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.

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.

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.

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.

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.

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.

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.

Professor Quibb's Picks – 2020

My personal prediction for the 2020 North Atlantic hurricane season (written May 13, 2020) is as follows:

20 cyclones attaining tropical depression status,
18 cyclones attaining tropical storm status,
9 cyclones attaining hurricane status, and
5 cyclones attaining major hurricane status.

I predict that the 2020 Atlantic hurricane season will continue the active trend of the last few years and likely feature a well above-average number of cyclones, although uncertainty in my forecast is higher than average. Note that the average Atlantic hurricane season (1981-2010 average) has 12.1 tropical storms, 6.4 hurricanes, and 2.7 major hurricanes. The main factor in support of this claim is the potential for a neutral to negative El Niño Southern Oscillation (ENSO) index this summer and autumn. This index, a measure of equatorial sea temperature anomalies in the Pacific ocean, has an inverse correlation with Atlantic hurricane activity: negative indices (corresponding to a La Niña) tend to favor more active seasons.

Unlike the last two years, there is a great deal of spread in the model forecasts for this year's ENSO. Some models have the index remain positive, while others show a strong La Niña event developing by season's end. The dynamical models (whose average is represented by the thick red line) lean toward La Niña more than the statistical models (green), which favor using historical data for prediction over simply modeling future changes in weather. Due to a rapid plunge of Pacific equatorial sea surface temperature anomalies in the last few weeks, I'm putting more weight on the dynamical solution. This would suggest a very active season, especially since 2018 and 2019 were active despite neutral to slightly positive ENSO index conditions.

Meanwhile, Atlantic water temperatures are running uniformly warmer than average for this time of year, and this is expected to continue through the summer. No particular regions stand out though: the Gulf of Mexico, Caribbean, and subtropical Atlantic will all have moderately high anomalies, and the tropical Atlantic just a little bit less so. Expect tropical cyclone formation in many areas this year, and not quite as much emphasis on the subtropical Atlantic.

The above image shows the strength of the Saharan Air Layer (SAL) of the atmosphere as it appeared on May 12, 2020. Tropical trade winds carry dust particles and dry air from the Sahara desert over water throughout the year, but this has particular relevance to the development of tropical waves emerging off of the west African coastline: the more dry air and dust, the more thunderstorm activity in the region is suppressed. The SAL is quite strong as of now (though there is a typical decline throughout spring), so expect a late start for long-track hurricanes this year. Come September, however, other factors indicate high risk for Cape Verde hurricanes.

I'll discuss a few more smaller-scale factors in association with assigning risks to different parts of the Atlantic basin. My estimates are on a scale from 1 (least risk) to 5 (most risk):

U.S. East Coast: 4
The position of the Bermuda/Azores high pressure system correlates with the ENSO index and has a great influence on tropical cyclone tracks. I predict that this high will remain further east than typical La Niña events, increasing the risk to the east coast but lowering it for the Gulf of Mexico. The threat to the east coast will start early to the season, in July and August.

Yucatan Peninsula and Central America: 4
The western Caribbean is at higher risk for tropical cyclones than in any of the last 3 seasons, particularly when early fall rolls around. Some of the Atlantic's highest ocean temperatures will be in this region, and wind shear will be below normal. Expect some low-latitude cyclones, possibly affecting Nicaragua and Honduras.

Caribbean Islands: 4
The Caribbean, too, looks to be at greater-than-average risk this year. Once the tropical wave train gets going in earnest, look for long-track storms approaching from the east (primarily in September). The east Caribbean should be less of a hurricane graveyard than the last few seasons, so cyclones following tracks similar to Hurricane Matthew and approaching Hispaniola and Puerto Rico from the south are likelier than average.

Gulf of Mexico: 3
Waters in the Gulf are once again very warm, and some "home-grown" cyclone development is likely. Nevertheless, I forecast that the strongest cyclones will track elsewhere for most of the season. Come October, however, there is still a moderate chance for storms moving south to north from near the Yucatan Peninsula toward the United States gulf coast, similar to Hurricane Michael.

Overall, I expect the 2020 Atlantic hurricane season to feature well above-average activity. Nevertheless, this is just an amateur forecast. Individuals in hurricane-prone areas should always have emergency measures in place. For more on hurricane safety sources, see here. Remember, devastating storms can occur even in otherwise quiet seasons.

Sources: https://www.tropicaltidbits.com/analysis/models/, https://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf, http://trackthetropics.com/saharan-air-layer-sal-tracking/

Hurricane Names List – 2020

The name list for tropical cyclones forming in the North Atlantic basin for the year 2020 is as follows:

Arthur
Bertha
Cristobal
Dolly
Edouard
Fay
Gonzalez
Hanna
Isaias
Josephine
Kyle
Laura
Marco
Nana
Omar
Paulette
Rene
Sally
Teddy
Vicky
Wilfred

This list is the same as the list for the 2014 season because no names were retired that year.

The Gravitational Assist Maneuver

On October 7, 1959, the Soviet space probe Luna 3 passed over Asia and transmitted a handful of blurry black-and-white photos back to Earth. These were humanity's first glimpse of the Moon's far side (see below). The probe was launched on a circumlunar trajectory three days prior and its initial orbit about the Earth did not take it back on a path from which it could successfully relay its valuable data. However, Luna did not propel itself onto the necessary path. This groundbreaking probe was also the first to accomplish another feat: the gravitational assist.

Since 1959, spaceflight, especially interplanetary spaceflight, has relied upon gravity assists to reach destinations all over the Solar System. Apart from Luna 3, all other examples we will consider involve using gravity assists to change orbits around the Sun, rather than some other object (such as the Earth in the case of Luna 3). The basic principle is as follows: changing orbits around the Sun requires changing spacecraft velocity relative to the Sun, known as heliocentric velocity. Conventionally, this is done using thrusters aboard the craft itself (e.g. with chemical rockets; see ion propulsion for another example). However, onboard thrusters always require ejecting mass, and heavier rockets are much more difficult and costly to launch.

Gravity assists take advantage of planets' orbital velocity and gravitational influence to alter a spacecraft's heliocentric velocity. Take the diagram below, which shows a probe flying by the planet Jupiter.

Passing close to Jupiter puts our probe on a curved trajectory about the planet. Moreover, as it falls "downhill" into the gravity well of the giant planet, it picks up speed (as indicated by the longer arrows). This gain in speed is short-lived, however, because it loses kinetic energy climbing out of the well as it departs. As a result, though the velocity vector is pointed in a different direction than before, it seems we've made no progress in increasing our probe's speed.

But this is not true: all velocities in the above diagram are relative to Jupiter, i.e. measuring the rate at which an observer on Jupiter would see the spacecraft traveling. What matters for its orbit though, is heliocentric velocity.

Suppose that, with respect to the Sun, our planet has an initial orbital velocity toward the left. Then the total heliocentric speed can increase for our spacecraft if the velocity relative to the planet is rotated to line up more closely with the planetary velocity (as indicated by the addition of arrows in the above diagram). Two of the greatest outer Solar System missions in history relied on this principle: the Voyagers.

Both Voyager 1 and 2 made use of a gravity assist at Jupiter to accelerate them to Saturn, and Voyager 2 did the same to reach Uranus and Neptune. While a slight change in direction at each planet is visible in the above diagram, the following graph better captures Voyager 2's changes in speed.

The blue curve graphs the magnitude of Voyager 2's heliocentric velocity at various distances from the Sun along its journey. Note that at each flyby of a giant planet, the probe's speed sharply increased for a short time (as it plunged into the planet's gravity well) but also was higher after each flyby than before, with the exception of after its final encounter with Neptune. The other curve shown is the Solar System escape velocity, that is, the velocity required at a specified distance from the Sun to ultimately escape its gravitational influence. Remarkably, Voyager 2 did not even have sufficient velocity to escape the Solar System until after its first flyby with Jupiter! Without gravity assists, the spacecraft would not be headed toward interstellar space today.

As indicated by Voyager 2's encounter with Neptune, gravitational assists can also reduce a spacecraft's heliocentric velocity. This is necessary for certain missions to the inner Solar System; objects launched from the Earth do not simply fall toward the Sun - they have to lose the angular momentum they inherit from our own planet!

In October 2018, the European Space Agency spacecraft BepiColombo on a mission to orbit Mercury in 2025. See here for an animation of the probe's seven-year trajectory. If the probe had been launched directly toward Mercury, the additional velocity acquired from falling into the Sun's gravity well would have made orbit impossible. Instead, the mission trajectory incorporated two Venus flybys and six Mercury flybys, all to slow down the spacecraft without the use of too much thrust.

Finally, some space missions even use encounters with planets to leave the plane of the Solar System! The Ulysses spacecraft, launched in 1990, had the goal of studying the Sun. In particular, it aimed to measure the solar wind (the flow of charged particles) and magnetic field emanating from the Sun. Unlike previous missions, Ulysses had the opportunity to study the Sun from above its poles in an orbit that was inclined 80.2° to the plane of the Solar System. A vast majority of solar system missions remain in the nearly flat plane of the Sun's equator in which the planets lie.

To achieve this unusual orbit, the probe went all the way to Jupiter just to flyby the giant planet. Jupiter's large mass allowed for a more effective gravity assist and its distance from the Sun meant that the probe was traveling slower there and the maneuver required a smaller change in velocity. Ultimately, Ulysses made a great deal of new discoveries, including that the solar magnetic field "flips" every 11 years.

Faced with the difficulties of efficient Solar System navigation, numerous space missions have utilized creative solutions involving gravity assists to reach their targets, providing another example of the spectacular innovations necessary to explore worlds beyond our own.

Sources: https://solarsystem.nasa.gov/missions/luna-03/in-depth/, https://solarsystem.nasa.gov/basics/primer/, https://www.researchgate.net/publication/228803791_Design_of_Lunar_Gravity_Assist_for_the_BepiColombo_Mission_to_Mercury, https://medium.com/teamindus/daring-gravity-assist-maneuvers-of-past-space-missions-411643cd3d55, https://solarsystem.nasa.gov/missions/ulysses/in-depth/