Monday, August 29, 2016

Tropical Depression Nine (2016)

Storm Active: August 28-

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. As of 5:00 am EDT on August 31, 2016, Tropical Depression Nine had sustained winds of 35 mph, a central pressure of 1001 mb, and was moving north at 2 mph. Tropical storm warnings and hurricane watches are in effect for the northwestern coast of Florida.

Sunday, August 28, 2016

Tropical Depression Eight (2016)

Storm Active: August 28-

On August 26, an area of showers and thunderstorms developed in association with a low pressure system just south of Bermuda that had originated from a stalled frontal boundary. While the atmosphere was fairly dry, organization increased over the next day as the low moved toward the west-northwest. By the evening of August 27, the system had developed a roughly circular area of convection with the center of circulation on the eastern edge. The next morning, it was well-defined enough to be classified Tropical Depression Eight. Over the next day, shower activity blossomed only intermittently, leaving the circulation bare a majority of the time. Thus the system remained a tropical depression into August 29 as it turned toward the northwest. Its forward speed also decreased as it moved toward a break in the subtropical ridge to its north. As a result, it stalled off the coast, missing the Outer Banks of North Carolina by less than 100 miles. As a result, scattered showers and gusty winds affected portions of eastern North Carolina on August 30.

Later that day, Eight began to move slowly northeast away from the U.S. east coast.

As of 5:00 am EDT on August 31, 2016, Tropical Depression Eight had maximum sustained winds of 35 mph, a minimum central pressure of 1011 mb, and was moving toward the northeast at 5 mph.

Monday, August 22, 2016

Hurricane Gaston (2016)

Storm Active: August 22-

During mid-August, a vigorous disturbance developed well inland over the continent of Africa. The potential development of this system once it emerged into the Atlantic was identified by August 17, three days before it encountered water. By the time it had moved over the ocean on August 20, a small area of thunderstorm activity persisted near its circulation center. Organization continued over the next few days and Tropical Depression Seven formed over the eastern Atlantic during the afternoon of August 22. Initially, conditions were favorable for development as the cyclone moved westward, and it quickly strengthened into Tropical Storm Gaston. The one inhibiting factor for rapid intensification over the following day was the Saharan dry air mass located to the north. Some of this air entered the circulation on the 23rd, forming a large eye-like structure near the center of circulation. However, Gaston managed to develop a central dense overcast shortly thereafter and became a strong tropical storm. Meanwhile, the system took a turn toward the northwest. As it was nearing hurricane strength on August 24, wind shear increased, slowing development. Nevertheless, the system overcame increasing upper-level winds out of the southwest to achieve hurricane status early on August 25, becoming the third hurricane of the 2016 Atlantic season.

However, the deteriorating atmospheric conditions soon disrupted Gaston's circulation, weakening it back to a tropical storm later that day. Squeezed between a upper-level low (the source of the shear) and a ridge to its north, the system moved rather quickly toward the northwest for the remainder of that day and gradually weakened. The next day, shear abated and the tropical storm began to reorganize. As a result, strengthening began once again by the morning of August 27. While Gaston was still moving northwestward, its forward speed slowed considerably that day. An eye feature developed overnight, and steady strengthening continued through the morning of the 28th, bringing the cyclone to category 2 hurricane status. That evening, the system slowed to a standstill and became the first major hurricane of the season. Just afterward, it reached an intensity of 120 mph winds and a pressure of 957 mb.

By August 29, Gaston had gained enough latitude to be affected by the mid-latitude westerlies and began a turn toward the northeast. The cyclone came off its peak intensity that day as the eye clouded over temporarily. However, once the eyewall replacement cycle was complete that night, a very large, symmetrical eye appeared on satellite imagery. Therefore, Gaston strengthened once again during the day of August 30 and regained major hurricane status that night, matching its peak winds of 120 mph and beating its former minimum pressure by achieving a mark of 956 mb.

As of 5:00 am EDT on August 31, 2016, Hurricane Gaston had maximum winds of 120 mph, a minimum central pressure of 956 mb, and was moving to the east-northeast at 10 mph.

Wednesday, August 17, 2016

Tropical Storm Fiona (2016)

Storm Active: August 16-23

On August 14, a tropical wave formed near the western coast of Africa and began to produce a large region of disorganized thunderstorm activity. Over the next couple days, the system moved generally west-northwestward and developed a small but vigorous circulation. Late on the 16th, the system was designated Tropical Depression Six. Water temperatures were warm in the region and wind shear was abating. However, as with many tropical cyclones in the central Atlantic, the depression had to contend with low humidity values in the surrounding air that constantly threatened to overwhelm the small system.

On August 17, a small concentrated burst of convection replaced the larger but less powerful banding features, resulting in the cyclone's upgrade to Tropical Storm Fiona. Thunderstorm activity waxed and waned over the next day following the usual cycle by which convection preferentially forms at certain times of day due to solar heating of the ocean and atmosphere. Overall, Fiona's intensity remained virtually unchanged on August 18. Meanwhile, the cyclone moved generally northwest in towards a gap between high pressure systems centered in the east Atlantic and near Bermuda. Wind shear increased somewhat on August 19 but water temperatures continued to climb as Fiona traversed anomalously hot waters in the central Atlantic. As a result, bursts of convection continued to form. Fiona fluctuated in intensity a bit but this state of affairs remained mostly unchanged through the day on August 20. Overnight, the circulation center became exposed again, and the cyclone weakened to a tropical depression. Fiona struggled more to develop a convective canopy on the 21st, but it held its own enough to remain a depression through the day.

Over the following two days, the system continued to wane, with the circulation becoming elongated by August 23. This, coupled with a lack of persistent convection, prompted its downgrade to a remnant low that morning. The low continued west-northwestward for another few days before it dissipated over the western Atlantic.

Tropical Storm Fiona struggled with dry air for most of its lifetime.

Tuesday, August 2, 2016

Hurricane Earl (2016)

Storm Active: August 1-6

On July 25, a tropical wave formed just off of the African coast. It moved at an unusually fast rate (around 30 mph) across the eastern and central Atlantic, being among the first of the season to exhibit vigorous shower activity this far east. However, conditions were not very favorable and it is in any case more difficult for fast-moving systems to develop organized circulations. During the final two days of July, convection increased with the wave and it brought rainfall to the Lesser Antilles as it passed westward into the Caribbean Sea. Only toward the end of this period, however, did surface pressures begin to fall in the region. Organization increased significantly by early on August 1 as a large, roughly circular area of thunderstorm activity developed, bringing some rain and high winds to Puerto Rico as it passed to the south. However, it was not until August 2 that a closed circulation was identified. Since the system already had tropical storm force winds, it was designated Tropical Storm Earl as it passed quickly to the south of Jamaica.

Earl was quite disorganized at first, but shear lessened and the storm's forward motion slowed significantly later that day. Combined with the quite high sea temperatures of the western Caribbean, these factors allowed the cyclone to strengthen overnight as its southern bands swept across Honduras. The circulation of Earl had become quite broad by the afternoon of August 3, resulting in heavy rainfall for larger portions of Honduras even as the center remained offshore. Hints of a mid-level eye appeared intermittently throughout the day and by the late afternoon Earl had become a category 1 hurricane. It did not have much time to strengthen further, however. At around 2:00 am EDT, August 4, Hurricane Earl made landfall in central Belize at its peak intensity of 80 mph winds and a minimum central pressure of 979 mb. Hurricane-force winds affected an area near Belize City, and total rainfall amounts of 8-12 inches were common in the central peninsula.

Earl rapidly weakened as it moved inland. By the time it crossed into Guatemala, it was again a tropical storm. That evening, the center took a trajectory just north of west, bringing a the circulation toward the southernmost waters of the Bay of Campeche. As a result, weakening had halted by August 5, with Earl still clinging to minimal tropical storm strength as it continued generally westward. Significant convective bands redeveloped in association with the system as the center traveled over water that morning and through most of the afternoon. The tropical storm strengthened unexpectedly during this period, enhancing the rainfall totals for regions of Mexico. That evening, Earl made landfall in Mexico with maximum winds of 60 mph. Rapid weakening commenced over land, and the system dissipated over the mountainous terrain the next day. Having crossed Mexico, the remnants of Earl contributed to the development of Tropical Storm Javier in the Eastern Pacific basin on August 7.

Earl is shown above near peak intensity on August 3, about 5 hours prior to landfall in Belize.

The high ocean temperatures of the western Caribbean Sea were favorable for Earl's intensification, but its rapid movement and interaction with land prevented it from more than a minimal hurricane.

Sunday, June 19, 2016

Tropical Storm Danielle (2016)

Storm Active: June 19-21

A tropical wave moved over the eastern Atlantic from Africa during the first week of June. However, it being so early in the hurricane season, the system had no opportunity to develop further for nearly two weeks, when it began to produce thunderstorm activity near the coast of Nicaragua on June 15. The wave continued generally west-northwestward for the next few days, during which time land interaction inhibited organization. On June 18, it emerged into the Bay of Campeche, and a low pressure center formed in association with the wave. During the next day or so, moderate wind shear affected the system, hindering convective banding. However, the center of circulation became better defined on June 19 as shear gradually lessened, while increasing shower activity showed hints of banding in the northern semicircle. As a result, the system was designated Tropical Depression Four.

After a brief lull in convective activity that evening, a large area of deep convection blossomed overnight, albeit not all that organized about the center of circulation. By the morning of June 20, reconnaissance aircraft discovered gale force winds in the system, prompting an upgrade to Tropical Storm Danielle. This was the earliest naming of a fourth storm in the history of Atlantic hurricane seasons, beating out the record set by Tropical Storm Debby in 2012 by 3 days. Heavy rain began over the Mexican coast on the west side of the Bay of Campeche and continued as Danielle moved west-northwestward toward land. Danielle reached its peak intensity of 45 mph winds and a minimum pressure of 1007 mb before making landfall in Mexico that evening. The storm brought 8-12 inches of rain to a large area of Mexico as it quickly weakened over the mountainous terrain of Mexico. It dissipated early on June 21.

During its brief period as a tropical system, Danielle brought heavy rains to eastern Mexico.

Forming in the central Bay of Campeche, Danielle did not have time to strengthen significantly before it moved westward over land.

Sunday, June 5, 2016

Tropical Storm Colin (2016)

Storm Active: June 5-7

Around June 3, a large area of disturbed weather formed over the northwestern Caribbean Sea. The next morning, a broad low-pressure center developed in association with the system just east of the Yucatan Peninsula. Moving over land that day, it was unable to organize further and convection remained over water well to the east. On June 5, however, the center emerged into the Gulf of Mexico. Shortly after, the low had acquired enough organization to be classified Tropical Depression Three. Due to shear out of the west, the depression's thunderstorm activity, though significant, was located in a north-to-south linear band over 100 miles east of the circulation center. Despite this disorganization, hurricane hunter aircraft discovered winds to tropical storm force that afternoon, prompting an upgrade to Tropical Storm Colin. The formation of the year's third named storm on June 5 broke the record for earliest third tropical storm, set on June 12, 1887.

Colin exhibited a very curious structure for a tropical cyclone. Overnight and into June 6, it was apparent that there were at least two distinct low-level centers widely separated from one another, both of which lay outside the intense area of convection to the east. The size of the system and its disheveled state were prohibitive to significant strengthening. Despite this, the impacts of the storm remained: by June 6, with Colin still over water, heavy rains spread throughout much of Florida, Georgia, and even South Carolina. The system consolidated a bit that day into a single circulation, albeit with several small-scale gyres. Meanwhile, it was accelerating rapidly toward the northeast. That evening, the center of Colin made landfall in the Big Bend region of Florida (although most of the rainfall had moved off to the east by this time).

Though at its peak intensity as a tropical system of 50 mph, the system was more disorganized than ever as it sped off to the northeast, quickly emerging off the coast of the Carolinas early on June 7. At this point, Colin was rapidly losing tropical characteristics, and it become post-tropical near the Outer Banks later that morning. By the afternoon, the system had moved away from land, still intensifying as a post-tropical system. It continued to speed toward the far northern Atlantic before being absorbed a few days later.

Colin remained disorganized during its brief stint as a tropical system. The above satellite image, taken June 6, shows multiple vortices exposed to the west of the convective canopy.

Colin made landfall in Florida on June 6 before quickly moving out to sea.

Friday, May 27, 2016

Tropical Storm Bonnie (2016)

Storm Active: May 27-30, June 2-4

Around May 24, a broad area of scattered showers and thunderstorms began to develop in association with a low-pressure trough situated to the north of Hispaniola. Over the next few days, convection gradually became more concentrated near the low-pressure center as it deepened and moved generally toward the west-northwest. By the afternoon of May 27, the circulation had become well-defined with a curved band of strong thunderstorms just to the north and west of the center. This resulted in the designation of the system as Tropical Depression Two.

Overnight, the system continued west-northwestward toward a tongue of warmer Gulf Stream waters off of the coastline of South Carolina. Even as it encountered higher sea surface temperatures, however, significant wind shear out of the south kept the center exposed through the day of May 28. By that morning, rain bands had begun to affect the U.S. coastline. The convection deepened somewhat that afternoon and the circulation became better defined, so the system was named Tropical Storm Bonnie. Since Bonnie was named before the official start of the season on June 1, 2016 became the first year since 2012 to have multiple preseason storms. A few hours later, the storm reached its peak intensity of 45 mph winds and hesitated slightly in its forward motion.

Shortly afterward, however, a large burst of wind shear out of the south ripped the existing convection away from Bonnie, weakening it overnight just before its landfall in South Carolina as a tropical depression during the morning of May 29. Despite the decay of the system, heavy rainfall continued for a large region of the southeast U.S. through the day. Shortly afterward, the cyclone took its anticipated turn toward the north east over land. By the morning of May 30, the system had weakened sufficiently that it no longer met the criteria of being a tropical cyclone. Bonnie's remnants continued to generate shower activity as the center moved slowly northeast back over the Atlantic Ocean on May 31.

In fact, the system became more organized back over water, slowly redeveloping convective bands and increased sustained winds. By the morning of June 2, it had regained tropical depression status near the Outer Banks. That day, the Bonnie strengthened slightly over the warm Gulf Stream waters as it moved away from land, but remained slightly below tropical storm strength. Meanwhile, it began to accelerate eastward under the influence of a subtropical ridge. Overnight, the system seemed to be on the wane as it moved over cooler water and convection diminished, but a resurgence during the afternoon of June 3 prompted once again upgrading Bonnie to a tropical storm. However, very cold water finally took its toll on the storm on June 4: it lost all significant shower activity, weakening to a tropical depression, and shortly thereafter, a remnant low. The remnants moved east-southeast for another couple of days before dissipation.

The above image shows Bonnie shortly before landfall in South Carolina.

Bonnie's slow motion near the U.S. coastline as both a tropical and a non-tropical system brought torrential rains to the Carolinas.

Wednesday, May 18, 2016

Professor Quibb's Picks – 2016

My personal prediction for the 2015 North Atlantic Hurricane Season (written May 18, 2015) is as follows:

14 cyclones attaining tropical depression status*,
13 cyclones attaining tropical storm status*,
7 cyclones attaining hurricane status*, and
3 cyclones attaining major hurricane status.
*Note: Hurricane Alex formed on January 13, long before the official start of the season on June 1 and before I made these predictions.

This prediction calls for a nearly average Atlantic hurricane season, with predictions just barely exceeding historical averages in all categories.

The picture for the 2016 Atlantic hurricane season is unusually murky, due to several uncertainties regarding significant factors that influence tropical cyclone formation. First, the 2015-16 El Ninò event has continued to unfold, ranking in the top 3 historically in both intensity and duration. Positive sea surface temperature anomalies have persisted into May in the equatorial Pacific, indicating the continuation of the event. The chart below compares El Ninò events since 1950.

The 2015-16 event (black line) is probably most comparable to the 1997-98 event in its qualities, so if this trend were to repeat, the 2016 season would end with the ENSO in a negative phase. However, it has occurred that El Ninò events persist to the end of the second year, or that they become roughly neutral. A neutral ENSO (El Ninò Southern Oscillation) index, all else held equal, would lead to an average hurricane season, and a negative index to a more active season. The latest predictions indicate that neutral conditions will in fact prevail during the season's peak in September and October, but there is a great deal of uncertainty.

Second, the Atlantic Multi-Decadal Oscillation (AMO) (an empirically observed trend in tropical cyclone activity that has decades-long period) appears to be wrapping up the positive phase that led to busier hurricane seasons during the 2000's and early 2010's. However, this trend is harder to predict than the ENSO, and while some meteorological experts believe that it is now entering its negative phase, it is difficult to know for certain. The combination of these two factors yield an expectation of an average season, but with an unusually high probability of deviance from this prediction.

Finally, we examine a few more proximate factors to cyclone formation in the Atlantic. Current mean sea surface temperatures, as with all global temperatures, are anomalously high relative to historical data. However, temperatures in the Gulf of Mexico and along the U.S. Eastern seaboard are lower relative to average than the southern Caribbean and central tropical Atlantic. These latter areas may therefore be especially favorable to cyclonogenesis. Normally, preseason wind shear tendencies would also be relevant to my forecast, but due to the possible rapid changes in the ENSO index, these observations would have little predictive power.

My estimated risks for different parts of the Atlantic basin are as follows (with 1 indicating very low risk, 5 very high, and 3 average):

U.S. East Coast: 2
Neither the jet stream nor the negative anomaly in sea surface temperatures is as pronounced in this region as in 2015. Nevertheless, wind shear may still inhibit development in this region, leading to a lower risk of landfalls.

Yucatan Peninsula and Central America: 4
The southern Caribbean has some of the most anomalously warm temperatures in the Atlantic, and could fuel tropical cyclones that traverse it. After upper-level winds subside about midway through the season, there is potential for dangerous hurricanes to develop in this region.

Caribbean Islands: 3
The Caribbean Islands are at about average risk this year, with moderately warm temperatures and a diminishing El Ninò that will lead to a fair, but not exceedingly high likelihood of westward-tracking cyclones. Expect 2-3 tropical storms, at least one of which is of hurricane strength, to affect the islands.

Gulf of Mexico: 2
The Gulf remains rather safe this year, continuing the trend from the previous two seasons. Rather low temperatures will limit the potential for significantly damaging landfalls.

Overall, the 2016 season is expected to be around average, but there is an unusually low degree of confidence in this forecast due to expected shifts in climate throughout the year. Regardless, everyone should take sufficient preparedness measures, since dangerous storms can occur even in quiet seasons.


Saturday, May 14, 2016

Hurricane Names List – 2016

For the North Atlantic Basin, the list for naming tropical cyclones in 2016 is as follows:


This list is the same as that for the 2010 season, with the exception of Ian and Tobias, which replaced the retired names Igor and Tomas, respectively.