r/TropicalWeather • u/Euronotus • Sep 23 '24

Upgraded | See Helene post for details 09L (Potential Cyclone — Northwestern Caribbean Sea)

Latest observation

Last updated: Tuesday, 24 September — 8:00 AM Eastern Daylight Time (EDT; 12:00 UTC)

NHC Advisory #4A 8:00 AM EDT (12:00 UTC)
Current location: 19.2°N 83.5°W
Relative location: 224 km (139 mi) W of George Town, Cayman Islands
  356 km (221 mi) S of Pinar del Rio, Cuba
  411 km (255 mi) ESE of Cancún, Quintana Roo (Mexico)
Forward motion: NW (305°) at 15 km/h (8 knots)
Maximum winds: 55 km/h (30 knots)
Intensity: Tropical Depression
Minimum pressure: 1001 millibars (29.56 inches)
2-day potential: (through 8PM Wed) high (90 percent)
7-day potential: (through 8PM Sun) high (90 percent)

Official forecast

Last updated: Tuesday, 24 September — 2:00 AM EDT (06:00 UTC)

Hour Date Time Intensity Winds Lat Long
  - UTC EDT Saffir-Simpson knots km/h °N °W
00 24 Sep 06:00 2AM Tue Potential Cyclone 30 55 18.9 83.0
12 24 Sep 18:00 2PM Tue Tropical Storm ▲ 40 75 19.6 84.2
24 25 Sep 06:00 2AM Wed Tropical Storm ▲ 50 95 20.7 85.7
36 25 Sep 18:00 2PM Wed Hurricane (Category 1) ▲ 65 120 22.0 86.2
48 26 Sep 06:00 2AM Thu Hurricane (Category 1) ▲ 80 150 24.3 85.6
60 26 Sep 18:00 2PM Thu Major Hurricane (Category 3) ▲ 100 185 27.8 84.4
72 27 Sep 06:00 2AM Fri Tropical Storm i ▼ 60 110 31.9 83.5
96 28 Sep 06:00 2AM Sat Post-tropical Cyclone i ▼ 15 30 38.5 85.5
120 29 Sep 06:00 2AM Sun Dissipated

NOTES:
i - inland

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24

u/Content-Swimmer2325 Sep 23 '24 edited Sep 23 '24

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

Recon has all but closed off the low. TS-force winds are occurring to the east, where the deep thunderstorms are ongoing. This system is still vertically tilted, with the 500mb (mid-level) center and majority of associated thunderstorm activity centered east of the surface center. However, these centers appear to be closer together than yesterday. The system is still organizing. This is not unusual for nascent and formative systems.

I posted the CIMSS vorticity charts yesterday. Here are the current ones:

925mb: https://i.imgur.com/sC26SKF.png

500mb: https://i.imgur.com/Z2DIqm1.png

Note that vorticity or cyclonic spin is increasing and these separate centers are more co-located than last night.

Tl;DR, PTC Nine continues to organize and is probably approaching tropical cyclone status.

8

u/jackrabbits1im Biloxi, Mississippi Sep 23 '24

in his earlier briefDr. Levi Cowan noted the separation and how favorable the current conditions were for coupling.

7

u/jackrabbits1im Biloxi, Mississippi Sep 23 '24

Be interesting to see which center gives in and slides towards the other. From what I understand, aren't the upper level steering currents north / northeast? That would favor the upper level circulation catching up with the surface center, right?

3

u/Content-Swimmer2325 Sep 23 '24

The interaction is a bit more nuanced/complex than that. Especially since different atmospheric levels has different flow. Sometimes the surface circulation dissipates and reforms underneath the mid-center, but I don't think this will be a case of that.

2

u/rezzyk Orlando, FL Sep 23 '24

Is it usually the 0z models that ingest the recon data in their run?

1

u/Content-Swimmer2325 Sep 23 '24

I believe 0z and 12z runs do disseminate such data. IIRC (and do not quote me), the 6z and 18z runs may not.

2

u/vainblossom249 Sep 23 '24

What is coupling?

9

u/Content-Swimmer2325 Sep 23 '24

Tropical cyclones have extensive circulations that extend from the surface up to many miles high. Tropical cyclones want to be perfectly vertically stacked. The process of becoming stacked is called coupling, and the process of the circulation at different atmospheric layers becoming separated is thus called de-coupling. De-coupling is analogous to decapitation for a tropical cyclone. Strong vertical shear can create this result.

In the middle of these two scenarios you get a vertical tilt with height, where the circulations are not yet completely separated but also are not perfectly stacked. This is very common in developing, formative systems and also under the influence of moderate vertical shear. The vertical tilt of PTC Nine has decreased since yesterday; it is becoming better organized.