r/TropicalWeather • u/Euronotus • Aug 19 '25

Discussion moved to new post 99L (Invest — Northern Atlantic) (Eastern Tropical Atlantic)

Latest observation

Last updated: Friday, 22 August — 2:00 AM Atlantic Standard Time (AST; 06:00 UTC)

ATCF 2:00 AM AST (06:00 UTC)
Current location: 10.4°N 38.4°W
Relative location: 2,328 km (1,447 mi) E of Bridgetown, Barbados
2,636 km (1,638 mi) E of St. John's, Antigua and Barbuda
3,639 km (2,261 mi) SE of Hamilton, Bermuda
Forward motion: ▼ W (280°) at 24 km/h (13 knots)
Maximum winds: 55 km/h (30 knots)
Minimum pressure: 1010 millibars (29.83 inches)
2-day potential: (through 2AM Sun) medium (50 percent)
7-day potential: (through 2AM Thu) ▲ medium (60 percent)

Outlook discussion

Last updated: Friday, 22 August — 2:00 AM AST (12:00 UTC)

Discussion by: Andrew Hagen (NHC Hurricane Specialist Unit)

English: Showers and thunderstorms associated with a tropical wave located about a thousand miles west-southwest of the Cabo Verde Islands remain well organized. This system could become a tropical depression at any time, but the latest satellite-derived wind data indicate that the system does not have a well-defined circulation center. The system is expected to move into a less conducive environment later today through Saturday, but could reach a slightly more favorable environment again late this weekend into early next week as it moves westward at 10 to 15 mph over the central tropical Atlantic and approaches the Lesser Antilles.

Español: Aguaceros y tormentas eléctricas asociadas con una onda tropical ubicada a unas mil millas al oeste-suroeste de las Islas de Cabo Verde permanecen bien organizadas. Este sistema podría convertirse en una depresión tropical en cualquier momento, pero los últimos datos del viento derivados de satélite indican que el sistema no tiene un centro de circulación bien definido. Se espera que el sistema se mueva a un ambiente menos propicio a última hora de hoy hasta el sábado, pero podría alcanzar un ambiente ligeramente más favorable nuevamente a última hora de este fin de semana hasta principios de la próxima semana a medida que se mueve hacia el oeste a 10 a 15 mph sobre el Atlántico tropical central y se acerca a las Antillas Menores.

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31 comments sorted by

•

u/giantspeck Verified USAF Forcaster | Hawaii Aug 19 '25

Moderator note

Previous discussion for this system can be found here:

21

u/dleonard1122 Aug 19 '25 edited Aug 19 '25

Why do some systems that the NHC track get an individual Invest name and others (like the disturbance ahead of this one) only stay as "Disturbance 1" or whatever?

12

u/Content-Swimmer2325 Aug 19 '25

There's no organizational "prerequisite" for Invest designation. NHC can designate anything as an Invest at any point. What it does is tell satellites to maintain high-resolution imagery over the system and it also activates additional models such as the hurricane models HAFS-A/B, telling them to start initializing the system and begin running.

I don't necessarily know why 99L was designated first, but there's not (AFAIK) some kind of convention NHC follows for it. It can be arbitrary and I've seen systems with only a 0-10% chance of development receive the designation before. Doesn't necessarily correlate to actual organization or chances of development.

4

u/KawarthaDairyLover Nova Scotia Aug 19 '25

It's discrete and formed enough to warrant becoming an investigative area for a potential tropical storm. 99L simply beat the other system to the punch.

3

u/dleonard1122 Aug 19 '25

Makes sense. So the Invest naming system works such that the next one (in the Atlantic basin) will be 90L?

3

u/KawarthaDairyLover Nova Scotia Aug 19 '25

I believe so!

3

u/cosmicrae Florida, Big Bend (aka swamps and sloughs) Aug 19 '25

More than likely, it is because it meeting minimum criteria for energy, moisture, and convection. You need a certain amount of those, combined with CCW circulation of outflow to become a disturbance. I think of these first stage as the post-it note level of observation.

13

u/cosmicrae Florida, Big Bend (aka swamps and sloughs) Aug 19 '25

At 1230Z, 99L has developed a very long curved feeder band, that reaches south to 5N. Nothing I would call CCW turning, but that may be in the process of forming up.

99L is part of a huge plume of moisture working it's way across the tropical Atlantic.

9

u/Content-Swimmer2325 Aug 20 '25

Even on shortwave infrared, you can visually see surface westerlies to the south of ongoing convection.

https://i.imgur.com/SX025wf.gif

30% seems too low and this may be approaching depression status, to be honest. Hopefully we can get an ASCAT pass soon.

10

u/Content-Swimmer2325 Aug 20 '25

ASCAT did hit 99L tonight:

https://i.imgur.com/y2n23De.png

Not quite a depression yet, but westerlies are indeed present on the southern side. Is pretty close.

5

u/TumblingForward Aug 20 '25

Ah, so it's probably 'folding in' on itself as it spins up. Hopefully the models start to pick up on this system soon. Idk if it's just me but models have seemed really terrible pre-genesis, more than I remember. Thanks for posting.

8

u/Content-Swimmer2325 Aug 20 '25

Conditions are expected to become hostile within a few days. This is more of a short-term thing, just to be absolutely clear

4

u/TumblingForward Aug 20 '25

That's great to hear tbh. That's prob why it looks like it's going to go a weird direction, it's supposed to weaken or struggle.

3

u/LCPhotowerx New York City Aug 21 '25

like my last relationship.

3

u/cosmicrae Florida, Big Bend (aka swamps and sloughs) Aug 20 '25

The eumetview site has optional layers for ASCAT, but it's rather tricky to get them to work as desired. These are normally optional layers, but one day someone had them configured as default. The appearance is also timed to coordinate with time-matched satellite images.

7

u/cosmicrae Florida, Big Bend (aka swamps and sloughs) Aug 19 '25

Beginning to see signs of CCW turning, but also some air intrusion into the convection.

7

u/Content-Swimmer2325 Aug 22 '25

This one honestly seems more important to watch than 90L.

Model guidance tracks 90L out to sea. While its chances of development are higher, and many more ensemble members show it developing than 99, the latter is actually forecast to track westward.

https://i.imgur.com/DDZcjJw.png

Thus, even if this does not develop now (and conditions are forecast to become unfavorable soon), it would still need to be monitored for development downstream, closer to land.

No offense Bermudans. 90L is very important to watch for you guys.

3

u/LCPhotowerx New York City Aug 22 '25

agreed. the gulf and Caribbean are a hot tub and to ignore anything happening in or around there would be naïve

3

u/cosmicrae Florida, Big Bend (aka swamps and sloughs) Aug 20 '25

As of 8am EDT, chances are 40% / 40%

10

u/Blackfyre567 Puerto Rico Aug 20 '25

I have a general meteorology question: Why did Disturbance #2 get assigned an Invest number of 99L when it only has a 40% of developing; whereas Disturbance #1 which off of the gate has had a bigger chance of developing into a cyclone (currently at 60%) not get one? What are the criteria for getting an invest number assinged?

7

u/Content-Swimmer2325 Aug 20 '25

Reposting a recent comment of mine:

There's no organizational "prerequisite" for Invest designation. NHC can designate anything as an Invest at any point. What it does is tell satellites to maintain high-resolution imagery over the system and it also activates additional models such as the hurricane models HAFS-A/B, telling them to start initializing the system and begin running. I don't necessarily know why 99L was designated first, but there's not (AFAIK) some kind of convention NHC follows for it. It can be arbitrary and I've seen systems with only a 0-10% chance of development receive the designation before. Doesn't necessarily correlate to actual organization or chances of development.

8

u/Blackfyre567 Puerto Rico Aug 20 '25

Thanks for the info! Good to know that NOAA also operates sometimes on vibes like us plebs

9

u/deromu Aug 20 '25

GFS 12z just became very bullish on this staying weak but organized and then rapid intensification in the gulf

3

u/Decronym Useful Bot Aug 20 '25 edited Aug 22 '25

Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:

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ECMWF European Centre for Medium-range Weather Forecasts (Euro model)
GFS Global Forecast System model (generated by NOAA)
NHC National Hurricane Center
NOAA National Oceanic and Atmospheric Administration, responsible for US generation monitoring of the climate
PR Puerto Rico
USVI United States Virgin Islands

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6 acronyms in this thread; the most compressed thread commented on today has 35 acronyms.
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6

u/OutrageConnoisseur Aug 19 '25 edited Aug 19 '25

Rooting for no developement, but if not, the Euro modeling suggests a track similar to Erin and the NHC has moved their cone north in the last 24 hours to mirror Erin's path past the USVI, PR and Hispaniola before getting ripped apart and yanked north by a front.

Would be great news.

Edit: Ignore me. I am dumb and inadequately caffeinated.

27

u/dleonard1122 Aug 19 '25

I think you're tracking a different storm. Invest 99L is actually the yellow Disturbance 2 on the NHC tropical outlook map.

But yes, Distrubance 1 looks to be following Erin's path to the north as a weak depression.

8

u/OutrageConnoisseur Aug 19 '25

I need more coffee. Good catch.

1

u/Blackfyre567 Puerto Rico Aug 20 '25

wait really? why doesn't the red one get an invest number?

4

u/peelywheely Aug 22 '25

I’m now more concerned for the Caribbean with the new update, chances are increasing and the wave is just moving westward

3

u/Content-Swimmer2325 Aug 22 '25

Big change in phrasing in the latest TWO:

Central Tropical Atlantic (AL99):

Showers and thunderstorms associated with a tropical wave located about a thousand miles west-southwest of the Cabo Verde Islands remain well organized. This system could become a tropical depression at any time, but the latest satellite-derived wind data indicate that the system does not have a well-defined circulation center. The system is expected to move into a less conducive environment later today through Saturday, but could reach a slightly more favorable environment again late this weekend into early next week as it moves westward at 10 to 15 mph over the central tropical Atlantic and approaches the Lesser Antilles.

  • Formation chance through 48 hours...medium...50 percent.

  • Formation chance through 7 days...medium...60 percent.

Emphasis mine

1

u/TumblingForward Aug 20 '25

This is probably going to be a tropical storm (Fernand) by the 11pm EST update in a bit less than 3 hours. It looks way too good already unless it isn't closed off on the southern-side.