PTC 9 on July 29
Visible GOES-17 satellite image of Potential Tropical Cyclone Nine at 15:20Z (11:20 a.m. EDT) Wednesday, July 29, 2020. (Image credit: NOAA/RAMMB)

A large and vigorous tropical wave in the eastern Caribbean, designated Potential Tropical Cyclone Nine (PTC 9) by NOAA’s National Hurricane Center, was spreading heavy rain showers on Wednesday over much of the Leeward Islands, Virgin Islands, Puerto Rico, and the Dominican Republic, as seen on long-range radar out of Puerto Rico and Martinique.

Rainfall amounts in the islands as of 11 a.m. EDT Wednesday were mostly less than an inch, though a personal weather station on Dominica received 3.13″ in 24 hours.

As of that time, PTC 9 had top sustained winds near 45 mph and a central pressure of 1006 mb, and was headed west-northwest at 23 mph. PTC 9 is predicted to bring tropical storm conditions with heavy rains of 3 – 6 inches to the Leeward Islands, the Virgin Islands, Puerto Rico, Dominican Republic, northern Haiti, and Turks and Caicos Islands over the next few days. Higher rainfall amounts of 4 – 8 inches were predicted for the Inagua Islands.

Figure 1
Figure 1. A heavy rain shower from PTC 9 moving in over St. Barts in the Leeward Islands at 9:09 a.m. local time July 29, 2020. Thanks go to Caribboy for this link. (Image credit: St-Barth.com webcam)

An Air Force hurricane hunter aircraft investigating the storm Wednesday morning was unable to find a well-defined surface circulation, so PTC 9 retained its ungainly moniker. Satellite images on Wednesday showed that PTC 9 was steadily growing more organized, with more low-level spiral bands and an area of intense thunderstorms consolidating near what appeared to be a developing surface circulation center. The storm is also expected to slow down in forward speed through Thursday, favoring development and making it likely that PTC 9 will be named “Isaias” (pronounced ees-ah-EE-ahs) by Wednesday night. If so, that would beat the record for earliest ninth storm in Atlantic tropical cyclone history set by Irene on August 7, 2005.

PTC 9 had favorable conditions for development on Wednesday afternoon, with sea surface temperatures (SSTs) near 29 degrees Celsius (84°F) and light wind shear of 5 – 10 knots. The system was embedded in a moderately dry atmosphere with a mid-level relative humidity of 55%, but the light wind shear to a large extent was allowing the storm to wall off the dry air of the Saharan Air Layer to its north.

Figure 2
Figure 2. Predicted surface wind (colors) and pressure (black lines) at 6Z (2 a.m. EDT) Friday, July 31, 2020, from the 6Z Wednesday, July 29, 2020 run of the HWRF model. The model predicted that PTC 9 would be approaching the Bahamas as a disorganized tropical storm with peak winds of 48 knots (55 mph, yellow colors) and a central pressure of 999 mb. (Image credit: Tropical Tidbits)

Forecast for PTC 9

The prospects that PTC 9 eventually will attain hurricane strength look dimmer than they did on Tuesday, but the uncertainty in the future of the storm remains very high until it forms a well-defined center and gets named.

As PTC 9 moves west-northwestward, it will have to contend with passage over Hispaniola on Thursday, and the mountainous terrain on the island could significantly disrupt PTC 9’s circulation. Passage close to the mountainous terrain of eastern Cuba on Friday may also act to disrupt the storm. In addition, PTC 9 will also have to contend with dry air from the Saharan Air Layer, located to the north. The 12Z Wednesday run of the SHIPS model predicted that the atmosphere surrounding PTC 9 would slowly dry to a relative humidity of 45% by Thursday evening. The model also predicted that wind shear would rise to a moderately high 15 – 25 knots on Thursday through Saturday, which should retard development.

However, SSTs for PTC 9 will warm to 30 degrees Celsius (86°F) by Saturday, which will aid the intensification process. Also favoring intensification will be a large-scale region of ascending air over the Atlantic, caused by passage of an atmospheric disturbance called a Convectively Coupled Kelvin Wave.

The 11 a.m. EDT Wednesday National Hurricane Center (NHC) intensity forecast called for PTC 9 to remain below hurricane strength, peaking with 60 mph winds this weekend. Nearly all of the top intensity models also predicted that PTC 9 would not become a hurricane.

The three best models for predicting tropical cyclone genesis – the European, GFS, and UKMET models – all support intensification of PTC 9 into a tropical storm by Thursday, July 30. In an 8 a.m. EDT Wednesday Tropical Weather Outlook, NHC gave PTC 9 two-day and five-day odds of formation of 90%.

PTC 9’s rains beneficial for drought-stricken Caribbean islands

Many of the Caribbean islands in the path of PTC 9 will welcome its heavy rains, since they will help alleviate moderate to severe drought conditions from an unusually dry spring and first half of summer.

Figure 3
Figure 3. Palmer Drought Severity Index (PDSI) for Puerto Rico and the U.S. Virgin Islands for July 23, 2020, showed much of the islands in moderate to severe drought. Regions marked with an “S” imply mostly short-term impacts to agriculture. (Image credit: U.S. Drought Monitor)

Climate change is likely partly to blame for the drought, as explained in a 2018 study, Exacerbation of the 2013–2016 Pan‐Caribbean Drought by Anthropogenic Warming. The authors of that research found that human‐caused warming contributed to 15 – 17% of the severity of the intense 2013 – 16 drought in the Caribbean, and 7% of its spatial extent. The findings “strongly suggest that climate model projected anthropogenic drying in the Caribbean is already underway, with major implications for the more than 43 million people currently living in this region,” they concluded.

This year’s drought is particularly acute in Puerto Rico, where over half of the island was in drought on July 23, according to the U.S. Drought Monitor product. The governor declared a state of emergency in late June and ordered water rationing, subjecting residents in affected areas to 24-hour water shutoffs every other day. Loss of running water makes living through a pandemic deadlier, since residents are unable to wash hands, bathe, and disinfect surfaces as often as needed to avoid spread of the coronavirus.

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According to Princeton’s Latin American Flood and Drought Monitor, the Dominican Republic is experiencing the worst drought in the Caribbean. To illustrate, rainfall amounts between June 15 and July 27 at Barahona, on the southwest coast of the Dominican Republic, were just 0.09″ (2.2 mm). The capital, Santo Domingo, received 0.44″ (11.3 mm) during the same period. Haiti is also under severe drought, putting much of the nation into a food crisis, according to the Famine Early Warning Systems Network.

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Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...

365 replies on “Rain from Potential Tropical Cyclone Nine brings drought relief to Caribbean islands”

    1. Sure looks like the center has relocated to just North of the island and looks like it is tightening up and getting ready to strengthen!

    1. I remember and greatly appreciated David Begnaud’s great reporting on Puerto Rico after Hurricane Maria. And there’s more flooding/severe weather conditions footage below that tweet. Stay safe everyone! Cuídate!

  8. Interesting to watch that SW tower replace it. And (about 18 hours ago) I thought the wave coming off Africa and entering the MDR was spinning AND organized enough it might’ve been named by around now to three hours from now. Whoops, guess not. Okay by me. : )

  10. Jeez, that jump N of Hispaniola looks like it may give Izzy a chance to get its act together sooner. Let’s hope that the size of the circulation can make any intensification slow until the shear starts ramping up. Also, the northerly point suggests a romp more through the Bahamas than Florida. Those poor people…

    1. Always be wary of an “I” storm. This thing looks like it has the potential to get real mean.

  17. Haven’t seen Windy.com graphics posted. Curious how GFS and ECMWF can vary so greatly on accumulated precip totals and forecast totals. Even precipitation imaging has the heaviest bands in different locations. I know Windy shouldn’t be trusted as an ‘absolute’ or ‘verified’ measure for wind speeds, they use a sort of levelized average algorithm if I understand correctly. But their visualizations are so pretty!

    https://uploads.disquscdn.com/images/102f7371990aeebd2f0808b61a1a0e75bccf292d4ef6d04c75f1f1796e0d7e8c.gif

  18. With path now far more conducive for short term intensification, RI on the table next day or so. Could be similar to what happened to Florence when it hit continental dry air and high shear, when Isaias gets closer to Florida. We may be in for a show today as explosive development is possible. The energy of this massive system has been extraordinary, and we’ve seen this far too many times recently to discount it.

    1. Was there ever one on this site? I don’t remember seeing one, but if that was put at the top of the comments that would be a nice touch (if the full list of page navigation was not available).

    2. Easiest for now…click the address bar and remove #comments after the last slash / and then hit refresh for the page.

    1. HItting the refresh button now brings up newest comments too. Yale’s moving fast with the suggestions.

      1. Not for me. If I’m on comments page 6, it will only refresh that comments page 6. Doesn’t refresh to newest comments. I have to delete the “comments-page-6___” portion of the URL and press “enter” to refresh newest comments.

    1. Maybe concerned about the mountains. I’m not sure I would want to venture too close to 10,000 foot peaks in the middle of a storm. They are pretty brave, but local terrain could affect their choices.

  25. Also they are not at all done setting up the commenting platform here. It will continue to improve.

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