Sunday, January 1, 2017

Voronoi Diagrams and Metrics

In mathematics and visual art, a Voronoi diagram is a type of partition on a surface (usually a plane). Such a diagram is determined from some set of points (called "seeds") on the surface and a notion of distance on the surface by assigning each point a "cell," namely the region in the plane within which the given seed is closer than any other seed. The diagrams are named for the Ukrainian mathematician Georgy Voronoy.

Our first example of a Voronoi diagram consists of only two seeds (the black dots) and two cells, where the line connecting the two seeds is also shown. The maroon region contains the points in the plane closest to the left-hand seed, and the blue region the right. The divider between the two regions bisects the line between the two seeds (since the midpoint is by definition equidistant from the two endpoints) and is in particular the perpendicular bisector of this line. We now present a more complicated example.

In this image, the dots again represent the seeds, while the differently colored regions are the cells of the diagram. The inner region is bounded by a polygon (specifically a pentagon) whose sides are perpendicular bisectors of the lines connecting each of the outer seeds to the center seed. Note also that the central region is finite, since the center seed is surrounded by other seeds, while the other regions extend outward forever. Finally, each point at which three regions meet is the circumcenter of the triangle formed by three nearby seeds. The image below illustrates this fact for our example with six seeds.

Three of the seeds have been connected to form a triangle (white). The circumcenter of the triangle is the center of the circle containing the triangle's three vertices (black). By the definition of a circle, the circumcenter (red) is equidistant from the three seeds and is therefore the point at which the three neighboring regions meet.

Further, regular patterns of seeds produce correspondingly regular patterns of the cells. For example, a repeating square lattice of points produces a repeating pattern of square cells, as shown below.

The reader may experiment with different seed placements using the interactive feature found here. There are many ways to generalize the Voronoi Diagram concept beyond the two-dimensional plane. For example, it is possible to construct three-dimensional Voronoi diagrams, again using points as seeds, except that space will now be divided into three-dimensional cells instead of two.

The above image shows a number of seeds scattered in three-dimensional space and a single cell corresponding to the seed at the center. The lines connecting the center seed to the surrounding ones are also shown. Instead of a polygon, the cell is a polyhedron, bounded by faces which are sections of the planes that form the perpendicular bisectors of the line segments connecting the seeds.

In mathematics, Voronoi diagrams are useful for visualizing the notion of a metric. Metrics are generalizations of the familiar concept of distance to a number of different spaces in addition to the normal Euclidean plane and space (which we have worked with so far). For example, consider the surface of a sphere, such as the Earth. Typically, we define the distance between two points to be the length of the straight line connecting them (which in Euclidean space is the shortest path between the points). However, given two points on the Earth (a sphere), the line connecting them might go through the interior. When we speak of "distance" on the sphere, we want the shortest path along the surface between the two given points, or in other words the fastest travel route from one to the other!

The shortest distance between the points A and B above on the sphere is not the latitude line that they share (though this would be the straight path between them on the 2D map projection) but the arc of a circle passing through the sphere's center. These circles are known as great circles. The distance between two points on a sphere is defined to be the length of the great circle arc connecting them. This is also why planes take what appear to be inefficient paths on two-dimensional maps: they are in fact following a great circle (see below).

Having defined a metric for the sphere, we may choose some collection of points on it and create Voronoi diagrams, just as before. The diagram below takes major airports around the world as seeds and constructs a Voronoi diagram on the Earth's surface (which, of course, is nearly a sphere).

Voronoi diagrams also have a number of applications outside mathematics in settings where understanding distances from a fixed set of sources is important. They are used in modeling the spread of disease, the growth of forests, cell development, the distribution of minerals in the Earth's crust, and rainfall maps, among other things. They are a beautiful visual tool for comprehending the relative positions of points in a given space.


Thursday, December 29, 2016

2016 Season Summary

The 2016 Atlantic hurricane season had below-average activity, with a total of

16 cyclones attaining tropical depression status,
15 cyclones attaining tropical storm status,
7 cyclones attaining hurricane status, and
3 cyclones attaining major hurricane status.

Before the beginning of the season, I predicted that there would be

14 cyclones attaining tropical depression status,
13 cyclones attaining tropical storm status,
7 cyclones attaining hurricane status, and
3 cyclones attaining major hurricane status.

The season was just a bit above average, since the average numbers of tropical storms, hurricanes, and major hurricanes are 12.1, 6.4, and 2.7, respectively. My predictions at the beginning of the season were close to the actual result, with the only difference being in the number of tropical depressions and tropical storms. The Accumulated Cyclone Energy of the 2016 season was 132, an above average value, since there were a few relatively long-lived and intense hurricanes. In particular, Hurricane Matthew had an individual ACE of about 48, the highest for a single Atlantic hurricane in a dozen years.

The biggest story in 2015 for Atlantic hurricane development was the 2014-5 strong El Nino event. Indicated by anomalously warm ocean temperatures over the equatorial East Pacific region, this event limited the 2015 season to below-average activity. However, by the end of that year, it was diminishing, and fairly neutral conditions prevailed throughout the 2016 hurricane season. Generally speaking, El Nino inhibits tropical cyclone formation and La Nina conditions (with anomalously cool ocean temperatures over the same Pacific region) support it. The near-average 2016 season was consistent with this general rule.

Atlantic ocean temperatures were also not quite as extremely warm as in 2015, but more favorable atmospheric conditions in the Caribbean Sea and Gulf of Mexico allowed systems to form in areas likely to result in landfalls. Hurricane Matthew was by far the most notable hurricane of the season, strengthening into a category 5 in the Caribbean at an unprecedentedly low latitude. It was also the first category 5 in the basin since 2007. The system went on to devastate Haiti with a category 4 landfall, and later affected Cuba, the Bahamas, and the southeastern United States. Some other notable facts and statistics concerning this season include:
  • Hurricane Alex formed in January, making it the first hurricane to form in the month since 1938
  • Following the rapid start to the season with Alex and Bonnie, Tropical Storm Colin and Tropical Storm Danielle became the earliest forming 3rd and 4th tropical storms in an Atlantic season in recorded history
  • Tropical Storm Julia actually formed with its center of circulation over land, marking the first occurrence of this phenomenon since 1988
  • When Hurricane Nicole achieved category 4 status, 2016 became the first known season to have two category 4 hurricanes (the other was Matthew) exist in the month of October
  • The season ended on another unusual note, with Hurricane Otto becoming the southernmost landfalling hurricane in Central America ever recorded, and the first Atlantic storm to cross into the East Pacific basin since 1996
  • Unfortunately, the death toll of the 2016 season exceeded 1700 people (with most of these occurring in Haiti due to Hurricane Matthew), making 2016 the deadliest Atlantic hurricane season since 2005

The 2016 season was not extraordinarily active, but saw a relatively high number of landfalling systems in the Caribbean, the Gulf, and elsewhere.


Monday, November 21, 2016

Hurricane Otto (2016)

Storm Active: November 21-26

On November 12, a trough of low pressure making its way across the Caribbean stalled in the southwestern part of the sea north of Panama. The system organized very gradually without moving significantly over a period of many days. Thunderstorm activity increased on the 14th, leading to the formation of a low pressure the next day. The low drifted toward the coastline of Central America, halted, and reversed direction over the following couple of days, while conditions became less favorable and disorganized the system. However, it was still over the same region of warm waters on November 19 when conditions began to improve and the circulation became better defined. On November 20, the low deepened and was better defined still, but thunderstorm activity remained too weak for a tropical cyclone classification. Finally, early on November 21, Tropical Depression Sixteen formed in the southwestern Caribbean.

Located over warm ocean waters and surrounded by an environment of diminishing upper-level winds, the cyclone steadily strengthened. That afternoon, it was upgraded to Tropical Storm Otto. Otto was meanwhile trapped in an area of very weak steering currents and barely moved that day, managing only a slow southward drift. But while the system was nearly stationary, its structure improved considerably: strong thunderstorm activity increased and large curved banding features formed overnight. By midmorning of November 22, Otto was close to hurricane strength and its southern quadrant was bringing heavy rains to the northern coast of Panama. A little more intensification that afternoon resulted in Otto becoming a category 1 hurricane, breaking the record for latest hurricane formation in the Caribbean Sea set by Martha in 1969. Meanwhile, the system began to slowly move westward as a ridge established itself to the north.

The next day, wind shear and dry air weakened Otto slightly back to a tropical storm. This setback was temporary, however, as atmospheric moisture was increasing, and a large burst of intense convection that evening restored hurricane strength. Otto was also moving more steadily westward toward the Central American coast by this time as the ridge amplified. Tropical storm conditions began to affect southeastern Nicaragua and northeastern Costa Rica by the early morning hours of November 24. Around the same time, Otto quickly developed an eye feature and intensified into a Category 2 hurricane, reaching its peak intensity with 110 mph winds and a central pressure of 975 mb. With this intensification, Otto became the latest forming category 2 hurricane on record in the Atlantic basin. A few hours later, around 1 pm EST, Otto made landfall just north of the border of Nicaragua and Costa Rica. This was also the southernmost hurricane landfall in Central America on record. As a result, the storm affected some areas less prepared for landfalling hurricanes.

Soon, however, land interaction began to quickly weaken Otto. By the evening of the 24th it had weakened to a tropical storm. The weakened system, now moving more quickly westward, emerged into the East Pacific basin overnight. The Pacific waters were fairly warm, but Otto faced dry air and significant wind shear. It moved steadily just south of west and weakened until dissipation occurred on November 26.

The above image shows Hurricane Otto at category 2 intensity just before landfall.

Otto's track was unusual in many respects. In addition to becoming the southernmost landfalling Central America hurricane ever recorded, it was also the first cyclone to survive crossing from the Atlantic to the Pacific since 1996.

Wednesday, October 5, 2016

Hurricane Nicole (2016)

Storm Active: October 4-18

During late September, a tropical wave was traversing the central Atlantic. Strong upper-level winds made it difficult for the system to organize, but by October 1, a low pressure center developed and the disturbance was producing strong winds. It moved northwest over the following few days and slowly organized as conditions became somewhat more favorable. By October 3, thunderstorm activity had become more concentrated. The next day, the system was producing winds to tropical storm force, and the circulation became better defined. Therefore, the system was designated Tropical Storm Nicole late in the morning on October 4.

It exhibited organized banding features and a well-defined circulation through the next day despite moderate shear. Meanwhile, the small system moved generally west-northwest into October 5. The cyclone continued to defy somewhat unfavorable atmospheric conditions and rapidly intensified over the following day, achieving hurricane status during the afternoon of October 6. An eye appeared on satellite imagery at the same time, and Nicole sped to an intensity of 105 mph winds and a pressure of 968 mb. The cyclone then stopped in its tracks and reversed course toward the south by October 7. Wind shear increased drastically and weakened Nicole as quickly as it had strengthened in addition to periodically exposing the center. By October 8, the system was again a fairly weak tropical storm, though vigorous convection bursts appeared intermittently throughout the day.

Nicole once again slowed to a standstill overnight and upper-level winds slowly relaxed. This allowed the system to recover some organization and initiate a slow strengthening trend. Later on October 9, Nicole finally assumed a more typical northward motion after meandering over the open Atlantic for several days. The next day, the system was again a strong tropical storm. However, the system still struggled with some dry air aloft even as it passed over very warm waters. It managed to develop a ragged eye feature during the morning of October 11, and intensification restarted. In fact, the system rapidly intensified into a hurricane that evening, and a category 2 storm by early on October 12. By this point, the cyclone was moving north-northwestward toward Bermuda. Later that day, the eye became quite large and symmetric and the system had outstanding outflow in all quadrants. These new increases in organization merited an upgrade to a major hurricane that evening and Nicole reached category 4 status briefly that night. Its peak intensity of 130 mph winds and a minimum pressure of 950 mb occurred as the system accelerated toward the north and tropical storm force winds began to engulf Bermuda.

Just after achieving peak intensity, the cyclone experienced a huge increase in wind shear, causing significant weakening to begin early on October 13. Nicole's center passed within 10 miles of Bermuda that morning with the cyclone still at category 3 strength, bringing hurricane force sustained winds to the island. By this time, the system was turning toward the northeast and gaining forward speed. Meanwhile, shear and decreasing ocean temperatures quickly weakened Nicole back to category 1 strength. The system acquired some extratropical characteristics the next day, but rather than transitioning fully, it became a sort of hybrid cyclone: the windfield and size of the system were typical of an extratropical system, but the inner core remained that of a tropical cyclone.

Late on October 14, Nicole briefly weakened to a tropical storm, but baroclinic processes reintensified the system the next day to a very powerful north Atlantic hurricane. It also turned toward the east on October 15 and slowed in forward speed. By the next morning, it had maximum winds of 85 mph and a pressure of 958 mb and was generating large ocean swells throughout the north Atlantic. Some weakening ensued the following day, but Nicole remained a hurricane through the morning of October 17. Diminishing ocean temperatures finally caught up to the system that night, purging the remaining tropical characteristics, and Nicole became post-tropical early on October 18. The cyclone remained extremely powerful for the next few days as it interacted with a frontal low and brought unusually strong winds to the southern coast of Greenland before being absorbed.

The above image shows hurricane Nicole at peak intensity shortly before impacting Bermuda. Since Matthew and Nicole both were category 4 hurricanes in October, the 2016 season was the first in recorded history to have two storms of at least category 4 strength in this month.

Nicole's meandering track brought it southward before doubling back toward Bermuda, stalling over the far northern Atlantic, and finally having impacts as far north as Greenland as an extratropical system.

Thursday, September 29, 2016

Hurricane Matthew (2016)

Storm Active: September 28-October 9

On September 22, a tropical wave exited in the African coast and moved rapidly westward across the Atlantic. For the next few days, the wave remained south of 10°N and embedded in the Intertropical Convergence Zone. Combined with the dry air over the east Atlantic, this factor precluded development initially. By the 26th, thunderstorm activity had increased and spiral bands had begun to appear on the north side of the disturbance. The system was still too far south to acquire spin, but it began to move west-northwest over the following day and acquired additional organization. By September 27, the wave was generating winds to near tropical storm force, but had not yet developed a closed circulation. Early on September 28, the system appeared on satellite imagery to be more organized, and aircraft reconnaissance confirmed the presence of a closed circulation later that day. Therefore, advisories were initiated on Tropical Storm Matthew. The aircraft also estimated that surface winds of 60 mph were already present, making the newly formed Matthew already a strong tropical storm.

At the time of formation, Matthew was passing through the Lesser Antilles and entering the Caribbean, bringing strong storms to the region. During the morning of September 29, moderate shear out of the southwest exposed the center briefly. However, high ocean temperatures and increasing humid air near the system allowed an inner core to quickly develop. By the middle of the afternoon, Matthew had strengthened to a hurricane. Meanwhile, the cyclone veered slightly south of west. Overnight, strengthening continued, bringing Matthew to category 2 status by the morning of September 30. Conditions began to deteriorate in the northernmost areas of Columbia later that morning. It is very unusual for tropical cyclones to affect South America, but Matthew's track was quite far south through the Caribbean. Around the same time, an eye appeared on satellite imagery and further rapid intensification took place. That afternoon, the system vaulted through category 3 and up to category 4 status. During the late evening, Matthew peaked as a category 5 hurricane with winds of 160 mph and a central pressure of 941 mb, making it the first category 5 hurricane in the Atlantic since Felix in 2007. Achieving this intensity at 13.3°N latitude, it was also the southernmost category 5 hurricane ever recorded in the Atlantic.

The forward speed of the system had slowed considerably at this point, although it was still moving west or just south of west. Early on October 1, the eye shrunk and the inner core became less organized, resulting in some weakening that day. This weakening was temporary, however, for after completing a small cyclonic loop that afternoon, Matthew regained strong category 4 strength and its new lowest pressure of 940 mb. The cyclone's motion remained slow and somewhat erratic through the following day, averaging to a general northwestward track during the day of October 2. Later that day, the outer rain bands of Matthew began to affect Jamaica and Haiti as it approached from the south. The system turned to the north and experienced slight weakening that night, but still maintained category 4 intensity. Matthew's structure did not change much the next day as it steadily approached the Greater Antilles. Extremely heavy rains began over Haiti on the 3rd and continued as the storm came closer and closer. Early on October 4, the center passed well to the east of Jamaica, though the island still experienced tropical storm conditions. Meanwhile, Matthew's pressure dropped to 934 mb, though the winds remained within category 4 intensity. Around 7:00 am EDT on October 4, Matthew made landfall in southwestern Haiti with maximum winds of 145 mph, the strongest hurricane to make landfall in the country in over 50 years.

Interaction with land began to slowly weaken the system, though the inner core remained largely intact. The cyclone emerged over water a few hours later and traversed the channel between Cuba and Haiti that day. It remained a category 4 with 140 mph winds when it made landfall near the eastern tip of Cuba at 8:00 pm EDT that evening. The cyclone stalled its northward motion slightly over land and weakened more substantially, becoming a category 3 storm early on October 5. By that time, it had again emerged over water and was entering the Bahamas. An amplifying ridge to the cyclone's east caused a northwest turn later that day. Meanwhile, Matthew began to recover from its land interactions that evening, with the pressure dropping and winds increasing as the eye passed through the Bahamas. Later on October 6, Matthew peaked as a category 4 once again. Rain bands had begun to affect Florida's east coast at this point and the cyclone continued to move closer to land, turning toward the north as it did so. Overnight, the center moved roughly parallel to the central and northern Florida coastline, with the outer eyewall bringing strong winds and heavy rains to the coastline from about 30 miles offshore.

Land interaction and increasing shear also started to weaken the storm as it moved northward. By the time it passed the coast of Georgia very early on October 8, the winds had diminished to category 2 strength. Matthew continued its turn and began to move northeast parallel to the U.S. coastline before finally crossing the shore late that morning as a category 1 hurricane in central South Carolina. Though the maximum winds had decreased by this point, very high levels of moisture in the atmosphere contributed to a huge rainfall event for the Carolina coasts, with over 10 inches of rain falling over a large swath of the region. In addition, the cyclone produced a large storm surge that inundated some low-lying areas. However, by this point, the hurricane was quickly acquiring extratropical characteristics, and transition was completed early on October 9. Though the system was moving eastward away from the coast, rains continued in the Outer Banks of North Carolina through the day before tapering off. The extratropical system was absorbed by a front the next day.

Hurricane Matthew was the first category 5 hurricane in the Atlantic in 9 years. The storm killed over 1000 people, a majority of whom lived in Haiti, making it the deadliest cyclone in the Atlantic since 2005. It also caused over five billion dollars in damages and was the costliest Atlantic hurricane since Hurricane Sandy of 2012. The above image shows the cyclone at peak intensity in the Caribbean Sea.

Matthew formed and strengthened unusually far south in the Caribbean before turning sharply northward and impacting the Greater Antilles, the Bahamas, and the U.S. east coast.

Tuesday, September 20, 2016

Tropical Storm Lisa (2016)

Storm Active: September 19-24

On September 16, a tropical wave moved off of Africa into quite favorable conditions in the eastern Atlantic. As a result, an area of low pressure developed almost immediately and an impressively broad circulation developed. After initially having some trouble consolidating, the circulation became well-defined on September 19 and the system was classified Tropical Depression Thirteen. Shortly after formation, the depression took a west-northwest turn toward a weakness in the ridge to its north. Steady organization followed over the next 24 hours as the system strengthened into Tropical Storm Lisa during the morning of September 20 and then experienced more strengthening through that evening.

Due to the presence of an upper-level low pressure system located to the northwest of Lisa, shear out of the west and southwest began to increase dramatically on September 21, gradually exposing the center of circulation as convection retreated to the east. Lisa held its own, however, continuing to produce very deep convective bursts. It even rebounded from a momentary weakening that day by strengthening to its peak intensity of 50 mph winds and a pressure of 999 mb during the morning of September 22. There was evidence by this point that the circulation was becoming elongated in response to the continued shear. Meanwhile, thunderstorm activity began to wane and Lisa weakened through the rest of the day and overnight.

The storm experienced once last resurgence of thunderstorms during the morning of the 24th and in fact was upgraded back to tropical storm status as a result. However, this was only a temporary reprieve. By that afternoon, the circulation was entirely bare. Lisa was downgraded to a tropical depression that afternoon and a remnant low that night. The circulation produced shower activity for an additional few days before dissipation.

The image shows a disorganized Lisa over the eastern Atlantic.

A break in the ridge to Lisa's north allowed it to veer north quite far east and encounter hostile atmospheric conditions quite quickly.

Thursday, September 15, 2016

Tropical Storm Karl (2016)

Storm Active: September 14-25

On September 12, a tropical wave near the African coastline began to show signs of development. Beginning the next day, gusty winds and locally heavy rain began to affect the Cape Verde Islands as the disturbance moved west-northwest toward them. The system acquired a surface circulation fairly rapidly and was classified Tropical Depression Twelve on September 14, while still over the islands. Moderate wind shear out of the west kept the center exposed and prevented strengthening over the next day. Late on September 15, however, organization markedly increased and the depression intensified into Tropical Storm Karl.

The next day, thunderstorm activity was pushed a bit farther away from the center by the wind shear, and Karl lost some organization. Additionally, the system took a turn to slightly south of due west by the morning of the 17th. Slight weakening followed, though Karl remained essentially steady state through September 18. The system's direction did change toward the west-northwest, and Karl began to gain latitude. Late on September 19, it appeared that the system was finally starting to organize as shear diminished, but thunderstorm activity collapsed into disorder again the next morning. Upper-level winds had not abated as anticipated, and the Karl in fact weakened to a tropical depression late on September 20th.

The long-awaited relaxation of shear commenced the following day, and the overall structure of Karl became much more symmetric, with banding features and deep convection on all sides of the circulation. This large-scale improvement was unusually not accompanied by any development of the inner core. Rather, it remained broad, rather like a pre-tropical system. As a result, Karl remained a tropical depression through the morning of September 22, still moving northwestward over the open Atlantic. Later that day, the cyclone finally began to strengthen, regaining tropical storm strength that evening and continuing to intensify through the morning of September 23 as outflow drastically improved. The system began to round the edge of the subtropical ridge that day and turned toward the north.

Conditions worsened in Bermuda throughout the day as Karl approached, eventually escalating to tropical storm force winds and heavy rains overnight. The center turned to the northeast just short of Bermuda early on September 24 and the system began to quickly accelerate away from the island. Wind shear was high across the system, but as it was beginning to undergo extratropical transition, there was some increase in intensity that day, bringing Karl to its peak of 70 mph winds and a pressure of 990 mb that night. By the morning of September 25, the cyclone was speeding northeast at almost 50 mph and was declared extratropical.

The above image shows Karl just after passing Bermuda.

Despite existing as a tropical cyclone for 11 days, Karl was never able to intensify into a hurricane.

Wednesday, September 14, 2016

Tropical Storm Julia (2016)

Storm Active: September 13-18

On September 8, a tropical wave located east of the Lesser Antilles began to exhibit some signs of organization. However, atmospheric conditions were rather hostile, so the system moved west-northwest and then northwest for several days without significant change. On September 11 and 12, the disturbance brought some storms to the Bahamas as it passed through. Surface pressures remained high, so not development occurred during that time. That night, the circulation center of the low moved over the Florida peninsula. Despite this, the warm waters just to the east fueled a huge increase in convection just offshore and caused the system to strengthen. Late on September 13, the low was upgraded to Tropical Storm Julia even while the center was just inland over northeastern Florida. Julia was the first Atlantic storm to form over land since 1988.

After moving northward overnight and crossing inland into Georgia, the center reformed farther east on the 14th and moved over water. Though land interaction lessened, wind shear increased out of the west. This made it difficult for Julia to develop thunderstorm activity over the center and the system weakened to a tropical depression by the morning of September 15. Now moving slowly eastward, Julia put some distance between itself and the coast, ending significant rainfall over the Carolinas and Georgia and averting a potential flooding event. That night, new convective bursts including gale force winds indicated that the system was once again a tropical storm.

Over the next day, the forward motion of Julia slowed to a standstill and it lost organization as hostile conditions continued. As a result, it was again downgraded to a depression by the morning of September 17. The cyclone maintained no persistent convective features that day, but intermittent bursts were enough to maintain the cyclone as tropical. In the meantime, the center of circulation adopted a slow northwestward drift. Soon, however, dry air finally overwhelmed the system, and it was downgraded to a remnant low the evening of September 18. The low continued northwestward slowly until it merged with a front near the coast of North Carolina.

The above image shows Tropical Storm Julia shortly after formation over Florida. Most convection associated with the system remained offshore to the east.

Julia meandered near the U.S. southeast for several days before wind shear and dry air destroyed the system.

Monday, September 12, 2016

Tropical Storm Ian (2016)

Storm Active: September 12-16

Near the end of September's first week, a tropical wave developed over the central Atlantic. Soon, a low pressure center developed along the southern portion of the wave and began to produce a large area of showers and thunderstorms. Despite conditions being very favorable for development, the system organized only slowly as it moved northwestward. By late on September 11, the low was producing gale-force winds. However, it did not yet possess a well-defined center of circulation. Despite the fact that upper-level winds had increased somewhat, the low managed to organize into Tropical Storm Ian during the morning of September 12.

Convection remained confined to the northeast of the center through the remainder of that day. Meanwhile, the system turned toward the north-northwest well to the east of the Lesser Antilles. The area of tropical storm force winds did experience an expansion that evening, and maximum winds increased somewhat, but Ian's structure did not meaningfully change until the afternoon of the 13th. At that time, convection managed to cover the center for the first time. However, the cyclone began to lose tropical characteristics shortly afterward, with the circulation becoming broader and losing concentrated thunderstorm activity near the center. By the morning of September 15, it was accelerating north-northeast over the open Atlantic. Ian actually appeared more tropical later that day than it had for a couple days, with deep convection reappearing close to the circulation center. However, it was racing toward the north Atlantic by this time, briefly achieving a forward speed of over 50 mph that night. At the same time, it experienced some intensification, reaching 60 mph winds and a pressure of 994 mb by the morning of September 16. The cyclone in fact continued strengthening as it became extratropical that afternoon. It merged with another strong low over the north Atlantic shortly afterward.

The image above shows Tropical Storm Ian shortly after formation.

Ian did not affect any landmasses over its short lifetime as a tropical cyclone.

Monday, August 29, 2016

Hurricane Hermine (2016)

Storm Active: August 28-September 3

On August 18, a tropical wave just southwest of the Cape Verde Islands began to display scattered shower activity. The system remained rather broad and disorganized during its trek westward across the Atlantic for most of the following week. Throughout much of this period, the wave and associated low were little more than an large swirl of sparse clouds with limited convection farther south. This was due to the influence of the Saharan air mass to the north. Organization increased some around the 22nd and 23rd as the low passed through the Lesser Antilles, but a well-defined circulation did not yet develop. In fact, organization decreased markedly over the following several days, as the convection became scattered across many of the islands of the Caribbean away from the center. It was not until the system passed just south of the Florida Keys on August 28 that it finally acquired the status of Tropical Depression Nine.

The location of the center of circulation remained difficult to spot on satellite imagery through the next day, but convection increased overall, particularly to the south of the center. This caused heavy rains for much of western Cuba. Organization increased markedly on August 30 and the subsequent overnight period: a large area of very deep convection developed over the southeastern Gulf of Mexico surrounding the center and the central pressure dropped appreciably. This did not immediately correspond to an increase in maximum winds, but aircraft data collected the afternoon of August 31 indicated that the system had finally strengthened into Tropical Storm Hermine.

By this time, the cyclone had assumed a track toward the north-northeast due to the influence of a trough over the southeastern United States. At the same time, atmospheric conditions improved, allowing Hermine to take advantage of the very warm Gulf waters and intensify quickly. That evening, the storm developed more organized banding features, with a huge band extended from northwest to southeast of the center. Shortly afterward, this band began to affect the Florida panhandle. By the morning of September 1, heavy rain was moving across western Florida as far south as Tampa. An eyewall appeared later that morning and completed its circumnavigation of the center that afternoon. Hermine became a category 1 hurricane soon after. A few hours later, the cyclone reached its peak intensity of 80 mph winds and a central pressure of 984 mb and made landfall in the Florida panhandle, causing storm surges of several feet, strong winds, and over 5 inches of rain for many areas.

Hermine weakened to a tropical storm overnight and move inland over Georgia by the morning of September 2. The cyclone traversed South Carolina that afternoon and entered North Carolina that evening. All the while, it weakened and gradually lost tropical characteristics. This did not prevent it from causing flooding rains from the Carolinas north to the Delmarva peninsula by the morning of September 3. Around this time, Hermine became post-tropical and moved east-northeast back over water. There it reintensified some and developed winds near hurricane force. While storm surge remained a threat to the coast, the system moved farther away from land over the next day, lessening direct impacts. Early on September 5, Hermine changed course, moving first slowly northward, and then toward the west-northwest by the afternoon. Though it remained post-tropical, the system generated some shallow convection to the north and west of the center, bringing continued shower activity to coastal New England. The circulation began to spin down on the 6th, and while it moved a little closer to the coast, impacts diminished. By September 7, the weakening cyclone had switched direction and was moving generally east-northeast. It dissipated shortly after.

The above image shows Hermine shortly after it became a hurricane.

Hermine continued to affect the U.S. east coast even after transitioning to a post-tropical cyclone (post-tropical track points are indicated by triangles).