structural change necessitated a reclassification of Melissa as a tropical storm. As the cyclone moved east, it encountered more hostile atmospheric conditions, which stripped away most of the thunderstorm activity. Melissa weakened into October 13.

As of 5:00pm EDT on October 13, 2019, Subtropical Storm Melissa had maximum sustained winds of 40 mph, a central pressure of 1003 mb, and was moving east-northeast at 18 mph. For more up-to-date information, please consult the National Hurricane Center.

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.