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.

Website visitors can comment on “Eye on the Storm” posts (see below). Please read our Comments Policy prior to posting comments. View all EOTS posts here.

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”

  1. May, no way, (Was) June too soon (Not) July, Stand by (We Stood, almost fell, some did.) August, Beware you must. (Beware) September, Remember, October, not quite over, November, all over. Not necessarily. These are just witticisms that have some validity. Mother Nature doesn’t care about ditty’s. If certain conditions exist, anything can happen at any time. Always better to be OVER prepared than under.

  2. Wow, definitely interesting how the “I” storms seem to generally have it in for land masses…I know it’s just a coinkydink, but it’s still one hell of a coinkydink, ya know? I knew just from looking at satellite presentation this afternoon (remember, no degree here but I’ve got a minor in atmospheric science lol) that it had improved in organization; most recent NHC forecast looks pretty similar to what we’ve seen with many ‘canes the past few years (Matthew & Dorian come to mind), which of course means that my home state of NC is gonna get clobbered in at least one location or another. I mean, granted, we DO kinda stick out from the East Coast like Florida! Anyway, I guess I’m moving over to the newest update, maybe I’ll legit be “first”! 😉

  3. Dry weather coming for FLORIDA . ISAIAH gone’ fishin’

    Land mass wins again !

  4. Seems we’ve seen this movie before. Everyone gets all fired up for Florida and, when it just barely misses, kinda lets their breath out. Then Morehead City takes a shot and Ocracoke / Hatteras get nailed from the backside surge and nobody pays attention.

  5. Japan Meteorological Agency
    Tropical Cyclone Advisory #7 – 3:00 AM JST August 1 2020
    TROPICAL DEPRESSION 03
    ———————————-
    South China Sea

    At 18:00 PM UTC, Tropical Depression (996 hPa) located at 17.6N 111.0E has 10 minute sustained winds of 30 knots. The depression is reported as moving west northwest at 9 knots.

    Dvorak Intensity: T2.0-

    Forecast and Intensity
    ========================
    24 HRS: 19.4N 107.2E – 35 knots (CAT 1/Tropical Storm) Gulf of Tonkin

    1. WTPQ20 BABJ 311800
      
      SUBJECTIVE FORECAST
      TD  02 (02) INITIAL TIME 311800 UTC
      00HR 17.4N 111.5E 998HPA 15M/S
      MOVE WNW 20KM/H
      
      P+12HR 18.1N 110.0E 998HPA 15M/S
      P+24HR 18.8N 108.4E 995HPA 15M/S
      P+36HR 19.5N 106.7E 992HPA 18M/S
      P+48HR 20.2N 105.6E 998HPA 15M/S=
      
      China Meteorological Administration now forecasts a tropical storm as well.
      
  6. Would you believe my power just went out? This doesn’t bode well for when the real winds get here …😒

  7. Hey Keep. I would like to register so I can be a part of the forum. Can’t seem to find the way. Could you render some assistance? Thank you.

  8. My parents aren’t preparing for this storm, they think it’s gonna be a weak storm that misses them, what do I show them to make them prepare? Models trending west and the eyewall wrapping around in the radar made me real scared. In Dade county area.

    1. Maybe mention that with the virus around, even a weak storm could have some bad repercussions. Better to be ready than to try to make it up on the fly. Good luck!

    1. A lot of folks (myself included) use Levi Cowan’s website, http://www.tropicaltidbits.com , for the latest models; he’s got a tab at the top you can click on to view them: GFS, Ensemble models, Euro, etc. as well as view storm information, satellite/radar imagery, live-feed dropsonde and recon plane info…all sorts of fun stuff. Please check it out, Levi is a great tropical meteorologist and his vids (linked to his site) are really detailed and I’ve learned SOOOOO much from watching them!

  9. 000
    NOUS42 KNHC 301815
    REPRPD
    WEATHER RECONNAISSANCE FLIGHTS
    CARCAH, NATIONAL HURRICANE CENTER, MIAMI, FL.
    0215 PM EDT THU 30 JULY 2020
    SUBJECT: TROPICAL CYCLONE PLAN OF THE DAY (TCPOD)
    VALID 31/1100Z TO 01/1100Z AUGUST 2020
    TCPOD NUMBER…..20-065 CORRECTION

    I. ATLANTIC REQUIREMENTS
    1. TROPICAL STORM ISAIAS
    FLIGHT ONE – NOAA 49 FLIGHT TWO – TEAL 74
    A. 01/0000Z A. 31/2330Z,01/0530Z
    B. NOAA9 0809A ISAIAS B. AFXXX 0909A ISAIAS
    C. 31/1730Z C. 31/2000Z
    D. NA D. 23.0N 76.5W
    E. NA E. 31/2300Z TO 01/0530Z
    F. 41,000 TO 45,000 FT F. SFC TO 15,000 FT

    FLIGHT THREE – NOAA 43 FLIGHT FOUR – NOAA 49
    A. 31/2230Z A. 01/1200Z
    B. NOAA3 1009A ISAIAS B. NOAA9 1109A ISAIAS
    C. 31/2000Z C. 01/0530Z
    D. 22.7N 76.0W D. NA
    E. 31/2200Z TO 01/0400Z E. NA
    F. SFC TO 15,000 FT F. 41,000 TO 45,000 FT

    FLIGHT FIVE – NOAA 43 FLIGHT SIX – TEAL 75
    A. 01/1000Z A. 01/1130Z,1730Z
    B. NOAA3 1209A ISAIAS B. AFXXX 1309A ISAIAS
    C. 01/0800Z C. 01/0830Z
    D. 24.0N 78.0W D. 24.5N 78.5W
    E. 01/0930Z TO 01/1530Z E. 01/1100Z TO 01/1730Z
    F. SFC TO 15,000 FT F. SFC TO 15,000 FT

    2. OUTLOOK FOR SUCCEEDING DAY:
    A. BEGIN 3-HRLY FIXES AT 01/2030Z.
    B. POSSIBLE NOAA P-3 TDR MISSIONS DEPARTING KLAL
    AT 01/2000Z AND 02/0800Z.
    C. POSSIBLE NOAA G-IV SYNOPTIC SURVEILLANCE MISSON AROUND
    THE STORM FOR 02/0000Z, DEPARTING KLAL AT 01/1730Z.

    3. REMARK: CORRECTED MISSION IDENTIFIERS IN ALL FLIGHTS.

    II. PACIFIC REQUIREMENTS
    1. NEGATIVE RECONNAISSANCE REQUIREMENTS.
    2. OUTLOOK FOR SUCCEEDING DAY…..NEGATIVE.

    $$
    WJM

    NNNN

  10. 000
    ABNT20 KNHC 311728
    TWOAT

    Tropical Weather Outlook
    NWS National Hurricane Center Miami FL
    200 PM EDT Fri Jul 31 2020

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

    The National Hurricane Center is issuing advisories on Hurricane
    Isaias, located near the southeastern Bahamas.

    Showers and thunderstorms remain separated from the center of a
    small area of low pressure located a couple of hundred miles east of
    the Cabo Verde Islands. The system is moving northward toward less
    favorable environmental conditions, and significant development of
    this system appears unlikely.
    * Formation chance through 48 hours…low…20 percent.
    * Formation chance through 5 days…low…20 percent.

    A westward-moving tropical wave located about 950 miles east of
    the Lesser Antilles is producing disorganized showers and
    thunderstorms. Some slow development of this system is
    possible while it turns northwestward over the western Atlantic
    by early next week.
    * Formation chance through 48 hours…low…near 0 percent.
    * Formation chance through 5 days…low…30 percent.

    $$
    Forecaster Brown

  11. The trailing blob like we saw with Matthew has become a regular traveler with hurricanes in recent years. The southern coc of Isaias early on was very impressive, even last night the southern portion was intensive, had a nice spin, and was continually feeding the blow up north of DR. Once that trailing feeder died last night, Isaias stopped getting fed, the dry air and shear were allowed to come on in. This new center blow up is incredibly intense, rapid deepening may be beginning. Cloudtops over center rapidly cooling, that’s not good. https://cdn.star.nesdis.noaa.gov/GOES16/ABI/SECTOR/taw/13/1800×1080.jpg

Comments are closed.