Tropical storm Isaias radar
Radar image of Tropical Storm Isaias at 11:14 a.m. EDT Thursday, July 30, 2020. Intense rainbands were affecting the island of Puerto Rico. (Image credit: National Weather Service via Mark Nissenbaum/Florida State University)

Tropical Storm Isaias (pronounced ees-ah-EE-ahs) formed on Wednesday night over the eastern Caribbean, bringing torrential rains and tropical storm-force winds to Puerto Rico and the Dominican Republic on Thursday. Isaias’s formation date of July 30 (Greenwich time) comes nearly a week earlier than the previous earliest appearance of the Atlantic’s ninth storm of the year, Irene, on August 5, 2005.

Tropical storm warnings and watches were in effect Thursday from Puerto Rico and the Virgin Islands into the Bahamas. A warning means tropical storm conditions are expected in the next 36 hours, and a watch means tropical storm conditions are possible within 48 hours. A tropical storm watch will likely be issued for portions of Florida later Thursday, since Isaias is likely to spread heavy rains into southeast Florida beginning on Saturday morning.

Figure 1
Figure 1. Visible GOES-17 satellite image of Isaias at 15:15Z (11:15 a.m. EDT) Thursday, July 30, 2020. (Image credit: Tropical Tidbits)

Long-range radar out of Puerto Rico on Thursday afternoon showed very heavy rainbands from Isaias were affecting Puerto Rico and the Dominican Republic. Numerous flash flood warnings and severe thunderstorm warnings were issued on Thursday for Puerto Rico. As of 9:30 a.m. EDT Thursday, the National Weather Service in San Juan, Puerto Rico, listed seven rivers on the island that were above flood stage, and 15 others near flood stage. Rainfall was heaviest on the western end of the island, with six Weather Underground personal weather stations reporting over 5 inches of rain. The peak rainfall amount was measured at Haciendas del Club on the west coast of the island, where 19.05″ had fallen as of 1:05 p.m. EDT Thursday.

At 11 a.m. EDT Thursday, Isaias was located near the southeastern coast of the Dominican Republic, with top sustained winds near 60 mph and a central pressure of 1003 mb. Punta Cana, on the eastern tip of the Dominican Republic, reported a wind gust of 60 mph late Thursday morning. Satellite images showed that Isaias was steadily growing more organized, with a large area of intense thunderstorms building near the surface circulation center, with additional low-level spiral bands forming. Isaias had favorable conditions for development, with sea surface temperatures (SSTs) near 29 degrees Celsius (84°F) and moderate wind shear of 10 – 15 knots. The system was embedded in a moderately dry atmosphere with a mid-level relative humidity of 60%, but the absence of strong wind shear to a large extent was allowing the storm to wall off the dry air of the Saharan Air Layer to its northwest.

Figure 2
Figure 2. Predicted surface wind (colors) and pressure (black lines) at 12Z (8 a.m. EDT) Saturday, August 1, 2020, from the 6Z Thursday, July 30, 2020 run of the HWRF model. The model predicted that Isaias would be passing through the northwestern Bahamas as a strong tropical storm with peak winds of 56 knots (64 mph, orange colors) and a central pressure of 991 mb. (Image credit: Tropical Tidbits)

Forecast for Isaias

Isaias will pass over Hispaniola on Thursday, and the mountainous terrain on the island will likely significantly disrupt Isaias’s circulation. The uncertainty in the future of the storm remains very high until it becomes apparent how the encounter with the island affects Isaias’s circulation and center location.

Isaias will also have to contend with dry air from the Saharan Air Layer, which surrounds it on three sides. The 12Z Thursday run of the SHIPS model predicted that the atmosphere surrounding Isaias would remain dry during the next five days, with a mid-level relative humidity of 50 – 55%. The model also predicted that wind shear would rise to a moderately high 15 – 25 knots on Friday through Monday, which should interfere with intensification by driving the dry air surrounding the storm into its core.

Figure 3
Figure 3. Infrared satellite image of Isaias at 8 a.m. EDT Thursday, July 30, 2020. Superimposed in warm colors are the location of dust and dry air of the Saharan Air Layer (SAL), which surrounded Isaias on three sides. (Image credit: University of Wisconsin)

However, SSTs for Isaias 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 Thursday National Hurricane Center (NHC) intensity forecast called for Isaias to remain just below hurricane strength, peaking with 70 mph winds this weekend. Most of the top intensity models predicted that Isaias would peak as a strong tropical storm or low-end category 1 hurricane. A good uncertainty range for Isaias’s peak intensity this weekend off the Southeast U.S. coast is 60 – 85 mph, and I expect that we will see Isaias peak as a category 1 hurricane.

Figure 4
Figure 4. Visible satellite image of 93L at 11:05 a.m. EDT Thursday, July 30, 2020. (Image credit: Tropical Tidbits)

Invest 93L off the coast of Africa close to tropical depression status

A strong and well-organized tropical wave off the coast of Africa was designated Invest 93L by NHC on Thursday morning. A Thursday morning pass by the ASCAT satellite showed that 93L had a well-defined surface circulation, and satellite images revealed a well-organized system with plenty of heavy thunderstorms. In an 8 a.m. EDT Thursday Tropical Weather Outlook, NHC gave 93L two-day and five-day odds of formation of 20%. I think that these odds are too low, and should be 60%.

The system is expected to head north-northwest into a stable air mass and over cooler waters this weekend, and it likely will dissipate early next week.

The next name on the Atlantic list of storms is Josephine. The current record for earliest 10th Atlantic named storm formation is August 22, when Jose formed in 2005.

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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...

358 replies on “Tropical Storm Isaias forms over the eastern Caribbean”

    LOCATION…21.7N 74.5W
    A Hurricane Watch has been issued for portions of the Florida east
    coast from north of Deerfield Beach northward to the
    Volusia-Brevard County Line.

  2. Took this about 10 a.m. as that first fast band of storms went through.
    Disqus is being disqusting and not letting me post there from my phone …

  3. Hmm…couldn’t get the image to work the first time…trying again…
    A very lop-sided storm (most recent pass is SW–>NE):

  4. Here in South Florida I’m not doing too much prep work for this. Technically we’re still in the cone but the trend is eastward. Fingers crossed that holds.

  5. (couldn’t get the image to attach, and apparently there’s no way to delete comments…)

  6. testing tweet embed…
    <blockquote class=”twitter-tweet”><p lang=”es” dir=”ltr”>En Hato Mayor del Rey, varias viviendas inundadas producto del paso de la tormenta Isaías. <a href=””></a></p>&mdash; Pedro pablo cruz(@pedroescuela) <a href=””>July 30, 2020</a></blockquote> <script async src=”” charset=”utf-8″></script>

      1. Me either. I feel better knowing some others are also having difficulty posting anything other than text.

      1. I’m just so giddy that I found your new blog. I was devastated when Cat6 shuttered and I hope all the familiar faces show up here to expand on your posts and provide real-time insights and conversation as we watch the radar returns frame-by-frame hahaha

      1. I haven’t been able to find a log in area either. I think we just post under our email. This is certainly not as good as what we’ve been used to. Guess this is a “new normal” as well.

      2. I imagine that as traffic increases, they’ll likely incorporate a discord solution here as well, and then we can log in with our old usernames.

  7. wunderground totally sucks now. the Hurricane section is not what it used to be (even without Dr. Masters and Bob Henson!

    1. This blog format sucks as well. IDK why they just didnt install a opensource FREE Facebook like interface. I have installed OSSN on a couple of my sites and it is much better than this

      1. I think this is just a wordpress blog. I imagine they’ll incorporate something more robust if traffic increases.

  8. Hey, is there a way to “login” on here, besides just posting a comment? I can’t find any link to setup an account.

  9. What’s up, team! Good to see everyone again. Watching this thing from the Florida Keys nervously. Remember too well the last-minute dog-leg Charley did in Punta Gorda/Port Charlotte area in ’04. We generally don’t take anything less than a 3 too seriously here; but all that warm water up there is making me anxious.

    1. I am in Port Charlotte. Arrived here the Spring after Charley. No one is batting an eyelash at this thing. A few older people made the news in Cape Coral for boarding up their houses, but that’s about it. The only activity you really see today is people cutting their grass ahead of the rain

  10. Japan Meteorological Agency
    Tropical Cyclone Advisory #5 – 21:00 PM JST July 31 2020
    South China Sea

    At 12:00 PM UTC, Tropical Depression (998 hPa) located at 17.3N 111.9E has 10 minute sustained winds of 30 knots. The depression is reported as moving west northwest at 6 knots.

    Dvorak Intensity: T1.5

    Forecast and Intensity
    24 HRS: 19.5N 108.6E – 35 knots (CAT 1/Tropical Storm) Gulf of Tonkin

    1. Bulletin issued at 20:45 HKT 31/Jul/2020

      Tropical Cyclone Warning Bulletin

      Here is the latest Tropical Cyclone Warning Bulletin issued by the Hong Kong Observatory.

      The Strong Wind Signal, No. 3 was issued at 8:40 p.m.

      This means that winds with mean speeds of 41 to 62 kilometres per hour are expected.

      The monsoon depression over the central and northern parts of the South China Sea has intensified into a tropical depression.

      At 9 p.m., the tropical depression was estimated to be about 520 kilometres south-southwest of Hong Kong (near 17.8 degrees north 112.8 degrees east) and is forecast to move west-northwest at about 18 kilometres per hour towards the vicinity of Hainan Island to Beibu Wan.

      According to the current forecast track, the tropical depression will maintain a distance of 500 kilometres or above from Hong Kong tonight (31 July) and tomorrow (1 August) and gradually move away from the territory afterwards. Local winds are not expected to strengthen significantly and the chance of issuing the Gale or Storm Signal, No. 8 will be relatively low.

  11. Virginia Beach here. The current track drops it real close to us overnight Monday into Tuesday.

    1. That’s the shear the NHC was talking about as their reasoning not to forecast higher intensity for Isaias.

  12. must of been an isolated location on the west coast of PRico that received almost 20inches. most of the locations i saw 6-8 inches.

    1. That’s entirely possible …. if a band of particularly heavy rainfall sets up. 8 inches of rain is already a lot. 20 inches can mean catastrophic flooding.

  13. Good morning all. Currently at my location in New Providence it’s sunny with light winds out of the east. Feels very much like a storm is traveling, tho …. the air is sultry in a way that I associate with tropical air masses.

    I’ll be in and out of the blog[s] today. Hopefully we’ll retain power tomorrow, but that remains to be seen.

Comments are closed.