Monday, September 28, 2015

Hurricane Joaquin (2015)

Storm Active: September 27-October 8

On September 25, a trough of low pressure situated over the tropical western Atlantic began to exhibit some signs of organization as it moved slowly toward the north-northwest. The next day, a surface low formed in association with the system and thunderstorm activity became more concentrated. On September 27, the low deepened and became better defined. Persistent deep convection near the center of circulation therefore led to the designation of Tropical Depression Eleven late that evening.

Immediately after formation, Eleven already faced increasing shear out of the north-northwest that exposed the center of circulation. Situated to the south of a high pressure system over the western Atlantic, the cyclone moved slowly and erratically over the next day, heading generally westward. Late on September 28, the cyclone's center suddenly moved rapidly southwestward, coming closer to the deep convection. As a result, organization increased, and the system was upgraded to Tropical Storm Joaquin. Shear lessened on September 29 while the cyclone was situated over very warm waters. As a result, Joaquin began a period of fairly rapid strengthening. Deep convection slowly became more symmetric about the center, and the system reached strong tropical storm intensity that afternoon. Meanwhile, the ridge over the northeast Atlantic grew stronger, pushing Joaquin southwest toward the central Bahamas. Early on September 30, healthy outflow was established in the northern semicircle and the beginnings of an eye appeared on satellite imagery. As a result, the system was upgraded to a category 1 hurricane.

Joaquin's central pressure continued to plummet that day. Even though a stable eye had yet to appear on infrared imagery, cloud tops in the eyewall cooled significantly and the system became more symmetric overall. The outer bands of the system affected the central Bahamas during that morning as the storm approached. During the evening, Joaquin exploded in intensity to become a major hurricane, the most powerful of the season. By the morning of October 1, the center of the storm entered the central Bahamas. The cyclone made a little more progress southwestward that day, walloping the Bahamas with high winds, a sustained period of torrential rain, and substantial storm surge in some locations. That afternoon, Joaquin achieved category 4 status, reaching its peak intensity intensity of 130 mph winds and a minimum pressure of 931 mb. This was the lowest pressure observed in an Atlantic hurricane since Hurricane Igor in 2010. Early on October 2, Joaquin began its long anticipated turn toward the north and then northeast as a trough moving eastward over the United States began to influence its motion. Meanwhile, though Joaquin never directly affected the U.S. east coast, tropical moisture from the hurricane interacted with a frontal system moving eastward to bring immense amounts of rain to the Carolinas.

Joaquin finally moved away from the Bahamas later that day. The eye clouded over but reappeared again on October 3, causing some fluctuations between category 3 and 4 intensity. For a brief period that afternoon, the eye of Joaquin became very well-defined and winds were estimated to reach 155 mph, just under category 5 strength (these were the strongest winds by an Atlantic hurricane since Igor). However, the hurricane did not beat its previous minimum pressure. After this secondary peak, wind shear began to weaken Joaquin. Meanwhile, acceleration toward the north and east continued, and the system began to affect Bermuda as a category 2 hurricane on October 4. During the evening, around 8:00 pm EDT, Joaquin made its closest approach to Bermuda, passing about 65 miles to the west-northwest. The hurricane was close enough to bring tropical storm force winds and hurricane gusts to the island.

Wind shear abated while the system was still over marginally warm water overnight and through the morning of October 5, halting weakening at category 1 status. Joaquin continued to maintain a well-defined eye even as it gained latitude. On October 6, the hurricane turned northeastward and accelerated toward the open northern Atlantic. That night, the system began to lose tropical characteristics as it moved north of the Gulf Stream into colder waters and convection became asymmetric. It weakened to a tropical storm on October 8 and became post-tropical late that evening.

Joaquin was an unusually powerful hurricane for the El Nino conditions in which it formed. Anomalously warm ocean temperatures contributed to its intensity.

Hurricane Joaquin was very difficult to predict early in its lifetime, leading to large forecast errors in the extent of its southwestward motion and timing of its northeastward turn.

Friday, September 18, 2015

Tropical Storm Ida (2015)

Storm Active: September 18-27

Yet another tropical wave entered the eastern Atlantic on September 13, bearing generally westward south of the Cape Verde Islands. A low pressure center appeared in association with the system soon after. By the 15th, the disturbance assumed a more west-northwestward heading as the ridge to its north remained somewhat weak. Convection remained vigorous throughout the next few days, but the low was unable to take advantage of mainly favorable conditions. However, thunderstorm activity became much more concentrated near the center early on September 18, and the formation of Tropical Depression Ten followed later that morning. The depression strengthened into Tropical Storm Ida that night.

By the morning of September 19, the newly named tropical storm began to experience wind shear out of the northwest. Though banding features and deep convection remained healthy on the south and east quadrants, the center of circulation became exposed to the northwest. Over the next day, Ida continued to lose organization. During the afternoon of September 20, the cyclone's forward speed suddenly increased, but this trend was to reverse almost immediately as the storm approached an area of weak steering currents by that evening. Meanwhile, shear relaxed and Ida's convection came roaring back: a huge area of intense convection appeared and covered the center. As a result, the system began to strengthen.

However, this new trend was stopped by a new feature in the environment - a pronounced increase in mid-level shear. Throughout the day of September 21, the shower and thunderstorm activity slowly separated from the center. In addition, the lower- and upper-level centers themselves became separated, making the system difficult to pinpoint. By September 22, the influence of a large trough to the north of Ida caused it to cease progress towards the west-northwest and instead drift in the entirely opposite direction, toward the east-southeast. The havoc caused by the wind shear caused the system's circulation to fragment somewhat, with multiple low-level vortices evident as cloud swirls on satellite imagery west of the convective canopy. Overnight, as it hovered around minimal tropical storm strength, Ida even took an unusual southward turn. On September 23, however, the though began to carry Ida on a more steady path toward the east.

Though the trough began to move away from the system, the destructive influence of dry air and wind shear did not abate, causing Ida to weaken to a tropical depression on September 24. Shortly afterward, a ridge to the system's north forced it to switch directions again, this time to the north-northwest. The cyclone continued to persist in the face of unfavorable conditions, pulsating periodically with new convection. Therefore, it maintained tropical depression status into September 26. Ida's motion turned more toward the west that day and began to degrade further as bursts of shower activity became less frequent. By the afternoon of September 27, Ida had been devoid of convection so long that it no longer met the criteria for a tropical cyclone and degenerated into a remnant low. The low continued generally northwest for several days, even showing some signs of organization near the end of September. However, no redevelopment occurred and the system eventually merged with a frontal boundary.

The above image shows the disorganized tropical storm Ida over the central Atlantic.

Ida did not approach any landmass throughout its lifetime as a tropical cyclone.

Thursday, September 17, 2015

Tropical Depression Nine (2015)

Storm Active: September 16-19

On September 10, a tropical wave began to produce some shower activity as it moved off of the coast of Africa. Atmospheric conditions were initially unfavorable for any further development, but the system's environment improved on September 12, allowing it to organize. Meanwhile, it passed well south of the Cape Verde Islands, moving on a west-northwestward trajectory. On September 14, dry air interrupted the system's development and convection collapsed. The low associated with the system continued to persist and deepen, however, as upper-level winds remained low. After making a turn to the northwest, the system rebounded on September 16 and was classified Tropical Depression Nine.

Almost immediately after becoming a tropical cyclone, Nine entered an area of strong upper-level winds as it moved closer to an upper-level low to the northwest. Despite its convection being displaced well to the east of the center, Nine still managed a little strengthening early on September 17. In the midst of powerful wind shear associated with the strong El Nino event, the depression did little besides generate limited convection east of the center throughout that day and the next. By September 19, the system had become devoid of convection for the final time and the circulation became elongated along the southeast-northwest axis. Shortly afterward, the system lost tropical cyclone status. Its remnants continued generally northwestward until losing their identity.

Tropical Depression Nine was at its most organized immediately after formation (as shown above) while it remained in an environment that supported development.

The above image shows the track of Nine.

Wednesday, September 9, 2015

Tropical Storm Henri (2015)

Storm Active: September 8-11

On September 7, a low pressure system formed near the tail of a stationary front situated over the central Atlantic, well to the southeast of Bermuda. Despite inhibitive upper-level winds, the system began to produce a concentrated area of shower and thunderstorm activity while it remained nearly stationary. The low became better defined on September 8 and though convection was largely confined to the eastern side of the circulation, it had acquired enough organization that night to be classified Tropical Depression Eight. At the time of its classification, Eight had a large area of maximum winds and some other subtropical characteristics, but was more tropical than subtropical.

Though the satellite presentation remained extremely disorganized, with a broad circulation barely in contact with thunderstorm activity to the east, the system's maximum winds did increase somewhat over the next day and Eight was upgraded to Tropical Storm Henri late on September 9. Soon after, the cyclone began to move northward as the ridge holding it place slowly eroded. It passed well east of Bermuda during the morning of September 10, accelerating toward the north as it did so. Wind shear relaxed later that day, but the poorly organized system, still battling incursions of dry air, was unable to take advantage and remained at minimal tropical storm strength. The circulation became elongated by September 11 with multiple low-level swirls evident on satellite imagery. Henri lost its well-defined center during that afternoon and degenerated into a trough. Its remnants merged with a frontal boundary the next day.

Never in its lifetime did Henri develop any convective structure that circumnavigated the center of circulation. This contributed to its inability to significantly intensify.

Henri (and its progenitor system) remained stationary for a few days due to a ridge to the north before an incoming front allowed it to accelerate poleward.

Saturday, September 5, 2015

Tropical Storm Grace (2015)

Storm Active: September 5-9

Yet another tropical wave moved off of the coast of Africa on September 3, exhibiting signs of organization shortly thereafter. By the next day, the system developed a vigorous circulation, but lacked a closed circulation. However, as it moved past the Cape Verde Islands to the south on September 5, the disturbance gained enough organization to be classified Tropical Depression Seven. Just a few hours later, Seven intensified into Tropical Storm Grace over the eastern Atlantic. Deep convection remained limited over the next day as dry air tried to invade the circulation, but the system gradually deepened nonetheless through the day of September 6, reaching its peak intensity of 50 mph winds and a central pressure of 1002 mb.

A strong subtropical ridge maintained Grace's rapid westward motion as shear out of the west-southwest began to increase later that day. This shear impeded the outflow and convective banding of Grace, and began to induce weakening during the morning of September 7. The system maintained some deep convection through the morning of September 8, but it lacked any organizational structure; by this time, Grace had weakened to a minimal tropical storm. It weakened further into a tropical depression later that day as it entered a more stable airmass hostile to thunderstorm formation. By the morning of September 9, Grace was little more than a swirl of low clouds. Shortly afterward, it lost closure entirely and degenerated into a tropical wave. Moisture associated with Grace eventually reached the Caribbean islands a few days later.

Grace never exhibited much convective organization, remaining a weak tropical storm (as shown above) for most of its short lifetime.

As with Danny and Erika before it, Grace formed over the east Atlantic but ultimately suffered its demise amidst the harsh El Nino atmospheric conditions of the Caribbean and central Atlantic.