Visible satellite image of Tropical Depression 8, predicted to intensify into a tropical storm. (Image credit: tropicaltidbits.com)

The tropics of the Atlantic and Eastern Pacific have burst into action this week, with multiple landfalls expected during the final weekend of July.

Tropical Depression 8, moving through the central Gulf of Mexico on Thursday, is predicted by NOAAs National Hurricane Center (NHC) to arrive on the Texas Coast as Tropical Storm Hanna on Saturday, July 25. The depression could be upgraded to tropical-storm status as soon as Thursday night. A large field of showers and thunderstorms (convection) was slowly consolidating across the Gulf on Thursday.

A tropical storm warning was issued Thursday afternoon for the lower and middle Texas coast from Port Mansfield to San Luis Pass, with a tropical storm watch eastward to High Island, including Galveston Bay. The size of this system means that intensification will be gradual, so it likely will not have time to attain hurricane strength before reaching Texas.

As the system moves closer, widespread heavy rains will stretch across the Gulf Coast from Texas to Louisiana, pushing inland across southern Texas this weekend. Localized rainfall totals could exceed 10 inches, and flash flooding will be possible.

Hurricane Douglas surged to Category 3 strength on Thursday, with top winds of 125 mph as of 5 pm EDT. The powerful storm was located more than 1,000 miles east-southeast of Hilo. A brisk and steady west-northwest course is expected to bring Douglas near Hawaii on Sunday. Douglas is most likely to pass over or near the Big Island and/or Maui, but squalls and heavy rains could affect other islands too. Hurricane or tropical storm watches may be issued on Friday.

Douglas will begin to gradually weaken on Friday as it encounters higher wind shear, drier air, and cooler water en route to Hawaii. However, Douglas could still be a minimal hurricane when it reaches the islands. Only a handful of tropical cyclones have made landfall in Hawaii, and the Big Island has never recorded a landfalling hurricane.

A hurricane watch was in effect Thursday for Barbados, St. Vincent, and the Grenadines as compact Tropical Storm Gonzalo churned westward through the tropical Atlantic. Gonzalo is a tiny tropical storm, noted NHC on Thursday. Winds of tropical storm strength (sustained at 39 mph) extended out just 25 miles from Gonzalos center.

Gonzalos small size will make it especially prone to rapid increases and/or decreases in strength. Overall, Gonzalo will have its best chance to reach hurricane strength between Thursday and Saturday as it approaches the Windwards, passing through the islands between Saturday afternoon and early Sunday.

Once Gonzalo enters the graveyard of the eastern Caribbean, increased wind shear and stable air will likely force the storm into a weakening trend.

A small but well-structured disturbance in the central tropical Atlantic developed quickly on July 21 into Tropical Depression 7, then strengthened to become Tropical Storm Gonzalo early Wednesday morning. Packing sustained winds of 50 mph at 11 a.m. EDT Wednesday, Gonzalo was chugging west at about 14 mph, according to the National Hurricane Center (NHC).

The newborn storm is expected to chart a steady west to west-northwest course, likely reaching the Windward Islands on Saturday, July 25, and moving on into the southeast Caribbean.

Gonzalo is the earliest seventh (G) storm on record in the Atlantic. As noted by Phil Klotzbach of Colorado State University, the prior record holder, Tropical Storm Gert, was named on July 24 during the astonishingly hyperactive year of 2005, when the Atlantic spat out 27 named storms. Gonzalo follows the tropical storms Cristobal, Edouard, and Fay, which were the earliest third, fifth, and sixth named storms for any Atlantic season in records going back to 1851.

Gonzalo is also this years first tropical cyclone in the Main Development Region, or MDR, the low-latitude area from the Caribbean to West Africa where about 75% of all major Atlantic hurricanes (Category 3/4/5) originate. Up to now, all of this years named Atlantic systems have been relatively weak tropical storms that formed north of the MDR. The MDR typically is most active in August and September, so the development of Gonzalo signals an early start for this key region.

Sea surface temperatures (SSTs) across the tropical and subtropical Atlantic dipped slightly in the wake of a mammoth, dust-laden, sun-blocking Saharan Air Layer during late June. Since then, however, SSTs have rebounded and are now above average for this time of year throughout most of the tropical and subtropical Atlantic, especially toward Central and North America (see graphic below).

The warm water will provide extra fuel for any tropical cyclones that head toward the Gulf of Mexico, Caribbean, and U.S. East Coast. Klotzbach noted that this months warm SSTs are similar to those typically seen during July in active Atlantic hurricane seasons.

Small tropical cyclones affect smaller areas, but they are also famously more difficult to predict. For example, a small system may find a favorable pocket for growth within a larger-scale pattern that would clearly inhibit a more sprawling system. This seems to be the case for Gonzalo, which is moving just south of a large zone of dry air. Despite the arid surroundings, Gonzalo appears to be nestled within a moist pouch that could allow the storm to continue strengthening as long as it doesnt ingest too much dry air.

Gonzalos path forecast is straightforward, but intensity outlook unclear

Small tropical cyclones can also gin-up and spin down more quickly than larger, lumbering systems. Gonzalo therefore could reach the Windward Islands as a hurricane or as a weakening tropical storm, and computer model guidance has spanned this spectrum of possibilities. The official NHC forecast as of Wednesday morning split the difference, projecting that Gonzalo most likely will move through the island chain as a mid-strength tropical storm on Saturday, July 25. The high-resolution Hurricane Weather Research and Forecasting model used by NHC, one of the most skillful models for intensity in the two- to four-day time range, predicted Wednesday morning that Gonzalo would become a Category 2 hurricane before reaching the islands.

In short, for Gonzalo, the path forecast is fairly straightforward but the intensity outlook is more challenging.

What happens to Gonzalo after this weekend is riddled with even more uncertainty. A continued west-northwest track is likely. However, Gonzalo will be entering the graveyard of the eastern Caribbean, where high wind shear and stable air often but not always conspire to weaken or dissipate tropical cyclones. Here again, Gonzalos survival may hinge on whether it can maintain itself against potentially hostile larger-scale conditions.

Another tropical disturbance vying with Gonzolo for attention could affect the U.S. before the week is out. Located in the southeast Gulf of Mexico early on July 22, this tropical wave, dubbed Invest 91L, may gradually strengthen as it heads toward the northwest Gulf over the next couple of days. This wave is not especially well organized, and any growth should be gradual. Still, with the help of light to moderate wind shear and very warm SSTs, this system has a chance of becoming a tropical depression or perhaps even a weak Tropical Storm Hanna by late in the week, when steady steering currents likely will be driving it toward the Texas coast. Even if it does develop, it appears 91L will not have enough time to become a hurricane.

The earliest eighth (H) storm on record in the Atlantic was Tropical Storm Harvey another product of the hyperactive 2005 season, assigned a name on August 3. That name was used again in 2011 and once more in 2017 before being retired. Famously, its last incarnation, Hurricane Harvey, became a Category 4 behemoth that caused massive rains and catastrophic damage in Texas.

As if there werent enough going on in the Atlantic alone, newly upgraded Hurricane Douglas in the Eastern Pacific will be heading in the direction of the Hawaiian Islands over the next few days. Douglas vaulted to hurricane status early Wednesday, and it is predicted to reach at least Category 2 strength over the next couple of days as it travels over warm SSTs of around 28C (82F).

Douglas will eventually reach cooler waters late this week. Wind shear is expected to remain light (below 10 knots), which may allow any weakening to be gradual. Models suggest that Douglas could affect Hawaii around July 27 while still a tropical storm, albeit a weakening one, and this was reflected in the official NHC forecast on Wednesday.

June 2020: Earths third-warmest June on record

It was once a rare event for a tropical cyclone to reach the Hawaiian Islands, but in recent years that rule of thumb has been overturned. Tropical storm landfalls in Hawaii over the past decade include Iselle (2014), Darby (2016), and Olivia (2018). Hurricane Lane, which peaked at Category 5 strength just before passing south and west of the Big Island in 2018, dumped the heaviest rains (58) ever recorded from a tropical cyclone in Hawaii.

A study published in May in the Proceedings of the National Academy of Sciences found an increased frequency of tropical cyclones from 1980 to 2018 over a zone extending east and south from Hawaii, amid larger-scale trends that could be explained only by factoring in human-induced climate change. We show for the first time that this observed geographic pattern cannot be explained only by natural variability, lead author Hiroyuki Murakami said then in a NOAA news release.

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7/23 update: Landfalls expected this weekend in Texas, Hawaii, Windward Islands - Yale Climate Connections