Thursday, August 17, 2017

Tropical Storm Harvey (2017)

Storm Active: August 17-

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.

As of 11:00 am EDT on August 18, 2017, Tropical Depression Harvey had maximum winds of 35 mph, a minimum central pressure of 1007 mb, and was moving to the west at 22 mph.

Sunday, August 13, 2017

Hurricane Gert (2017)

Storm Active: August 12-17

On August 2, a vigorous tropical wave left Africa and moved westward over the Atlantic Ocean. While environmental conditions seemed conducive for development, the system was unable to consolidate. Dry air interfered with the production of deep convection, and the associated circulation remained highly elongated. Competing vortices on the northeast and southwest sides vying for dominance cost the wave the chance to organize over the next several days. By August 7, conditions had become unfavorable due to the presence of an upper-level low to the northeast. Nevertheless, the system proceeded steadily west-northwestward, passing a bit north of the Lesser Antilles on August 10. Wind shear diminished and the wave encountered more humid air soon after, giving it another chance at development. Convection increased and the circulation became better defined over the next two days, and Tropical Depression Eight finally formed late on August 12.

The system was experiencing some wind shear out of the north, but conditions were otherwise supportive of intensification. August 13 saw the naming of Tropical Storm Gert when the system lay well east of the Florida coastline and was turning toward the north. The inner core structure improved considerably that night and into August 14. The first hints of an eye appeared that afternoon, and Gert was upgraded to a hurricane that night. The cyclone then began to feel the influence of a frontal system moving off of the U.S. east coast, and turned northeast on August 15, accelerating as it did so. Even as it gained latitude, Gert still took advantage of warm Gulf Stream waters to continue strengthening. A compact eye feature became apparent on both visible and satellite imagery by the morning of the 16th. That evening, the system reached its peak intensity as a category 2 hurricane with 105 mph winds and a minimum pressure of 967 mb near 40° N. Cooler waters and deteriorating atmospheric conditions finally caught up with Gert overnight and it weakened, beginning extratropical transition. The system became extratropical on the afternoon of August 17 as it sped east-northeastward over the open ocean.



The above satellite image shows Gert at peak intensity as a category 2 hurricane. Nova Scotia is visible at the top of the image.



A frontal boundary interacting with Gert steered it out to sea, minimizing land impacts.

Monday, August 7, 2017

Hurricane Franklin (2017)

Storm Active: August 6-10

Towards the end of July, a tropical wave tracked westward across the central Atlantic, showing some potential for development as it produced scattered showers and thunderstorms. Dry air and deteriorating atmospheric conditions stifled this potential by the time August had begun. Nevertheless, the tropical wave continued into the Caribbean. Significant convection flared up near the system on August 3 when it was located in the eastern Caribbean, and surface pressures began to slowly decline in the area. For the next day or two, however, it was contending with very high wind shear, and was unable to organize much. This changed early on August 5, when thunderstorm activity concentrated near its nascent circulation. Meanwhile, shear began to decline, allowing the system to take advantage of quite warm sea water. Late on August 6, Tropical Storm Franklin was named northeast of Honduras.

Franklin's environment steered it steadily west-northwest the following day. Its banding features steadily improved, resulting in steady strengthening. By that afternoon, its sustained winds had increased to 60 mph and its pressure had dropped to 999 mb. However, dry air infiltrated the circulation from the south that evening, weakening thunderstorm activity near the center and preventing additional intensification before Franklin made landfall that evening in the Yucatan Peninsula. The cyclone weakened over land into August 8, but the low-lying land did not disrupt the core much, and the system remained well-organized. Late in the afternoon, the center of circulation emerged over the Bay of Campeche and assumed a more westward trajectory. Strong outer bounds quickly formed and Franklin's core also quickly improved over the very warm ocean water. The following day saw the system intensify from a minimal tropical storm to a category 1 hurricane by the afternoon of August 9. It strengthened a bit further to its peak intensity of 85 mph winds and a pressure of 981 mb before making landfall in Mexico. Franklin's decay was swift over the mountainous terrain, and it dissipated by late morning on August 10.

The remnants of Franklin crossed over into the eastern Pacific Ocean over the next day and quickly reorganized over water. Late on August 11, they regenerated into a tropical storm. Since the system dissipated before reforming in another basin, it received a separate name from the Eastern Pacific name list: Jova.



The above image shows Hurricane Franklin at peak intensity just before landfall in Mexico.



Franklin strengthened quickly over the warm waters of the Bay of Campeche.

Monday, July 31, 2017

Tropical Storm Emily (2017)

Storm Active: July 31-August 1 During the last week of July, a cold front extending from Texas to the northeast U.S. pushed south and east, weakening as it did so. By July 30, the southern half of the cold front had moved over the Gulf of Mexico and stalled. A non-tropical low quickly formed along it just south of the Florida panhandle and moved slowly toward the east. Overnight, thunderstorm activity became clustered around the circulation center and the system became organized enough to merit tropical depression status early on July 31. Just two hours after formation, radar indicated that Tropical Depression Six's maximum winds had risen to 45 mph and it was upgraded to Tropical Storm Emily.

The storm moved just south of east through the morning and made landfall near Tampa, Florida before noon. Heavy rain fell even as the system weakened over land, with the heaviest south of the center. By the afternoon, the system had weakened to a depression. It accelerated and turned toward the northeast overnight and emerged over open Atlantic waters east of Florida early on August 1. Emily's grip on tropical cyclone status was quite tenuous by this time: only scattered bursts of disorganized convection remained. Late that night, it was downgraded to a remnant low. This low continued out to sea over the next several days before dissipation.



The above image shows Tropical Storm Emily making landfall in Florida within 12 hours of formation.



Emily was yet another short-lived tropical storm, the fifth of the 2017 season. In fact, the combined ACE (accumulated cyclone energy) of these storms was the lowest on record for the first five of an Atlantic hurricane season.

Monday, July 17, 2017

Tropical Storm Don (2017)

Storm Active: July 17-18

A mid-July tropical wave crossed westward from the coast of Africa to more than half-way across the tropical Atlantic without generating much thunderstorm activity. However, on July 16, the system began to organize, despite the proximity of dry air. A low pressure center formed shortly afterward, even as convection remained quite limited. During the afternoon of July 17, a curved band developed about the center and the circulation became better-defined. As a result, the low was upgraded to Tropical Storm Don about 500 miles east of the Windward Islands.

Over the next day, Don moved quickly toward the west. It strengthened briefly as a central dense overcast appeared, but increasing shear reversed this slight intensification as quickly as it had occurred. By midday on July 18, Don's disorganized thunderstorm activity was moving over the Windward Islands. That evening, before the system passed over the islands, Don lost its circulation center in the face of strong shear and dissipated. Scattered gale force winds and heavy rain did continue, however, as its remnants entered the Caribbean.



Tropical Storm Don was only a small cyclone for its brief existence, forming as it did on the edge of a dry air mass with limited moisture supply.



Don existed for less than two days before succumbing to high wind shear as it entered the Caribbean Sea.

Thursday, July 6, 2017

Tropical Depression Four (2017)

Storm Active: July 5-7

At the beginning of July, a tropical wave located southwest of the Cape Verde Islands began to organize. The system was moving rather slowly for its latitude over the next several days, allowing it to began circulating more easily than it otherwise would. Slowing development, however, was its interaction with the intertropical convergence zone (ITCZ). Even though this interaction generated a great deal of convection, the disturbance needed to separate from the ITCZ to initiate development. On July 4, the system began to veer toward the west-northwest and gain some latitude. The next day, it acquired a circular area of strong thunderstorms near its center and became Tropical Depression Four over the open tropical Atlantic.

Shortly afterward, however, the system began to feel the effects of a Saharan dry air encroaching from the north and east. On July 6, the depression continued to the west-northwest, but its thunderstorm activity slowly declined as it entrained dry air. In addition, the cyclone increased in forward speed, making it difficult for the circulation to persist. It did not persist long, in fact: the system lost a closed circulation and dissipated during the afternoon of July 7, far from any land.



The above image shows Tropical Depression Four over the open Atlantic.



The short-lived tropical depression fell victim to a large dry air mass quickly after formation.

Tuesday, June 20, 2017

Tropical Storm Cindy (2017)

Storm Active: June 20-22

On June 16, a large trough of low pressure formed over the western Caribbean Sea and the neighboring regions of central America. Heavy rainfall fell over adjacent landmasses as the system organized just east of the coast of Belize and moved slowly northward. By late on June 18, a huge north-south area of convection lay just to the east of the center of the disturbance, but it was still not organized enough to be considered a tropical cyclone. When it moved over the Gulf of Mexico shortly afterward, the open waters stimulated further development: finally, on June 20, it developed into Tropical Storm Cindy.

From its formation onward, Cindy did not look particularly like a tropical storm. The center remained largely devoid of convection, with several low-cloud swirls competing for dominance. Heavy rain was falling, but well away from the center in the northern semicircle. Much of this precipitation was already falling over land, from eastern Texas to the Florida panhandle. Cindy moved slowly toward the northwest as a medium-strength tropical storm through that night and June 21. Unfavorable wind shear prevented the storm from intensifying further. Early the next morning, Cindy made landfall near the border of Louisiana and Texas. After landfall, it quickly weakened to a tropical depression. The remnants of Cindy continued to bring rain over the U.S. as it traveled northeastward at a progressively faster clip over the following days.



Tropical Storm Cindy was a rather asymmetrical system with little to no convection near the center of circulation.



Cindy existed as a tropical cyclone only briefly in the Gulf of Mexico before making landfall.

Tropical Storm Bret (2017)

Storm Active: June 19-20

On June 13, a tropical wave formed just off the Atlantic coast of Africa and began rapidly moving toward the west. From the beginning, the system was located at a very low latitude, but was quite vigorous in its production of thunderstorm activity. Conditions were favorable for development in the low-latitude tropical Atlantic, and organization proceeded slowly over the next several days. By June 18, the wave had developed a broad circulation, but was having difficulty acquiring a well-defined center due to its rapid westward motion. The next day, a closed center was found; since gale force winds were already occurring north of the center, it was classified Tropical Storm Bret. At the time, it was centered just east of coastal Venezuela moving toward the west at a blustering 30 mph.

Early on June 20, the center of Bret crossed extreme northern Venezuela and moved into the much more hostile environment of the eastern Caribbean, where wind shear was quite high. The system's circulation, never well established, did not long survive these conditions, and Bret dissipated that same afternoon. Bret was the first known system to develop so early in the season within the low-latitude tropical Atlantic east of the Caribbean. It was also the lowest-latitude Atlantic tropical system in June since 1933.



The above image shows Tropical Storm Bret near the island of Trinidad.



Though short-lived, Bret was an unusual tropical storm. It was one of a rare class of tropical cyclones to make landfall in South America.

Monday, May 15, 2017

Professor Quibb's Picks – 2017

My personal prediction for the 2017 North Atlantic Hurricane season (written May 15, 2017) is as follows:

15 cyclones attaining tropical depression status*,
15 cyclones attaining tropical storm status*,
6 cyclones attaining hurricane status, and
3 cyclones attaining major hurricane status.
*Note: Tropical Storm Arlene formed on April 19, 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 slightly exceeding historical averages in all categories.

In contrast to 2016, the conditions for the 2017 season are fairly common and uncertainty is relatively low. The first condition taken into account is the state of the El Niño Southern Oscillation Index (or ENSO index), a measure of sea surface temperature anomalies in the Pacific Ocean that has a tendency to affect Atlantic hurricane activity. After the index took a brief dip into negative territory this past winter, the index has returned to nearly zero, or "neutral." As shown in the figure below from the International Research Institute for Climate and Society, a modest increase is expected over the coming months.



As a result, the conditions prevailing for the hurricane season are likely to be neutral or weakly El Niño. Since El Niño tends to suppress Atlantic activity and cause cyclones to, on average, take more easterly tracks, this factor would suggest a quieter hurricane season.

Sea surface temperatures, meanwhile, are a bit above average across the Atlantic basin, but the anomalies are not as great in magnitude as they have been over the past few years. The warmest areas are currently the Caribbean and the tropical portion of the Atlantic farther east. Parts of the Gulf of Mexico, meanwhile, are slightly cooler than average. Further warming of the current higher-than-normal areas is likely over the next few months, so these might be conducive to cyclonogensis. The tropical Atlantic has also been quite moist, as has the Caribbean, supporting the development of hurricanes. The Gulf of Mexico, in contrast, has been persistently dry. Finally, with the developing El Niño, increasing wind shear is likely across the Atlantic, especially at higher latitudes and near the United States. Such shear is hostile to tropical systems, so I predict limited activity near the U.S. east coast, despite a pocket of warm water there.

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: 3
The presence of an El Niño would tend to reduce risk, as stated above. However, seasonal forecasts indicate that high temperatures will prevail near the coast for most of the summer, resulting in higher oceanic heat content. Look for quick forming and quick hitting systems - long-lived hurricanes are likely to miss the coast this year.

Yucatan Peninsula and Central America: 3
Signs in the adjacent Caribbean Sea point to elevated tropical activity this year: warm waters, moist air, and limited wind shear. However, steering ridges will have a difficult time setting up along the Caribbean Islands to the north, preventing developing storms from tracking due westward for the most part and instead allowing them to gain latitude. A combination of these two opposing factors leads to the "average" designation for this region.

Caribbean Islands: 4
Complementing the previous point, the Caribbean Islands will be in the more likely path of tropical cyclones. Coupled with the fact that the tropical Atlantic is warm, there is significant risk for landfalling tropical storms and hurricanes this year.

Gulf of Mexico: 1
Atmospheric conditions about the Gulf are already dry and strong upper-level winds are moving across the region. With an increasing ENSO index, this state of affairs is likely to continue indefinitely. Combined with the slightly cooler waters, these signs indicate a very low risk for the Gulf coast.

Overall, the 2017 season is expected to be near or just slightly above average, but with a lower than average risk to landmasses (most storms should curve out to sea). While the confidence in this forecast is somewhat higher than last year, everyone in hurricane-prone areas should still take due precautions as hurricane season approaches. Dangerous storms may still occur in quiet seasons. Sources: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf, http://www.ssd.noaa.gov/PS/TROP/TCFP/atlantic.html

Saturday, May 13, 2017

Tropical Storm Arlene (2017)

Storm Active: April 19-21

During mid-April, a non-tropical low over the central Atlantic well east of Bermuda was producing a large area of tropical storm force winds and scattered thunderstorm activity. As the system drifted eastward, it more more organized, and began to show signs of subtropical development by April 18. Though convection remained mainly confined to the southeast quadrant by the next morning, the low had acquired enough organization to be classified Subtropical Depression One. At that time, the cyclone was moving north-northeast at a moderate clip as it interacted with an extratropical low.

Any cover the center of circulation had managed to develop that day was quickly stripped away by increasing wind shear by early on April 20. The system made a comeback later that morning, however, and in fact became more symmetric, resulting in its reclassification as a tropical depression. It turned toward west-northwest that afternoon and unexpectedly strengthened into Tropical Storm Arlene, only the second known tropical storm to form in April in the Atlantic. Further, its central pressure dropped to 993 mb, the lowest for a tropical system ever recorded in the month of April. Arlene's unusual run ended the next day as it became extratropical and was quickly absorbed by a larger system.



The above image shows Tropical Storm Arlene near its peak intensity over the open Atlantic.


Arlene did not approach any landmasses during its short lifetime. However, it was notable in that it was only the second Atlantic tropical storm known to form in April, after Ana in 2003.