Monday, August 26, 2019

Tropical Storm Erin (2019)

Storm Active: August 26-29

On August 21, an trough of low pressure near the eastern Bahamas began to produce a scattered area of shower and thunderstorm activity. It moved slowly west-northwestward, but developed only slowly due to wind shear. Just as the storm became more concentrated on August 23, the newly formed low pressure center moved over southern Florida. The land interaction temporarily hampered further progress toward tropical depression status and most rainfall remained offshore to the east. Within another day, the system felt the tug of an approaching cold front and turned northeast, emerging over water once again. The low pressure center remained elongated, though, and westerly shear kept the western half of the circulation dry. Nevertheless, Tropical Depression Six formed during the afternoon of August 26, well offshore of North Carolina. At that time, Six had slowed to a near standstill due to the influence of nearby mid-level high pressure systems.

For the next day, the cyclone moved little and the circulation center remained exposed to the northwest of the cloud cover. Nevertheless, it became a little more organized on August 27 and was upgraded to Tropical Storm Erin. That night, it picked up speed toward the north under the influence of an upper-level trough and passed about halfway in between Bermuda and the coast of North Carolina. Shear increased again and Erin weakened back to a tropical depression on August 28. The circulation began to lose definition too. By the morning of the 29th, Erin had started to merge with a nearby front and became post-tropical.

The above image shows Tropical Depression Six just before being upgraded to a tropical storm.

Erin did not affect any landmasses, but its remnants eventually brought heavy rainfall to Atlantic Canada.

Hurricane Dorian (2019)

Storm Active: August 24-September 7

Through mid-August, the presence of Saharan dry air in the main development region (MDR) of the Atlantic between west Africa and the Lesser Antilles stifled the tropical waves that periodically traversed the basin east to west. This finally began to change on August 23 when a low pressure system formed in association with a tropical wave a bit more than halfway to the Caribbean from Africa. Already, spin was evident on satellite imagery, but thunderstorm activity was at first quite limited. However, the low managed to battle the aforementioned dry air in its vicinity and consolidate further, becoming Tropical Depression Five late in the morning on August 24. Not long after, convection concentrated near the center of circulation and it strengthened into Tropical Storm Dorian.

At first, wind shear gave the cyclone a ragged appearance on satellite imagery and limited the upper-level outflow characteristic of a healthy tropical cyclone. This shear lessened on the 25th and Dorian took advantage, beginning to intensify slowly. Meanwhile, the system maintained its heading due west. Deep convection blossomed more consistently, keeping dry air at bay. The inner core was struggling, with multiple vortices and an ill-defined circulation center. This did not prevent it from bringing heavy rains to Barbados late on August 26, when Dorian became one of the only tropical cyclones recorded to pass directly over the island. The system also entered a weakness in the ambient subtropical ridge and turned west-northwest. Several factors favored intensification for Dorian, including low shear and warm ocean temperatures, but the circulation had dry air to contend with and could not get its center together; the mid-level and surface circulations were decoupled (displaced over 100km from one another). Further, the structure that Dorian managed to accumulate was torn apart when it passed directly over St. Lucia early on August 27 and entered the Caribbean.

The battered low-level center dissipated after contact with land and reformed farther north, resulting in a more vertically stacked circulation. This also had a significant impact on the eventual track, because it put Dorian on a northwestward heading. Late on the 27th, intensification resumed and an eyewall appeared on radar imagery. As the cyclone passed over the U.S. Virgin Islands early on August 28, a massive burst of convection appeared in the eyewall. Within hours, Dorian reached hurricane status. Some islands recorded sustained winds to near hurricane force and over 5 inches of rain. The storm's winds increased a bit further through the evening, though patches of dry air out of the south gave a few parting shots at Dorian's core as it moved northwestward into a moister atmosphere. By early on the 29th, Dorian had left the Caribbean behind.

Hints of an eye appeared on microwave satellite imagery that day, but the core only slowly improved. This was in part due to an upper-level low over the Bahamas imparting moderate shear out of the southwest. The limited shear did not prevent Dorian from reaching category 2 status that evening. For most of the day of August 30, the cyclone still had a curious “squashed” appearance, with excellent outflow channels toward the east and west, but a relatively narrow cloud shield north-to-south. The eye did begin to clear out in earnest though, and Dorian steadily intensified into the first major hurricane of the 2019 season. Simultaneously, the same upper-level low that had sheared Dorian helped it to turn back toward the west, in conjunction with a building ridge over the western subtropical Atlantic. Once the hurricane was north of the upper-level low and moving west, the flow was aligned with Dorian’s forward motion. Shear therefore diminished and left the system in near-perfect conditions for rapid intensification.

Late on August 30, it took advantage. A large, symmetric she cleared out at last on visible satellite. Dorian’s central pressure plummeted and its winds increased to match; within a few hours, it was a category 4 hurricane. The trends continued on the 31st as sustained winds reached 150 mph, high-end category 4 strength. Even colder cloud tops appeared in the now very circular storm, bringing Dorian to category 5 status early on September 1. The extraordinary bout of strengthening did not abate until that afternoon, when Dorian reached its peak intensity of 185 mph sustained winds and a central pressure of 910 mb. Only a handful of Atlantic hurricanes on record had ever achieved winds of this magnitude, and none had ever impacted the Bahamas at such an intensity. The eye of Dorian passed over Abacos Island of the northern Bahamas at peak intensity, causing catastrophic damage to the island. Even worse, steering currents collapsed that evening and the system stalled early on September 2 near the neighboring Grand Bahama. These two islands, especially the second, therefore spent many hours in the destructive eyewall of the hurricane.

Dorian’s strength finally began to ebb as the strong winds caused upwelling of cooler ocean water from beneath the surface (a common occurrence for stalled hurricanes). Late in the morning on the 2nd, it weakened to a category 4. This did little to improve the situation in Grand Bahama, however, for the eye remained stationary just north of the island that whole day. An eyewall replacement cycle occurred during the afternoon, having the net effect of enlarging wind radii but lowering the peak strength of the cyclone. Early on September 3, Dorian weakened to a category 3 and finally budged northwest as a weakness developed in the ridge. The core of the storm ingested some dry air that afternoon, though it was quickly incorporated into a ragged eye that evening. As often occurs for systems gaining latitude, Dorian continued to expand, and it passed close enough to the central Florida coastline to cause tropical storm force winds in its outer rainbands. Maximum winds dropped a little more though and the cyclone weakened to a category 2 later on the 3rd.

As Dorian moved away from its own cold wake, it produced colder cloud tops again, aided by the Gulf Stream waters. It picked up a little more speed and turned north-northwest during the morning of September 4. Fortunately for the U.S. southeast, an approaching front facilitated Dorian turning farther to the right. It traveled roughly parallel to the coastline that day, turning due north around the latitude of northern Florida and then north-northeastward as it moved toward the Carolinas. Meanwhile, its structural improvements allowed slight strengthening, and Dorian regained major hurricane strength late on the 4th, reaching 115 mph winds and a 955 mb pressure. During the morning of September 5, the western eyewall swept over coastal South Carolina and it weakened back to category 2 strength. The close brush caused significant storm surge inundation and hurricane force wind gusts. Overnight, Dorian's center made an even closer approach to North Carolina. The winds decreased to category 1 strength, but the central pressure remained steady just below 960 mb, and the inner core structure was remarkably stable. Late in the morning on September 6, the cyclone made an official landfall in Cape Hatteras, North Carolina as it accelerated northeastward. Even as it gained latitude, Dorian continued to generate new deep convection in the eyewall through that night.

Finally, early on September 7, the cyclone began to lose its warm core and extratropical transition began over the cold waters north of the Gulf Stream. Even as Dorian became asymmetric, it actually deepened and intensified. This was due to baroclinic processes by which extratropical cyclones intensify, such as large temperature gradients in the atmosphere along frontal boundaries. Dorian regained category 2 status that morning, peaking again at 100 mph winds and a pressure of 953 mb southwest of Nova Scotia. Hurricane force winds and heavy rain caused widespread damage and loss of power in Atlantic Canada. During the afternoon, Dorian at last became post-tropical just before making landfall near Halifax, Nova Scotia. Its effects continued overnight though as it moved across the Gulf of St. Lawrence toward Newfoundland. The extratropical cyclone's winds dropped below hurricane strength during the afternoon of September 8 before the center passed over northwestern Newfoundland. During the night, ex-Dorian finally moved away from land over the far northern Atlantic. It moved east-northeastward for a few days before dissipation.

The above shows Dorian at peak intensity over the Bahamas. It was the strongest Atlantic hurricane ever recorded that far north (at 26.6 °N).

The above shows the long track of Dorian. Its worst impacts were in the Bahamas, the coastal Carolinas, and Atlantic Canada (as a post-tropical storm).

Wednesday, August 21, 2019

Tropical Storm Chantal (2019)

Storm Active: August 20-23

In the middle of August, a frontal system moving toward the U.S. southeast coast began to stir up thunderstorm activity over the Carolinas and northeastern Florida. Near its southern end, a trough developed, and on August 17 a low pressure center. It moved steadily northeastward along the Carolina coastline and then out over the open ocean. Conditions were not very favorable but nevertheless a small area of thunderstorm activity persisted near the center. On August 19, the disturbance turned due east, traveling out to sea at a fast clip. The next day, the circulation became well-defined enough for the system to unexpectedly develop into Tropical Storm Chantal.

As the tropical storm moved slightly south of east, sea temperatures actually warmed modestly and wind shear decreased. However, these favorable factors were balanced by a major inhibitor: the atmosphere was very dry over the open northern Atlantic. As a result, Chantal struggled to maintain its convection and gradually weakened, becoming a tropical depression by August 22. The gradual spin-down continued the next day and the cyclone degenerated into a remnant low late on August 23. By this time, the low had turned southeast and slowed down. It dissipated soon after.

The above image shows Chantal moving eastward away from the North American coastline.

Chantal was a short-lived and weak tropical storm which did not affect land during its time as a tropical cyclone.