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: 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. Some people have been asking about the splitting wave …. I know we’ve had multiple named storms come from the same original Twave … but I also seem to recall that if there are two competing centres and one goes off at a tangent, it usually means that one is being spit out so the other can take control of the entire envelope of moisture and develop into a more cohesive entity. Again, I can’t remember which storm it was some years back that just kept coughing up and spitting out competing centres like a cat with hairballs …. In any case, both outcomes are possible. A third possibility is a kind of “Fujuwhara” effect, where one of the centres lines up and is absorbed by the other. I think this is what Levi has been hinting at in the last couple of videos.

    1. Thanks for the explanation. I’m viewing a few loops and the water vapor seems to pretty clearly be ‘splitting.’ I mean, it’s extended over a significant area of Venezuela. I would never doubt Levi’s expertise, but my mind seems to want to tell me, it won’t be re-combined/absorbed. Fascinating, though.

    2. Yes I remember the storm you are referring to! It kept spitting out huge centers as well as areas of convection. I can’t remember the name, I can barely remember my kids names! lol

  2. Really is splitting? Better not be mitosis. Or cloning. I mean, it IS 2020.
    Good. Weaken its ‘somewhat’ structure. And more rain for all the lands.

  3. Thank you. I’m in SE Florida, hoping this storm will either miss us, or stay at Tropical storm status. Wake up call for everyone in the cone right now.

  4. LOVE the old site, glad I found the new place! Y’all helped me thru many storms, THANKS!!

  5. Thanks Keep and all mods. I realize there isn’t a way to ‘report’ or ‘flag’ comments right now. I’m concerned someone could add comments (even lots of them) to older articles or pages back on the comments count. And personally, I’m exhausted by the few more recent from WU who do the maybe somewhat-subtle sealioning/trolling methods. I have great confidence you’ll have it all handled. You do great. Cheers.

  6. I’m not sure how to ‘log in’ to wordpress when I dont have an account so let’s try posting anyway 😀

    1. wait, so if I didn’t create an account, what’s to stop someone from posting under someone else’s name?

      1. Shhhhhhhh. Yup. Honesty, integrity, goodwill, a sense of inner peace and altruism.
        Oh yeah. And KEEP.

  7. As everyone may remain focused on PTC9, a slightly different note…

    Have people read (Great Lakes region) YCC contributor Daisy Simmons’ new article “What is Climate Justice?” I think it’s a nice overview, inclusive of many of the complex topics that are already affecting millions. Not a long or difficult read, but a good one. Anybody have thoughts or opinions on it that you’d care to share?

    For me, I’ll start. By saying I like the structure. 3 main talking points. Expanded and explained in bullets and some brief detail. I also love when writers keep paragraphs short (like 4-5 sentence max. for online reading without great eye strain).

    Not a lot of external links to sources, articles, etc. But well chosen. I’ve appreciated the WaPo article “Climate change is also a racial justice problem.” And am very pleased by how both air quality and healthy food access are mentioned. Transportation equity and emergency planning even get a nod, nice.

    Your thoughts?

    1. I saw that, but haven’t had a chance to look at it yet.
      It’s been, understandably, a rather busy day here.

    1. Oh cool!!! Thank you! See, I knew I’d learn something from your timelapse vids. First one, if I didn’t read the title/description, I would’ve sworn was filmed from a train. The motion is so smooth. Comfy suv/truck maybe higher vantage point? Plus, the cars on the frontage road kept making me think “train ride.”

      Second, I LOVE [timestamp 0:08-0:09 and thru 0:013] the 3-D effect of the low-lying clouds moving at a faster rate. Separation from the higher elevation clouds/cloud pattern. I’ve gotta find a way to use that! Sorry, but I guess I’m stealing from you? You’re awesome. Thank you for sharing. Stay safe. Cheers!

      1. Hi Carmot! messed with it a lot lol, I can’t wait to see your new timelapse videos!
        You’re welcome!
        it took some time lol, but will make more videos if I go on another roadtrip, maybe you can drive around the island and make a video!

      2. Well, no car. Pretty much don’t know anybody on island enough to get a ride anywhere. But I’ll keep working on it. I think you saw the last one I posted? Sunset from Capitol Hill looking down at (uninhabited) Managaha Island. RAIN. Much rain. On me.

  8. Hopefully recon gets there in time before the advisory. Seem to be having some technical difficulties.

  9. What people are missing (ignoring) is that first and foremost, the whole area is a trough.

    The tropical portion is secondary to the trough which is pushing forward. People are wishcasting a storm while ignoring the trough.

    1. I love viewing band-8 water vapor, thanks for posting. This thing is so massive. I’m concerned it can wall off adverse conditions… Until it slows its forward motion. Then (depending on many factors like land interaction, obviously) a tighter and more organized storm *could* really explosify. I still hold plenty of concern with PTC9 and all in its path.

  10. <a href=""><img src="" alt="click image for NHC Discussion">
    1. Since the quick response and makeover of this WP commenting system, many HTML tags no longer work. Like “a href” and all HTML entity tags. It should work (and automatically embed) if you simply “copy image” and paste OR you copy and paste the URL for the gif, jpg, or png image.

      Just copying and pasting your URL from "https___nhc.noaa____thru___line_and_wind.png":
    1. Well. To my non-expert eyes, seems like the CoC may develop just off the NE tip of Puerto Rico (El Yunque Nat’l Forest). Sure would be nice if PTC9 leaves plenty of rain without much accompanying damage. Maybe clears PR and just drifts OTS. Is that considered wishcasting? ; )

      1. It’s a good bit further South than that right now, so a whole lot is going to depend on landmass interaction.

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