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. 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. 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. 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. 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

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”

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  6. Already seeing breezy weather here in Key Largo. Waiting to see what direction this fella decides he wants to go.

    1. interesting yes! Sure going to be alot of rain up the coast! looks like it will meet up with a central plains low too meaning more rain!

  8. Wow! So good to see everyone again. Nothing like a good ol fashioned hurricane season to bring everyone together again.

    SW FL here. No one really cares. There’s a few people in Cape Coral who made the news for boarding up their houses, but other than that the only thing anyone is doing is cutting their lawns ahead of the rain.

  9. Recovery of structure is happening in earnest now. Big moisture envelope to the north of Isaias starting to fillout the structure of a large Isaias. Hot tower building in center of the strongest convection. If it can take off with convection, it’s going to be a problem. Hopefully the dry air and shear keep it so the engine can’t turn over. Wrap of moisture ever closer to letting this CDO blow up. Losing it’s southern blob feeder support early this morning must have taken quite the toll too. https://cdn.star.nesdis.noaa.gov/GOES16/ABI/SECTOR/taw/08/1800×1080.jpg

  13. 92W in the sea east of the Philippines has intensified into a tropical depression and was given the Philippine tropical cyclone name “Dindo”.

    At 10:00 PM PhST, the center of Tropical Depression “DINDO” was estimated based on all available data at 825 km East of Tuguegarao City, Cagayan (18.1N, 129.5E )

    Moving West Northwestward at 10 km/h
    Maximum sustained winds of 45 km/h near the center and gustiness of up to 55 km/h

  14. They haven’t updated the latest center fix, but drawing it in myself…WOW! Isaias is bookin’!

  16. Humidity and dew points remain well into the 70’s across the Bahamas. Can the moisture over the Bahamas make the cyclonic turn with Isaias to close off the western dry air? No reason to believe there’s not massive energy associated with Isaias still. Happy Isaias weakened, but this storm has made all the right moves to get to this point, it’s likely not done with the surprises.

  18. What are the chances of it being a cat 2 or 3 up into Carolina…if it intensifies over warm Gulf Stream waters?

    1. Quite possible. After the Bahamas, OTS just offshore all the way past the Outerbanks still on the table too.

  20. Here’s the weather now … No wonder pple used to be caught off guard in the old days …

    2. You are right about that, usually you get some of the best weather before and after a storm!

  21. A Tropical Storm Warning has been issued for portions of the
    Florida east coast from north of Ocean Reef northward to Sebastian Inlet and for Lake Okeechobee #Isaias

    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 #Isaias

  22. Very grateful to find this after WU implosion, which has been sad to witness. Good to see OG members here like Patrap and the like…..

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