Tuesday, November 7, 2017

Tropical Storm Rina (2017)

Storm Active: November 6-9

On November 3, a weak area of low pressure developed in the central tropical Atlantic, well away from any land areas. It moved slowly north and north-northeast over the following days and became better organized on November 5. Early in the morning on the 6th, the disturbance was organized enough to be classified as Tropical Depression Nineteen. The system was moving over marginal sea surface temperatures in an area of shear that was not too high, so modest strengthening occurred over the next day and the depression became Tropical Storm Rina overnight.

Rina began to accelerate northward on the 7th, passing the latitude of Bermuda almost 1000 miles to the east. Though sea surface temperatures were declining, the system's maximum winds increased some as it took on some subtropical characteristics. Rina reached its peak intensity of 60 mph winds and a pressure of 997 mb on November 8. Later that day it turned toward the north-northeast and began weakening as it transitioned to an extratropical system; it completed this transition during the morning of November 9. The system then turned eastward, eventually impacting the UK as a weak low before dissipation.

This image shows Tropical Storm Rina shortly after formation.

Rina did not affect any land areas during its lifetime.

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.

Sunday, September 17, 2017

Hurricane Maria (2017)

Storm Active: September 16-30

Maria formed from a tropical wave that first left Africa around September 10 or 11. At first, conditions did not support development of the broad system, but they steadily improved over the next several days. On September 15, the disturbance appeared much more organized on satellite imagery, and some rotation became evident. By the morning of the 16th, only a well-defined center of circulation separated it from tropical cyclone status. It cleared this hurdle during the afternoon, becoming Tropical Depression Fifteen. From this point, its maximum winds increased almost immediately, and the depression was upgraded to Tropical Storm Maria shortly thereafter.

The tropical storm was already quite large, though gaps remained in the satellite presentation of the cyclone in between rain bands. Despite this, the inner core strengthened fairly swiftly, and Maria became a strong tropical storm by the morning of September 17. That afternoon, a large burst of convective activity ignited near the center of circulation, overcoming the dry slot that had been hampering intensification. Soon, Maria had a well-formed eyewall and was upgraded to a hurricane. The outer bands were starting to affect the Lesser Antilles and the system continued west-northwestward toward the islands. September 18 saw incredibly rapid strengthening of Maria. In the morning, it strengthened into a major hurricane, and while an eye was apparent on radar, it had not yet cleared out on satellite imagery. The clearing came that afternoon; a very small "pinhole" eye developed, indicating a small core but extremely intense winds. Its intensity shot up through category 4, and Maria achieved category 5 intensity with 160 mph winds and a pressure of 924 mb during that evening. The eye then made landfall small island of Dominica in the Lesser Antilles.

Though small, the island was mountainous, and briefly disrupted Maria's core, bringing the intensity down slightly to category 4. However, as moved west-northwest into the Caribbean, its central pressure began to drop again, and the hurricane regained category 5 status early in the morning of September 19. Remarkably, the cyclone was not done intensifying: it became more symmetric on satellite imagery that day, and thunderstorm activity around the centered grew even further. That evening, Maria reached a peak intensity of 175 mph winds and a central pressure of 908 mb, one of the top ten lowest pressures ever recorded in an Atlantic hurricane at the time, even though its maximum winds were slightly weaker than those of Hurricane Irma a few weeks earlier.

By this time, the center was approaching Puerto Rico and the U.S. Virgin Islands. As is typical with powerful hurricanes, a secondary eyewall then formed and the inner one weakened somewhat, causing a decrease in maximum winds. When Maria made landfall in Puerto Rico early on September 20, it was a high-end category 4 hurricane with maximum winds of 155 mph, but the area of maximum winds had expanded in the wake of the eyewall replacement. Regardless, it was the strongest cyclone to make landfall in Puerto Rico since 1928. The hurricane brought extremely strong winds and damaging flooding rains to the island, causing several rivers to exceed their previous record stages. Nevertheless, land interaction took a significant toll on Maria and it quickly weakened over the next several hours. After traversing much of Puerto Rico from east-southeast to west-northwest, the center emerged over water early in the afternoon. The system had dropped to high-end category 2 strength, but reorganization began as it moved further northwest. A ragged eye developed by the evening and the circulation recovered some overnight, bringing Maria back up to major hurricane strength early on September 21. The southern portion of the circulation brought widespread tropical storm conditions and occasional hurricane conditions to the Dominican Republic that day.

The cyclone then veered northwest, moving away from Hispaniola. Some modest strengthening ensued, though the eye of the hurricane was quite unstable and actually clouded over that night. Wind shear out of the southwest was disrupting the system. Early on September 22, the center passed just east of the Turks and Caicos islands. Following the weakness in the ridge to its north left by Jose before it, Maria moved north-northwest that day. No significant changes in strength occurred through September 23, although the pressure and winds fluctuated. In fact, the central pressure decreased, but the maximum winds found in the eyewall were not as strong as before. As a result, Maria was downgraded to a category 2 on September 24. Later that day, Maria's structure took a more significant hit as the center moved over the cold wake left by Jose and the northwestern eyewall collapsed. As a result, the hurricane weakened to a category 1 overnight as it continued slowly northward. The center was nearly exposed late on the 25th, but the storm maintained minimal hurricane strength.

Maria finally weakened to a tropical storm on September 26, as the outer edge of its tropical storm wind field brushed the Outer Banks of North Carolina. Since almost all thunderstorm activity was displaced to the north and east of the center, there were few land impacts. Winds actually increased for a brief period on September 27, and the storm regained hurricane strength. This was short-lived though; it was a tropical storm again the next morning. After moving north at a crawl for several days, Maria finally began to turn eastward and accelerate as a cold front approached the U.S. east coast. The next day, the heading shifted back east-northeast. Shear also increased significantly as the circulation encountered colder waters, beginning extratropical transition. On September 30, Maria became post-tropical.

The above image shows Maria as a category 5 hurricane in the Caribbean sea.

Hurricane Maria brought devastating damage to Dominica, the U.S. Virgin Islands, Puerto Rico, and other parts of the Lesser Antilles.

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.

Wednesday, September 6, 2017

Hurricane Katia (2017)

Storm Active: September 5-9

An area of low pressure formed in the Bay of Campeche on September 1. At first, strong shear prevented the system from doing more than generating shower activity over the region as it moved little. Conditions gradually improved for development, however, and by September 4, the disturbance was producing a large and concentrated area of thunderstorms over water. The next day, Tropical Depression Thirteen formed. There was very little steering the system, so it initially drifted toward the east and then east-southeast overnight. The system strengthened into Tropical Storm Katia early in the morning of September 5.

Later that day, the system developed a very compact inner core, and maximum winds increased rapidly. Just 12 hours after becoming a tropical storm, Katia was already a hurricane. When Katia became a hurricane, Irma and Jose were also hurricanes, making 2017 the first Atlantic season to feature 3 simultaneous hurricanes since 2010. Meanwhile, the system became nearly stationary during the morning of September 7. During the day, hints of an eye were seen, and Katia's winds increased gradually. A developing ridge to the hurricane's north finally set it on a definite heading overnight, this time toward the southwest. The next day, as it approached the coastline, the system reached its peak intensity as a category 2 hurricane with 105 mph winds and a minimum pressure of 972 mb. During the afternoon and evening, however, dry air invaded the circulation, and Katia collapsed weakening rapidly before even hitting land. By the time it made landfall in Mexico, it was down to minimal hurricane strength. The storm's swift demise continued, and it dissipated on September 9. The main impacts of Katia were heavy rain over the mountainous terrain of central Mexico.

The above image shows Katia at peak intensity as a category 2 hurricane. Fortunately, rapid weakening just before landfall reduced impacts in Mexico.

Weak steering currents prevailed during Katia's lifetime in the Bay of Campeche, and the system moved very slowly throughout its existence.

Hurricane Jose (2017)

Storm Active: September 5-21

A new tropical wave entered the Atlantic basin right at the end of August and produced disorganized shower activity as it moved westward. Not much organization occurred until September 4, when conditions became more favorable and thunderstorm activity much more concentrated. By early on September 5, the disturbance was producing winds near tropical storm force. Hours later, the center of circulation was organized enough to name the system Tropical Storm Jose. The newly-formed storm was located about halfway between Africa and the Lesser Antilles and was moving west-northwest.

Moist air and low shear allowed Jose to begin strengthening immediately. A predecessor to an eye was already becoming evident during the afternoon of September 6, and the cyclone was a minimal hurricane by the evening. Simultaneously, Irma and Katia were also hurricanes, making 2017 the first Atlantic season to have three hurricanes at once since 2010. Jose's intensification continued well beyond category 1 strength. During the afternoon of September 7, it became another major hurricane as the eye became well-defined on satellite imagery. The next morning, it exploded into strong category 4 intensity. Jose's heading shifted northwest during the day, bringing on a track to just miss the northern Leeward Islands to the northeast. As it approached the islands during the evening, the storm reached its peak intensity of 155 mph winds and a central pressure of 938 mb, just below category 5 strength.

Partially due to the outflow of Irma, upper-level winds became less favorable for Jose overnight, and a weakening trend had begun by the morning of September 9. The hurricane made its closest approach to the northern Leeward Islands before noon. Fortunately for the islands, which had been devastated less than four days prior by Hurricane Irma, the center passed to the northeast, and wind radii were low on the southwest side. Nevertheless, tropical storm conditions did affect some areas for the better part of the day. The system moved northwest away from the Caribbean that evening, and Jose maintained category 4 strength through the morning of September 10. The eye disappeared that day, however, and the system steadily fell through category 3 strength into category 2 overnight. Very deep convection was still present around the center, however. Atmospheric steering currents were also weakening, causing Jose to lose forward speed and veer toward the north into September 11. During the day a developing mid-level ridge began turning the system toward the east, and its heading was due east by early the next morning. Although the hurricane maintained impressive thunderstorm activity during this time, little to no banding could form in the face of strong wind shear. As a result, Jose decayed into a minimal category 1 hurricane by September 12.

At this point, the system stabilized in intensity, and continued its slow clockwise loop over the western Atlantic by moving southeast overnight and into September 13. Warm waters allowed large pulses of deep convection to continue, offsetting the unfavorable upper-level winds and causing some upward fluctuations in intensity. Later that day, Jose turned sharply south and then west as the nearby ridge continued to evolve. The system lost some organization overnight, and weakened a bit on September 14 to a tropical storm. Another factor that inhibited strengthening later that day and early the next is that Jose was completing its loop and crossing over cooler ocean waters left in its wake when it traveled across the region a few days previously. Nevertheless, the storm remained at the brink of hurricane strength into September 15. That day, it recovered some deep convection as it turned northwest, and restrengthened into a hurricane. The storm still struggled some with dry air over the next day, but gradual strengthening occurred. Having reached the western periphery of the steering ridge, Jose also took an overall turn toward the north, although the center wobbled some to the east and west as it did so. On September 17, Jose reached its secondary peak strength of 90 mph winds and a pressure of 967 mb.

By that evening, Jose was beginning to display some characteristics of an extratropical cyclone. As it passed the latitude of North Carolina well offshore, its inner core weakened but its wind field expanded. Nevertheless, it hung on to minimal hurricane strength over the next day as it continued generally northward. Early on September 19, the cyclone's outermost rain bands swept across the mid-Atlantic coast and southern New England. Later in the day, the system turned toward the northeast, away from the coast, though the system was so large that some coastal rains continued. By this time, the center of circulation had moved north of the warm Gulf Stream waters, resulting in weakening. Jose finally lost hurricane status and became a tropical storm. Gradual decay continued into September 20. Meanwhile, the storm turned toward the east and slowed down, coming nearly to a standstill that night southeast of Cape Cod. Rain continued for portions of southern New England through the 21st. Late that evening, however, Jose no longer had enough convection to remain a tropical cyclone, and was classified post-tropical. The circulation continued to spin down offshore as it moved little over the next few days. It finally dissipated on September 25.

The above image shows Jose at peak intensity approaching the Lesser Antilles.

Jose's long track held it offshore of the U.S. east coast for many days, causing prolonged high surf and rip current risks.

Wednesday, August 30, 2017

Hurricane Irma (2017)

Storm Active: August 30-September 12

On August 28, another vigorous tropical wave emerged off of the coast of Africa. Saharan dry air did not hamper it as it had its predecessors, and the wave maintained thunderstorm activity as it passed near the Cape Verde Islands on August 29, bringing locally heavy rain and gusty winds. Before long, its circulation became better-defined and it developed gale-force winds. As a result, advisories were initiated on Tropical Storm Irma during the morning of August 30. Already, the storm had 50 mph sustained winds, and strengthening continued steadily as Irma's inner core became better defined. The system turned a bit toward the west-northwest on August 31, and an eye feature suddenly appeared on satellite imagery. Irma was undergoing extremely rapid intensification, and was upgraded directly from a tropical storm to a category 2 hurricane and then a major hurricane by late afternoon.

The circulation was quite compact, with hurricane-force winds very close to the center. As is often true with small cyclones, Irma was subject to short-term fluctuations in intensity. A few eyewall replacement cycles, in which the eye clouded over temporarily and then reappeared, occurred between August 31 and September 2. This caused Irma to alternate between category 2 and category 3, an impressive intensity as it traversed only marginally warm waters. Meanwhile, the subtropical ridge to its north began to build southward, and the cyclone turned west and even slightly south of west by September 2. Over time, this brought the system toward warmer waters and moister air. By September 3, though Irma's intensity remained in the low-end category 3 range, the cyclone was beginning to grow larger, displaying outer banding features. That evening, Irma's central pressure began to decrease. This trend continued into September 4, when the maximum winds began to strengthen as well.

Meanwhile, Irma began to round the bottom of the subtropical ridge and assume a due westward path near 17° N. This brought it over areas of higher oceanic heat content and even higher atmospheric humidity levels. As a result, intensification once again increased in speed: Irma became a category 4 that afternoon. During the evening, its eye cleared and became quite large in the wake of another eyewall replacement. During the morning of September 5, yet another burst of intensification brought it to category 5 strength, with winds of 175 mph. This was the farthest east such high wind speeds had ever been recorded in the Atlantic. The outer part of Irma's circulation began to affect the northeast Caribbean islands that day. The trend of remarkable deepening continued as the system turned west-northwest, bringing the hurricane to a peak intensity of 185 mph winds and a minimum pressure of 914 mb. These winds were tied for the second-highest ever recorded in an Atlantic hurricane, and the pressure was the lowest known in the Atlantic outside of the Caribbean and Gulf of Mexico. That night, the cyclone passed directly over the northern Leeward Islands.

Despite the continuing evolution of the inner core, Irma did not undergo an eyewall replacement on September 6, and maintained its incredible category 5 intensity, though with some pressure fluctuations. The center passed through the U.S. Virgin Islands and then just north of Puerto Rico that evening. Overnight, the circulation was a bit disrupted from land interaction, and the winds began to slowly decrease as Irma moved west-northwest just north of Hispaniola. That evening, the center of circulation passed among the Turks and Caicos Islands. During the night, the outer eyewall of Irma finally got the better of the inner, weakening the maximum winds down to category 4 strength, but expanding the overall windfield. The system moved west-northwest between Cuba and the Bahamas the morning of the 8th as a strong category 4 hurricane. Late in the afternoon, Irma turned back toward the west and approached the northern coast of central Cuba. Just before making a direct hit on the northern archipelago of Cuba, the system briefly regained category 5 strength, reaching winds of 160 mph and a minimum pressure of 924 mb.

After passing over the northern islands, the center of circulation did not pass inland, but rather turned just north of west and paralleled the coast overnight, slowing down as it did so. Land interaction took a significant toll on the storm for the first time early on September 9, dropping Irma down to high-end category 3 strength. The eye wobbled a great deal along its path during the day, but the overall motion was a turn toward the north-northwest by the evening. Meanwhile, as conditions improved in Cuba, they deteriorated in Florida, as intense outer bands swept across much of the peninsula. These include tropical storm conditions, flooding rains, and several reports of tornadoes. During the night, Irma regained category 4 intensity, reaching a final peak of 130 mph winds and a pressure of 928 mb before passing over the Florida Keys early on September 10. Increasing wind shear and land interaction steadily weakened Irma from then on. It made landfall as a category 3 hurricane in southwestern Florida that afternoon. As with many sheared landfalling systems, Irma became lopsided, with almost all rainfall occurring north of the center, and the northern eyewall much more intense than the southern. The size of the system was such that, located over western Florida, it brought tropical storm force winds from the Florida panhandle to parts of Georgia and even extending well into South Carolina.

On September 11, Irma weakened to a tropical storm while centered over the northwestern Florida Peninsula, having caused more than 6 inches of rain throughout much of the state. By the afternoon, the center had pushed into Georgia and rains spread northwestward as they ended in Florida. Overnight, the system weakened to a tropical depression and then became post-tropical. In addition to those already mentioned, Irma set many other records. Its 3.25 days spent as a category 5 hurricane were tied for the most ever recorded in the Atlantic with the 1932 Cuba hurricane. Its category 5 landfalls in the Leeward Islands and Cuba were among the strongest ever recorded. Finally, Irma maintained 185 mph winds for a period of 37 hours, which was the most ever recorded in the entire world.

This image shows Hurricane Irma at peak intensity on September 6 as it passed directly over the northern Leeward Islands, causing catastrophic damage.

Irma's long track as a powerful hurricane brought devastating impacts to the northern Leeward Islands, the U.S. Virgin Islands, the Turks and Caicos Islands, Cuba, and Florida.

Thursday, August 17, 2017

Hurricane Harvey (2017)

Storm Active: August 17-19, 23-31

On August 13, a large tropical wave entered the Atlantic from the west coast of Africa. As with many of the previous August waves, thunderstorm activity diminished as soon as it was over water. There was some spin associated with the system over the next few days, but the low pressure area remained elongated. The circulation improved greatly on August 15 and 16, leaving limited shower activity as its main barrier to development. Meanwhile, the system was moving due westward at a steady clip toward moister air, and thunderstorm activity increased significantly by the morning of the 17th. That afternoon, aircraft reconnaissance discovered a closed circulation and tropical storm force winds, prompting the naming of Tropical Storm Harvey.

After formation, Harvey moved at around 20 mph toward the west, steered by a ridge to its north. As with many systems moving at such speeds near the Lesser Antilles, the storm had difficulty maintaining a center of circulation and was rather disorganized. Nevertheless, it brought some localized heavy rain and gusty winds as it passed over the Windward Islands during the morning of August 18. Moderate shear also took a toll on the system as it continued quickly westward; during the afternoon of August 19, it was downgraded to a tropical depression. The system's deterioration continued through the evening and aircraft reconnaissance was unable to detect evidence of a closed circulation that evening. Harvey had opened up into a wave and was no longer a tropical cyclone.

The remnants of Harvey continued toward the west, where conditions for development began to improve again. Early on August 20, they passed near the coasts of Nicaragua and Honduras, bringing some rain to the shoreline. Shower activity increased significantly that afternoon. Before a new circulation could fully develop, however, the system began to interact with the Yucatan Peninsula. While Ex-Harvey crossed over land on August 21 and 22, it produced convection mainly over the Gulf waters to the north. Nevertheless, some spin reappeared on satellite imagery. The system became much more vigorous by the morning of the 23rd, once it was back out over warm Gulf waters. Aircraft reconnaissance soon found a well-defined center of circulation and advisories were reinitiated on Tropical Depression Harvey before noon.

During the afternoon, Harvey slowed to a standstill, and a central dense overcast feature appeared, indicative of a developing system. The central pressure began to slowly decline, but the circulation was still rather disorganized. Overnight, the depression was upgraded to a tropical storm. The upper-level low that was causing some shear over Harvey was weakening, meanwhile, leaving the storm in very favorable atmospheric conditions. Early on August 24, the system began a run of rapid intensification, reaching hurricane strength by early in the afternoon! Though the wind speeds leveled out through the evening, the central pressure continued to drop, presaging a corresponding increase of winds. This occurred in the middle of the night, bringing Harvey to category 2 strength. Around this time, a vigorous outer rain band swept across the Texas coastline. During the morning of August 25, it became evident that the hurricane was experiencing an eyewall replacement cycle (EWRC), where the innermost ring of thunderstorms about a powerful hurricane's eye contracts or dissipates, and a second ring (the outer eyewall) takes over. This put a temporary cap on Harvey's winds, but did not stop the central pressure from dropping steadily.

Meanwhile, the system was still moving at a moderate pace toward the northwest and approaching the Texas coastline. Squalls and even tornadoes occurred over land as the more vigorous rain bands swept across the coast. During the afternoon, Harvey's EWRC was completed, and a large eye cleared out on satellite and radar imagery. This was accompanied by another rapid increase in winds, bringing the hurricane to its peak intensity as a category 4 storm; it had maximum winds of 130 mph and a central pressure of 938 mb when it made landfall in Texas. This was first major hurricane landfall in the United States since 2005, the first hurricane landfall in Texas since 2008, and the strongest in the state since 1961. The storm slowed as it moved north-northwestward inland and weakened. By mid-morning on August 26, Harvey was a minimal hurricane, and it weakened to a tropical storm that afternoon. However, as the storm slowed to a standstill, the greatest concern was rain: the counterclockwise spin of the circulation continually brought new moisture from over the Gulf northward over areas east of the center, dumping feet of rain over a wide swath of Texas. Harvey reversed course and began to meander generally southward by the morning of August 27. Later, the direction of drift turned southeastward. Since part of the circulation remained over water, the storm was able to maintain minimal tropical storm strength through the next day. Meanwhile, the main source of rain was a large band wrapping around the northeast side of the circulation down to the Gulf, pulling moisture over eastern Texas and Louisiana. Early on August 28, Harvey's center reemerged over the Gulf of Mexico.

Some intermittent convection rekindled near the center once it touched warm Gulf waters, but the circulation was entraining dry air from western Texas, and wind shear was high. As a result, the inner core could not redevelop, and Harvey resembled an extratropical cyclone more than a tropical one. It had a comma-shaped tail of thunderstorm activity wrapping from north of the center eastward, so that rain continued over eastern Texas and Louisiana. The cyclone traveled first southeast, and then east over the next day. Winds increased modestly during this period to 50 mph in strong rain bands over water. Finally, on the 29th, Harvey turned toward the north and increased in speed somewhat, making its final landfall in western Louisiana early on August 30. The cyclone weakened over land as it moved north, but dealt a final burst of heavy rain to eastern Texas before becoming a tropical depression that evening. Soon, the system became extratropical, but rainfall continued to push northeastward, bringing moderate amounts of rain to the mid-Atlantic and northeast by September 2. A peak rainfall accumulation of 51.88" occurred in Cedar Bayou, Texas, the highest rainfall total every recorded from any tropical cyclone in the continental United States.

The above image shows Harvey shortly before landfall in Texas.

Harvey's unusual slow movement near and over the state of Texas brought unprecedented rains to south and east parts of the state.