2023 Unnamed Subtropical Storm

Storm Active: January 16-17

NOTE: This post concerns a subtropical storm of the 2023 Atlantic hurricane season which formed in January of that year, but was not identified at the time. Reanalysis confirmed that it was in fact a subtropical cyclone a few months later. Since it was not named, the naming of storms later in the season began with "A" (see Tropical Storm Arlene), but the numbering of subsequent tropical depressions began with "Two".

On January 14, a trough became separated from a larger frontal boundary just off the U.S. east coast. A well-defined but non-tropical low formed off the mid-Atlantic states the next day. Water temperatures over the Gulf stream, while cold, were much warmer than normal for the region, allowing deep convection to develop. This lead to the formation of a subtropical storm around the morning of January 16. The cyclone underwent significant deepening that next day and turned northeast, then north. It reached a peak intensity of 70 mph sustained winds and a central pressure of 976 mb late on the 16th. During the morning of January 17, the cyclone made landfall in far eastern Nova Scotia, with estimated sustained winds of 50 mph at landfall. The storm continued northward, became post-tropical later that day, and dissipated on the 18th.

The image above shows the unnamed subtropical storm on January 16, with a distinct, eye-like feature.

The 2023 unnamed subtropical storm had the lowest central pressure of any January tropical or subtropical cyclone ever observed in the Atlantic.

Tropical Storm Jose (2023)

Storm Active: August 29-September 1

On August 19, a tropical wave moved off the African coast over the tropical Atlantic ocean. It moved across the Cabo Verde islands on August 21, bringing scattered downpours. The system moved slowly west-northwestward for several days. Conditions were marginally favorable, but the disturbance could not quite take advantage enough to be designated a tropical cyclone. It nonetheless plodded along, turning toward the north and entering the subtropical Atlantic. At that time, development was no longer expected, but the system make an unlikely comeback and finally became Tropical Depression Eleven on August 29, by which time it was located around 850 miles east-southeast of Bermuda.

The depression meandered for the next day and changed little in organization. Late on August 30, though, it began to pick up speed toward the north, feeling the influence of the large Hurricane Franklin approaching from the west. Somewhat surprisingly, the system's thunderstorm activity became significantly better organized early on the 31st, and it was upgraded to Tropical Storm Jose. The storm even managed to strengthen some more that night, reaching a peak winds of 60 mph.

On September 1, the small storm began to rocket northward, rapidly spiraling in toward the much larger circulation of Franklin, which became extratropical that afternoon. Despite increasing shear from the larger system, Jose maintained its identity through that evening and produced a trail of thunderstorm activity in its wake. Shortly after that, however, the system was at last absorbed into ex-Franklin.

The above image shows small Tropical Storm Jose over the open Atlantic.

The disturbance that was to become Jose moved across the Atlantic for over a week before developing (triangle points).

Hurricane Idalia (2023)

Storm Active: August 26-31

On August 24, an area of low pressure formed in the far western Caribbean, extending over the neighboring Yucatan peninsula. It gradually made its way north and became better organized. On August 26, a closed circulation developed in the Yucatan channel between the northeast peninsula and western Cuba. The system was designated Tropical Depression Ten that afternoon. Steering currents near the depression were initially very weak. As a result, the newly formed center meandered in a counterclockwise loop over the next day, executing a turn toward the south, east, and then finally north by the next day. In the meantime, some dry air was entrained into the circulation from the north and west, but this was not enough to stop the gradual improvement of the cyclone, to the point that it was named Tropical Storm Idalia on the 27th.

Idalia was a large cyclone, and took some time to spin up. A large burst of convection covered the center that evening and slow strengthening ensued, though the center remained close to the northern edge of the convective mass. Meanwhile, a trough over the U.S. finally got the storm moving, generally toward the north. Idalia passed very close to the western tip of Cuba during the evening of August 28, bringing tropical storm conditions to that area. Overnight, a more substantial inner core structure developed and the storm reached hurricane strength early on August 29. By that time, it was accelerating a bit on its northward heading into the eastern Gulf of Mexico.

Gulf waters were extremely warm, near record high temperatures in many locations. It took Idalia some time to build an eyewall, but by late that day it was intesifying rapidly. Though the storm overall was large, the central dense overcast and hurricane-force wind radius were rather small. Likewise, the eye that cleared out on satellite imagery late on the 29th was small. Nevertheless, the storm had become an extremely formidable hurricane by early on August 30, reaching its peak intensity as a low-end category 4 with 130 mph winds and a central pressure of 940 mb. An eyewall replacement began just before landfall, weakening the cyclone slightly, and Idalia made landfall in the "big bend" of Florida around 7:45am local time with maximum winds of 125 mph and a central pressure of 947 mb.

The storm weakened quickly over land and moved northeast, crossing into Georgia while still a hurricane. By the 30th, the center was crossing South Carolina; it moved over the Atlantic later that day. The core had completely eroded by that point and Idalia was down to a tropical storm. It was also transitioning to extratropical thanks to an interaction with the trough to its northeast. The storm was completely devoid of central convection by August 31 and finished extratropical transition that afternoon.

The large circulation of Idalia moved generally eastward toward Bermuda. Its motion gradually slowed on September 1 and windy conditions began on Bermuda, though there was almost no rain associated with ex-Idalia. Some deep convection began to redevelop northeast of the center that evening. However, the storm remained entangled with a front and did not regain tropical characteristics. The storm began to move northeast away from Bermuda on September 3.

The above infrared image shows Idalia at peak intensity as a category 4 hurricane in the early hours of August 30.

Idalia caused significant storm surge in northwest Florida; the shape of the coastline and large continental shelf in that region compounds the risk of storm surge.

Tropical Storm Harold (2023)

Storm Active: August 21-23

During the third week of August, a tropical disturbance moved through the western Atlantic, across southern Florida, and finally into the Gulf of Mexico on August 20. The system was generating a large area of thunderstorms on its northern side, but the circulation was rather broad as it tracked quickly westward. Nevertheless, it acquired enough organization to be classified Tropical Depression Nine on August 21. Overnight, the cyclone strengthened into Tropical Storm Harold. However, its rapid pace of motion did not leave Harold much time to strengthen; it made landfall in south Texas during the morning of August 22.

The storm's broad circulation and ample moisture did slow its weakening as it moved inland, though, allowing the storm to bring a swath of heavy precipitation all the way across Texas and northern Mexico over the following day. It weakened to a tropical depression late on the 22nd and to a remnant low on the 23rd.

The image above shows Harold making landfall in south Texas on August 22.

A strong ridge of high pressure to Harold's north steered the cyclone quickly across the Gulf of Mexico and over land.

Hurricane Franklin (2023)

Storm Active: August 20-September 1

Around August 17, a disturbance began to consolidate in the tropical Atlantic several hundred miles east of the Lesser Antilles. It originated as part of a sprawling trough of low pressure which extended far to the east-northeast and spawned two other tropical cyclones, Emily and Gert. The system crossed into the Caribbean sea on August 19, bringing heavy rain to the Windward Islands as it passed over. The system was facing some wind shear but conditions were otherwise favorable. This led to the formation Tropical Storm Franklin on August 20.

Franklin struggled to maintain a well-defined center of circulation over the next few days, not an unusual battle for cyclones in the eastern Caribbean. It had plenty of warm water and atmospheric moisture to work with, but wind shear inhibited it from gaining much strength, and in fact the low-level center reformed in different locatinos a couple of times. The storm at first had moved west, but it turned sharply toward the north by the 22nd under the influence of a trough over the western Atlantic. This ultimately brought the center over the Dominican Republic during the morning of August 23. The main impacts to Hispaniola were heavy rain. Franklin emerged over the open Atlantic late that afternoon.

The atmospheric steering currents collapsed the next day and the system slowed down and began an unusual turn to the east. Persistent wind shear out of the northwest also kept deep convection primarily to the southeast of the center. This prevented Franklin from strengthening much through August 25. That morning, the storm turned southeastward under the influence of an upper-level anticyclone in the Caribbean. It is very rare for a storm to move on a southeast heading in the tropics, especially before the very end of hurricane season.

Franklin's environment soon changed rather significantly, though. An upper-level low cut off to its west and it turned rather sharply back toward the northwest over the next day under the influence of this low. At the same time, wind shear lessened considerably, allowing the cyclone to become more symmetric and strengthen. Moderately dry air near the storm kept the thunderstorm activity limited, but this wasn't enough to prevent significant deepening on August 26 as hints of an eye appeared on satellite imagery. Franklin intensified into a category 1 hurricane that morning.

Spiral banding in the storm improved throughout the day as it continued slowly northwestward. It reached category 2 on August 27 and turned toward the north. However, it was the next day that Franklin really broke through, clearing out a large and circular eye. In a significant burst of strengthening, it became the first major hurricane of the season early in the morning of the 28th and then a category 4 shortly after. Franklin reached its peak intensity that evening with 150 mph winds and a central pressure of 926 mb. This pressure reading was the lowest yet recorded in an Atlantic hurricane over the open Atlantic (not the Gulf of Mexico) so far north, at 29 ° N. It was the lowest pressure in any Atlantic hurricane overall in nearly 3 years, since Iota of 2020.

On August 29, the satellite presentation degraded somewhat as the storm experienced an eyewall replacement cycle. Franklin weakened to a category 3. For a time, the storm cleared out a large new eye, but wind shear began to increase out of the north the next day, due to a combination of a large trough moving off the U.S. northeast and trailing Idalia in its wake. The storm was still a category 2 when it passed north of Bermuda; its wind field was large enough to bring tropical storm force winds to the island.

Franklin weakened to a category 1 on the 31st and also began extratropical transition as it sped toward the northeast. This transition completed on September 1, at which time the storm became a hurricane-force extratropical low. This low absorbed the much smaller Tropical Storm Jose late that night. The system ultimately moved eastward across the northern Atlantic and combined with another low pressure system off the coast of Europe.

The image above shows Franklin shortly before reaching peak intensity on August 28.

Franklin's unusual track was difficult to forecast due to a complicated combination of steering influences.

Tropical Storm Gert (2023)

Storm Active: August 19-22, September 1-4

Around August 14, a tropical wave around halfway between west Africa and the Caribbean showed some modest signs of organization. It moved roughly west-northwestward over the next few days. By the 16th, a large area of vorticity was had developed across the eastern tropical Atlantic, with the aforementioned disturbance at its western end. Weak low pressure centers formed at both ends; the eastern one eventually became Tropical Storm Emily. Around August 17, another low actually formed even farther west of our system, still associated with the overall monsoon trough. That low moved into the Caribbean and eventually became Franklin. All of this interaction made it harder for the system to consolidate; in addition, the farther north the disturbance tracked, the closer it came to a large area of Saharan dry air. Nevertheless, the western low ultimately became better defined and deep convection persisted near the center long enough that it was classified Tropical Depression Six on August 19.

By the time it formed, Six was around 850 miles east of the northeasternmost Caribbean islands. However, there was an upper-level trough located near the greater Caribbean islands, which sheared the storm more strongly as it continued westward. It struggled to maintain thunderstorm activity over the center on August 20. Despite all of its struggles, the cyclone produced vigorous enough convection to generate gale force winds that night and was upgraded to Tropical Storm Gert early on August 21. The storm began to weaken again soon after and became a tropical depression later that day. Gert persisted longer than expected, but ultimately weakened into a remnant low on August 22, not too far from the Leeward Islands.

Gert's remnants turned north-northwest and very slowly moved toward the subtropical Atlantic. After about a week of this, a low-pressure redeveloped in association with the system, and it began to get better organized again. On September 1, a remarkable ten days after ceasing to be a tropical cyclone, the disturbance was once again Tropical Depression Gert. Nor did it stop there. That afternoon, its cloud pattern became more organized and it strengthened into Tropical Storm Gert. At first, the storm didn't move very much, but it soon felt the influence of an extratropical low to its west, which was the remnant of Hurricane Idalia. This steered Gert toward the north-northeast with increasing speed. Despite some northerly wind shear, it strengthened on September 2, reaching a peak intensity of 60 mph winds and a central pressure of 998 mb.

At last, the hostile atmospheric environment began to take a toll on Gert. It weakened a little and most thunderstorm activity was stripped away from the center on September 3. The small cyclone turned north and then west of north as it spiralled toward the powerful ex-Idalia located southeast of Atlantic Canada. It dissipated and merged with this larger storm late in the morning on September 4.

The image above shows the small but persistent Tropical Storm Gert on September 3. The edge of the cloud pattern of the extratropical low which was once Hurricane Idalia is visible at left.

Gert had an unusual history, with two periods of time as a tropical cyclone separated by 10 days as a disturbance wandering the western Atlantic.

Tropical Storm Emily (2023)

Storm Active: August 20-21

On August 15, a tropical wave emerged into the tropical Atlantic ocean. It moved west-northwestward over the next few days and produced disorganized shower activity as part of a large area of unsettled weather which included both it and another disturbance several hundred miles to its west-southwest. Both systems developed low pressure centers, but their progress toward becoming tropical cyclones was hindered by their proximity. The western system ultimately became Tropical Storm Gert. By that time, the eastern low was producing gale-force winds, but the circulation was not well-defined enough to yet be named. That changed on August 20, when it finally became Tropical Storm Emily.

By the time Emily was named, its window of opportunity to strengthened had already closed. Wind shear and drier air overwhelmed the system, stripping it of any deep convection by early morning on August 21. Later that morning, it became a post-tropical cyclone, only one day after forming. What was left of the storm tracked northwest for several more days. Along the way, it encountered somewhat more favorable atmospheric conditions, but it could not take advantage of them to the point of redeveloping.

Emily was a sheared tropical storm during its brief existence.

Most of the track shown here shows when Emily was not a tropical cyclone (triangle points). It was only a tropical storm for a day (circle points).