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. 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: 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. 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. 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.

How inequality grows in the aftermath of hurricanes

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

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. 000
    ABNT20 KNHC 300534

    Tropical Weather Outlook
    NWS National Hurricane Center Miami FL
    200 AM EDT Thu Jul 30 2020

    For the North Atlantic…Caribbean Sea and the Gulf of Mexico:

    The National Hurricane Center is issuing advisories on recently
    upgraded Tropical Storm Isaias, located over the northeastern
    Caribbean Sea.

    A broad area of low pressure, associated with a tropical wave,
    located a few hundred miles southeast of the Cabo Verde Islands is
    producing disorganized showers and thunderstorms. Some development
    of this system is possible during the next day or two before
    environmental conditions become unfavorable.
    * Formation chance through 48 hours…low…10 percent.
    * Formation chance through 5 days…low…10 percent.

    Public Advisories on Tropical Storm Isaias are issued under WMO
    header WTNT34 KNHC and under AWIPS header MIATCPAT4.
    Forecast/Advisories on Tropical Storm Isaias are issued under
    WMO header WTNT23 KNHC and under AWIPS header MIATCMAT4.

    Forecaster Cangialosi

  2. TXNT28 KNES 300601

    A. 09L (ISAIAS)

    B. 30/0530Z

    C. 16.2N

    D. 67.0W


    F. T3.0/3.0






  3. Such a sloppy system.. its hard to see how this thing will survive and become a bona-fide hurricane 🌀…


    FORECAST POSITION AT T+120 : 37.5N 44.1W


    ————– ——– ——– ——–

    00UTC 04.08.2020 37.5N 44.1W WEAK

    12UTC 04.08.2020 37.7N 41.5W WEAK LITTLE CHANGE

    00UTC 05.08.2020 37.2N 39.7W WEAK LITTLE CHANGE

    1. the gfs took a poop in the 12am run it jumped to the north after starting where the nhc has the center position the 6am run should be more in line maybe with the euro witch was spot on

  5. Looking at the Puerto Rico Satellite loop that circulation north of PR is really tightening up. Looks really good. & will be north of Hispaniola. If the SW portion gets hung up on Hispaniola then maybe the circulation north of PR becomes the main show.

  6. AL, 09, 2020073000, , BEST, 0, 157N, 663W, 45, 1004, TS, 34, NEQ, 300, 0, 0, 120, 1009, 180, 150, 0, 0, L, 0, , 0, 0, ISAIAS, M, 0, , 0, 0, 0, 0, genesis-num, 021,
    Leben, was ist das?

  7. I saw Levi Cowan’s video. It looks like we are close to having a closed low & it is likely going to be pulled more north & merge with the other low pressure area which is slightly north of Puerto Rico. If this happens then the COC would not pass over the most mountainous terrain over central & Western Hispaniola. This is why some of the computer models have trended more east.

  8. Good evening. Trying out the site using my phone.

    When I select previous pages of comments, it expands them for some reason.

    Other than that, seems to work fine.

    1. I like it alot.i just wish there was away to see new posts without stroking the screen to refresh


    Same question popped into my mind also when I looked at that vis loop. As you probably know, collapsing tstorms will look that way. Made me wonder if the wave is tearing down what it doesn’t want to build something that works better – maybe consolidating its centers?

    The link is a previous comment from Baha… How can I make it show Baha’s comment and not the link?

  10. I occasionally posted to WeatherUnderground until the blog was sold to The Weather Company (IBM). for some reason I couldn’t log in under the new format.

  11. A map from July 29, 2020, that shows NASA and SpaceX’s landing zones for the Crew Dragon Demo-2 mission amid the estimated path and conditions of brewing Tropical Storm Isaias.

    The outer-edge green shows a 5-10% chance of sustained tropical storm-force winds. Google Earth; NOAA; NASA; Business Insider

Comments are closed.