Friday, July 31, 2020

Tropical Depression Ten (2020)

Storm Active: July 31-August 1

On July 29, a tropical wave moved off Africa, but unlike its predecessors, made little westward progress. Almost immediately, the system had a pronounced spin, but there was little thunderstorm activity north of the center. The disturbance drifted north-northwestward over the next few days and gradually became more organized. With a new flare up of central convection, the system became Tropical Depression Ten east of the Cape Verde islands.

Overnight, the depression made its closest approach to the islands, but it was small enough that it brought only a few showers to the easternmost part of Cape Verde. By August 1, the system was encountering cooler water and more stable air. Thunderstorm activity waned and the shallow system turned left toward the west. That night, the depression dissipated.

The above images shows the exposed center of Tropical Depression Ten on July 31 with a bit of thunderstorm activity off to the west-northwest.

Tropical Depression Ten was small and short-lived; it did not affect land.

Thursday, July 30, 2020

Hurricane Isaias (2020)

Storm Active: July 29-August 4

On July 23, a large tropical wave exited Africa. It was perhaps the first of the season to really punch a hole in the dry Saharan air layer (SAL) to its north. The disturbance passed south of the Cape Verde Islands and pushed rapidly westward. Within a few days, the wave had a huge area of vorticity associated with it, but was highly disorganized. It's often the case that larger systems take longer to consolidate, and this was no exception. Though it developed a broad low pressure center, no well-defined center of circulation appeared for several days. At its fast clip, the outer rain bands of the tropical wave began to affect Barbados by July 28. By that time, tropical storm force winds were already occurring to the north of the center.

Overnight, the low passed over the Windward Islands at a remarkable 25 mph clip, bringing strong wind and heavy rains to nearly all the Lesser Antilles during that period due to its large size. As far as internal dynamics were concerned, the system was still struggling. Instead of a well-defined center of circulation, there was only a sharp trough axis oriented southwest to northeast, with competing mid-level vortices and convection clusters at each end. The large-scale cyclonic spin helped to turn the axis toward south to north on the 29th, but the system was still a mess on satellite imagery. It was only that evening that a new central area of thunderstorm activity formed atop a clear center and Tropical Storm Isaias was born, already packing 50 mph sustained winds. Not only did Isaias become the new earliest "I" storm (previous record-holder: Irene of 2005, which was named on August 7), but it also brought the number of named storms in July 2020 to 5, also tying an Atlantic record held by 2005.

Beginning later that day, the storm brought significant flooding and mudslides to Puerto Rico, the Dominican Republic, and some of the surrounding isles. The low-level center soon made landfall along the southern coast of the Dominican Republic, but the overall circulation was so large that the land encounter did not significantly disrupt the system. In fact, at the same time, a mid-level circulation moving east to west parallel to the northern coast became better defined. Soon, the surface center reformed under the nascent vortex and strengthening resumed. This is not to say interaction with Hispaniola did not affect Isaias at all: the central overcast became a bit separated from the eastern and southern portions of the circulation by a few tongues of drier air. Nevertheless, the detached bands pulled an immense amount of tropical moisture northward into the storm that fueled intense thunderstorms as well as the system's further development.

Overnight, Isaias moved northwestward into the Bahamas and strengthened into a category 1 hurricane. Some struggles with dry air caused the storm to level off in intensity on the 31st. Overnight, it peaked in intensity with 85 mph winds. The minimum pressure was 987 mb. Meanwhile, it continued northwestward at a fairly quick speed under the continuing influence of the neighboring subtropical ridge. During the morning of August 1, Isaias's center passed directly over Andros Island; around the same time, dry air, pushed into the circulation by upper-level winds out of the southwest, completely overcame the storm. A totally exposed center of circulation moved west-northwestward off of the island and nearly all convection was left on the other side of a north-south wall of low-humidity air. This caused Isaias to weaken back to a tropical storm that afternoon.

The storm continued to battle back, however, and convection blossomed again that evening. This sort of fluctuation continued, though less extreme, through the next few days. The shear persisted throughout and Isaias was not quick vertically stacked even thunderstorm activity was on the upswing: the mid-level center remained east of the low-level one. Fortunately for Florida, the system moved north-northwestward paralleling the coastline, and shear meant that the west half of the circulation was almost dry. Impacts to the state were confirmed to a few bad squalls. Late on August 2, the storm began to accelerate northward as it felt a cold front approach the U.S. east coast.

During the day of August 3, the interaction of the front with Isaias changed the shape of the storm: it took on the characteristic comma shape of a cyclone entering the mid-latitudes and the heaviest rainfall now fell northwest of the center. The outer bands of Isaias swept across South Carolina through the evening as the center approached. The inner core also recovered a bit from the latest intrusion of dry air and managed to build a more complete eyewall on radar. This resulted in the storm regaining hurricane strength.

Baroclinic forces (which are the mechanism by which non-tropical systems strengthen) also contributed to the intensification of Isaias: an anomalous jet stream out of the south fed unusually strong winds into the cyclone's eastern side a few thousand feet above ground. This phenomenon slowed the storm's weakening during and after landfall. Moreover, some of the winds mixed down to ground level, bumping up Isaias's intensity and matching its peak of 85 mph winds and a pressure of 988 mb. It maintained this through landfall just north of the Carolina border just before midnight. Isaias joined Bertha, Cristobal, Fay, and Hanna to become the earliest fifth named storm landfall in the United States on record.

By this time, the storm had begun to accelerate in earnest. On August 4, its forward speed exceeded 30 mph toward the north-northeast and it weakened back to a tropical storm. Nevertheless, rainfall totals of 3-6 inches (with isolated higher totals) were common along the storm track through the Carolinas and up the mid-Atlantic though Virginia, inland Maryland, and eastern Pennsylvania. Sporadic tornadoes broke out on the system's east side nearer to the coast. Isaias moved over the interior northeast and became post-tropical that evening. The remnant system brought rain and wind as far north as southern Quebec before dissipating.

The above image shows Isaias after achieving hurricane status for the first time on July 31st.

After moving across the tropical Atlantic as a tropical wave for nearly a week, Isaias was finally named in the Caribbean.

Thursday, July 23, 2020

Hurricane Hanna (2020)

Storm Active: July 22-27

During the third week of July, a tropical wave moving through the Caribbean began to exhibit thunderstorm activity near its northern end over the Greater Antilles. Due to land interaction and wind shear, the disturbance did not develop further for the next few days. Around July 21, a broad surface low formed in the Florida straits, bring scattered storms to much of the eastern Gulf region and Florida. It slowly consolidated over the next day, ultimately becoming Tropical Depression Eight late on July 22.

A ridge over the southern U.S. kept the depression moving west-northwestward across the open Gulf of Mexico, though at a slow forward speed. On July 23, Eight's structure improved drastically and the central pressure dropped, but winds lagged behind. It wasn't until late that night that the storm was upgraded to Tropical Storm Hanna. Hanna added to the growing list of records set by its predecessors, becoming the earliest "H" storm. The previous record-holder was Harvey of 2005, which formed on August 3 of that year (not to be confused with the devastating Hurricane Harvey of 2017, which retired the name). At the time of naming, Hanna had vigorous convection in the southern and eastern semicircles, but the northwest struggled some with dry air. This led to the formation of a curious mid-level eye feature, even with the cyclone still a minimal tropical storm! The mid-level center was not yet vertically aligned with the surface circulation due to a bit of wind shear, so Hanna wasn't quite ready to strengthen more rapidly.

On July 24, the storm turned more to the west and filled in more in the northwest quadrant, leading to faster intensification. On top of extremely warm ocean waters, these favorable factors were enough to boost Hanna to hurricane status by the morning of July 25. By that time, tropical storm conditions were affecting the south Texas coastline, and a large eye became increasingly evident on satellite and radar imagery. Hanna's central pressure dropped steadily through the afternoon despite increasing proximity to land. It reached its peak intensity as a high-end category 1, with 90 mph winds and a central pressure of 973 mb, just before landfall in south Texas.

As it made landfall, Hanna took a dive to the west-southwest, angling towards northern Mexico. It weakened to a tropical storm and soon crossed the international border, but maintained an impressive radar signature throughout the next day. By the afternoon of the 26th, the storm weakened to a tropical depression. The next day it weakened to a remnant low and dissipated over the mountains of northern Mexico.

The above images shows Hanna at landfall in south Texas.

Hanna's slow track through the western Gulf of Mexico allowed it ample time to strengthen into a well-organized category 1 hurricane.

Tuesday, July 21, 2020

Tropical Storm Gonzalo (2020)

Storm Active: July 21-25

Around July 17, a tropical wave associated with a monsoonal trough of low pressure near the equator entered the Atlantic from the east. To its north, plentiful dust-laden air still streamed westward off of the Saharan desert over the ocean. For the first part of July, this Saharan air layer (SAL) contributed to a very dry tropical Atlantic and stifled the season's early tropical waves. This wave, however, stayed low in latitude and produced persistent shower activity as the trade winds guided it westward.

There was an elongated area of enhanced vorticity associated with the system for a few days, but no single circulation center. Some spin became apparent early on July 20, when a surface low formed. Though the circulation became better defined by the evening, thunderstorm activity was still a bit spotty and winds remained lackluster. The system entrained more moisture from the inter-tropical convergence zone (ITCZ) to the south on July 21, and strengthened a little, earning the designation Tropical Depression Seven late that afternoon. Organization steadily increased overnight, and the storm became Tropical Storm Gonzalo during the morning of July 22. Following in the footsteps of earlier storms of the 2020 season, Gonzalo was the earliest ever seventh named storm, beating out Gert of 2005, which formed July 24 of that year.

The steering pattern near Gonzalo was quite simple: a strong subtropical ridge to its north kept it traversing the 10°N parallel at a gradually increasing speed. Environmental factors were more mixed, however. Ocean temperatures were quite warm and wind shear was fairly low, but the SAL lurked to the north and the system was fighting dry air. That day, Gonzalo lost some of its outer banding features even as the inner core improved and it strengthened some. The small cyclone peaked at 65 mph winds and a pressure of 997 mb that night, but the central dense overcast promptly collapsed a few hours later as dry air invaded Gonzalo. The storm ejected a blob of convection to the west, but the circulation was nearly bare apart from that by the morning of the 23rd, leading to some weakening.

A small shield of thunderstorm activity made a comeback later that day, once again covering the center of circulation, but the weakening trend continued throughout the next couple of days as the central pressure rose. Meanwhile, Gonzalo's westward motion had hastened, making it more difficult for the cyclone to maintain a closed circulation. During the morning of July 25, Gonzalo made landfall in Trinidad as a minimal tropical storm and weakened to a tropical depression shortly after. Land interaction further sealed the cyclone's demise and what was left of the circulation dissipated that afternoon. Though fast moving, Gonzalo brought heavy rain to portions of the southern Windward Islands and northern Venezuela. After dissipation, the remnant tropical wave continued west-northwestward through the eastern Caribbean.

The above images shows Gonzalo over the open tropical Atlantic. Despite warm waters, dry air and stable air eventually overwhelmed the small cyclone.

Gonzalo took an unusually southern track, ultimately affecting South America.

Thursday, July 9, 2020

Tropical Storm Fay (2020)

Storm Active: July 9-11

On July 4, a disturbance that had brought heavy downpours to Louisiana dipped southward into the Gulf of Mexico. Though a surface low formed over water, it was weak and moved northeast back over land without any tropical development. Over the next few days, it moved inland across Georgia and South Carolina. By July 8, the broad circulation began to feel the approaching Atlantic waters and generated large areas of thunderstorm activity offshore. By that evening, the system had moved over the ocean just off the border of the Carolinas, but it lacked organization.

The morning of July 9 saw the formation of a surface vortex, but convection still lay off to the northeast of the center, precluding tropical cyclone development. It wasn't until a new center of circulation took over under the thunderstorms near Cape Hatteras that afternoon that the system became organized enough to be classified. Aircraft data indicated the presence of tropical storm force winds east of the center, so the disturbance was designated Tropical Storm Fay. Upon formation, Fay continued the streak of broken records by becoming the earliest "F" storm recorded. The previous record was Tropical Storm Franklin of 2005, which formed on July 21.

While over the northern edge of the Gulf Stream, Fay managed to develop deeper convection that night, resulting in some modest strengthening. Soon, however, the center moved over colder waters and dry air invaded the circulation, displacing thunderstorm activity away from the center to the north or southeast. Fay reached a peak intensity of 60 mph winds and a pressure of 998 mb during the morning of July 10 as it approached the mid-Atlantic coastline. Well before landfall, heavy rains spread across the northeast. That afternoon, the cyclone's center crossed the coast in southern New Jersey. Once inland, Fay decayed rapidly and lost tropical characteristics by early on July 11 over New England. Later that day, the vortex was absorbed by another system approaching from the west.

The above image shows Fay a few hours before landfall in New Jersey. The cyclone had taken on a "hybrid" appearance between tropical and subtropical, with comparatively little cloud cover near the center of circulation.

Fay took a rather unusual track up the east coast: rather then veering east, it moved nearly due northward during its time as a tropical cyclone.

Sunday, July 5, 2020

Tropical Storm Edouard (2020)

Storm Active: July 4-6

On July 3, a low pressure system formed along a stationary front northeast of the Bahamas. Though atmospheric conditions were only marginally favorable, the small system managed to organize quickly and become Tropical Depression Five the next morning. Sandwiched between a ridge to the south and a trough to the north, the depression was steered rapidly east-northeastward. Very early on July 5, Five passed Bermuda, making its closest approach just northeast of the island. Thunderstorm activity at the time was minimal, however, so the island experienced little more than gusty winds and showers.

There was a resurgence of deep convection that evening and winds increased to tropical storm force, prompting the naming of Tropical Storm Edouard. With this upgrade, Edouard became the earliest "E" storm ever in the Atlantic, surpassing 2005's Hurricane Emily, which was named on July 11. The cyclone did not have much time as a tropical storm, however, as it accelerated northeast and began extratropical transition on July 6. By late that afternoon, the still vigorous system had merged with a nearby frontal boundary roughly 500 miles southeast of Newfoundland.

Edouard was a fast-moving and short-lived storm that took a typical track northeastward across the subtropical Atlantic.

The above image shows the track of Edouard, including the progress of its extratropical remnant across the ocean toward Europe.

Monday, June 22, 2020

Tropical Storm Dolly (2020)

Storm Active: June 22-24

On June 19, a non-tropical low pressure system formed off the southeastern United States. Initially, wind shear in the area was high enough to stifle any development. The system moved steadily northeast over the next few days and became better defined. During the day of the 21st, an area of convection popped up near the center, but was still disorganized. Atmospheric conditions improved some more the next day as the low moved closer to a tongue of warm ocean waters from the Gulf Stream. Later on the 22nd, the disturbance was classified Subtropical Depression Four several hundred miles east of the mid-Atlantic coastline.

The depression moved east-northeastward away from land at a moderate pace over the next day and crossed over the warmest ocean waters to be found at that latitude. As it did so, thunderstorm activity increased near the center and a curved banding feature set up in the northern semicircle, fanning eastward. By the afternoon of June 23, the cyclone had a more concentrated wind field with values in excess of gale force. Thus, it was upgraded to Tropical Storm Dolly. Its bout of strengthening was short-lived, however, for on its track lay the much colder waters of the open north Atlantic. Dolly's satellite presentation quickly degraded overnight and the storm weakened to a tropical depression on June 24. Later that morning, it became post-tropical well southeast of Nova Scotia. The remnant low picked up speed toward the northeast until it dissipated a few days later.

This image shows Dolly just after being classified as a tropical storm.

Dolly was a short-lived tropical storm that did not affect any land areas.

Monday, June 1, 2020

Tropical Storm Cristobal (2020)

Storm Active: June 1-9

During the last week of May, a central American gyre (CAG) set up over the southeastern Mexico, Guatemala, Belize, and the surrounding ocean areas. CAGs are broad areas of low pressure and enhanced rainfall, which clearly exhibit cyclonic rotation on satellite imagery. They are akin to monsoonal lows in other parts of the world, and typically occur near the beginning and end of hurricane season. Often, the rotation and ample moisture of CAGs can lead to tropical cyclone genesis in neighboring bodies of water. In this case, a disturbance embedded in the CAG developed into a tropical depression in the Eastern Pacific on May 30. The next day, it strengthened into Tropical Storm Amanda and made landfall along the Pacific coast of Guatemala. By that afternoon, the circulation had dissipated over the mountainous terrain.

Nevertheless, locally heavy rainfall had been occurring in the region for several days and continued as the remnants of Amanda moved inland. Flooding displaced many from their homes as over 10 inches of rain fell in some mountainous areas of Guatemala. Very early on June 1, a new convective outburst formed, unusually, over the southern half of the Yucatan peninsula. The new disturbance already had some spin to it, and further development occurred rapidly when as it moved westward into the Bay of Campeche that afternoon. Just a few hours later, it was classified Tropical Depression Three.

The depression was initially very broad, with little to mark its center of circulation on infrared satellite imagery. That night, it took some steps toward constructing a central dense overcast and gradually organized. On June 2, the cyclone strengthened into Tropical Storm Cristobal and broke the record for the earliest a third named storm had ever formed in an Atlantic hurricane season. The previous title-holder was Tropical Storm Colin in 2016. Cristobal was still embedded in the CAG and faced a strong ridge to its north, so it moved little that day, actually diving south toward the Mexican coastline that evening. Some strengthening occurred overnight and the cyclone reached an intensity of 60 mph maximum winds and a pressure of 994 mb before sliding slowly southeast across the coast of southern Mexico during the morning of June 3.

Since the storm did not move far inland, weakening was slow and flooding widespread over the areas that had already experienced rains for several days. Cristobal weakened to a tropical depression on June 4 and the center crossed the border into northwestern Guatemala. A weakness in the ridge over the Gulf states opened at last that night, causing the storm to turn northward. Cristobal traversed the spine of the Yucatan during the day on June 5 and recovered strength with more of the circulation over water. Nevertheless, land interaction had taken its toll; the circulation was broader and more asymmetric, with significant dry air entrainment on the southwestern side, a single (though powerful) curved band of convection to the north and east, and little to no activity near the center of circulation itself. The cyclone regained tropical storm status and entered the Gulf of Mexico early that evening.

Rainfall finally abated in Mexico as Cristobal moved northward, but storm totals exceeded 20 inches in regions bordering the Bay of Campeche. The storm strengthened modestly on June 6, but its structure was far too broad (with some subtropical characteristics) to allow rapid intensification. Its immense eastern band of thunderstorms swept across the coast of west Florida that evening, despite the fact that the center of circulation was near the center of the Gulf, 400 miles away! Cristobal's look became a bit more tropical the next day as the center approached the Gulf coast, with moderate convection popping up near the center. Maximum winds were near 50 mph when the center made landfall in southeastern Louisiana during the afternoon of June 7; at the same time, central pressure reached a minimum value of 992 mb. The large circulation brought measurable storm surge to a large swath of coastline stretching eastward to west Florida, with the worst impacts just east of the point of landfall.

Cristobal weakened to a tropical depression overnight but continued to bring heavy rain and tropical moisture northward. A trailing band lingering over coastal Louisiana brought some of the most severe storms to the region nearly a day after actual landfall as the center of circulation accelerated into Arkansas. Unusually, the system maintained tropical status throughout June 9 as it passed over Missouri and along the border of Iowa and Illinois and even entered Wisconsin before becoming extratropical that evening. The extratropical remnants merged with a front some time after.

The above image shows Cristobal just before its first landfall along the Bay of Campeche.

Taking into account the CAG that spawned Cristobal and the storm itself, southern Mexico was affected by flooding rains for nearly a week.

Wednesday, May 27, 2020

Tropical Storm Bertha (2020)

Storm Active: May 27-28

Around May 25, a weak low pressure center formed near southern Florida along a trough extending westward into the Gulf of Mexico and eastward into the Atlantic. Over the next few days, the rains associated with the system inundated portions of south Florida. More than five inches of rainfall fell in just a few hours in a few locations that day. On May 26, the low emerged over water east of northern Florida, but upper level winds were quite strong, hampering tropical development. The situation changed drastically in a short time, however, and wind shear relaxed. During the morning of May 27, thunderstorm activity blossomed near the center of circulation and the low suddenly became a tropical cyclone. Radar and buoy data indicated it already had gale force winds, so the system was classified Tropical Storm Bertha.

When it was named, Bertha was merely a couple dozen miles off the coast of South Carolina, moving northwest. It reached its peak intensity of 50 mph winds and a minimum pressure of 1004 mb at the time of landfall that same morning. Pushing inland quickly, the storm weakened to a tropical depression that afternoon. Localized flooding occurred along the storm's path over inland North Carolina into the southern Appalachians. Bertha was downgraded to a remnant low early on May 28 and dissipated later that day near western Pennsylvania.

This image shows Bertha near the time of its landfall in South Carolina.

Because it formed so close to land, Bertha spent only one day as a tropical cyclone. Nevertheless, with its formation, 2020 became the first season since 2012 to feature two named storms in the month of May.

Saturday, May 16, 2020

Tropical Storm Arthur (2020)

Storm Active: May 16-19

During the second week of May, a cold front stalled over the western Atlantic, its southwestern end threading the Florida Straits. Over the next few days, wet weather prevailed in that region and a broad circulation became evident around May 14. The same day, torrential rains soaked extreme south Florida and the Keys, with lesser impacts in Cuba and the Bahamas. The system moved northeast but lacked deep convection until May 16, when the center of circulation became better defined east of the Florida peninsula. That afternoon, it was classified Tropical Depression One.

The depression became rather asymmetric that evening, with nearly all thunderstorm activity in a semicircular band east of the center. Nevertheless, winds increased enough for it to become Tropical Storm Arthur, the first named storm of the 2020 season. 2020 therefore marked the sixth consecutive year in which a storm formed before the official start of hurricane season on June 1, a new record streak. Arthur churned steadily north-northeast through May 17. Its circulation became better defined as shear lessened, but cooler ocean waters limited convective activity, leading to just a bit of strengthening that day. Rainfall began that evening in eastern North Carolina as Arthur approached.

Near the coast, the storm encountered a deep pool of warmer water in the Gulf stream and strengthened some more. On May 18, Arthur passed just east of Cape Hatteras with peak sustained winds of 50 mph. By that time, the thunderstorm activity in the western semicircle had improved significantly, leading to heavy rains over a large swath of the coast and sustained tropical storm force winds in at least the easternmost barrier islands. The mid-latitude westerlies grabbed hold of Arthur, however, and accelerated it northeastward away from land that evening. Baroclinic processes strengthened the storm a bit more as it underwent extratropical transition overnight, bringing Arthur to its peak intensity of 60 mph sustained winds and a minimum pressure of 991 mb. It turned toward the east and became post-tropical during the morning of May 19. The remnant system turned sharply south later that day and angled toward Bermuda, but weakened to an extent that it brought only intermittent showers and gusty winds as the center passed the island on May 20. It dissipated shortly afterward.

The above image shows Tropical Storm Arthur just off the coast of North Carolina.

Arthur did not quite make landfall, but it brought heavy rain to south Florida as a tropical disturbance (before formation) and the Cape Hatteras region.