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: 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. 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. Hi all. It seems that the islands will get good rainfall from this system. It looking better for SE Florida and SE USA as well.

  2. test, I’m just making sure I figured it out. So glad to see everyone.
    Hey Baha and Pottery. This is Castastrophe Adjuster. Don’t know if this is going to post or not can’t figure it out yet.

  3. 93L INVEST
    As of 12:00 UTC Jul 30, 2020:

    Location: 12.8°N 20.7°W
    Maximum Winds: 25 kt

  4. Thanks for the update! It is really nice to have two comment sites on this storm.
    Good morning

  5. This is what we have for now,

    ZCZC MIATCPAT4 ALL
    TTAA00 KNHC DDHHMM
    
    BULLETIN
    Tropical Storm Isaias Intermediate Advisory Number 8A
    NWS National Hurricane Center Miami FL       AL092020
    800 AM AST Thu Jul 30 2020
    
    ...ISAIAS CAUSING LIFE-THREATENING FLASH FLOODING AND HIGH WINDS
    OVER PUERTO RICO...
    
    
    SUMMARY OF 800 AM AST...1200 UTC...INFORMATION
    ----------------------------------------------
    LOCATION...17.6N 68.5W
    ABOUT 125 MI...205 KM W OF PONCE PUERTO RICO
    ABOUT 105 MI...175 KM ESE OF SANTO DOMINGO DOMINICAN REPUBLIC
    MAXIMUM SUSTAINED WINDS...60 MPH...95 KM/H
    PRESENT MOVEMENT...NW OR 305 DEGREES AT 20 MPH...31 KM/H
    MINIMUM CENTRAL PRESSURE...1003 MB...29.62 INCHES
    
  6. hard to count on the models but gfs 06 the tropical storm kisses e cen flor The nhc cone a/t/m we are in the yellow. 40-50mph . good luck to our island neighbors in the bahamas . looks like they’ll get it much worse.

  7. It looks like we get the LLC slide today. The LLC will move into more favorable location under deep convection which will be north of Hispaniola.

  8. Maximum sustained winds are near 60 mph (95 km/h) with higher gusts.
    Little change in strength is anticipated until landfall in 
    Dominican Republic later today, with re-strengthening forecast on 
    Friday and Saturday.
    
    Tropical-storm-force winds extend outward up to 415 miles (665 km)
    from the center. A Weatherflow station in Yabucoa Tanque de Agua 
    reported sustained winds of 52 mph (83 km/h) with a gust to 59 mph 
    (94 km/h).
    
  9. Current Intensity Analysis

    UW – CIMSS
    ADVANCED DVORAK TECHNIQUE
    ADT-Version 9.0
    Tropical Cyclone Intensity Algorithm

    —– Current Analysis —–
    Date : 30 JUL 2020 Time : 055019 UTC
    Lat : 16:14:01 N Lon : 67:42:30 W

    CI# /Pressure/ Vmax
    3.2 / 987.6mb/ 49.0kt

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