Tropical Storm John Forecast Discussion

523 
WTPZ45 KNHC 240852
TCDEP5

Tropical Storm John Discussion Number   8
NWS National Hurricane Center Miami FL       EP102024
300 AM CST Tue Sep 24 2024

John moved inland at around 0315 UTC as a 105 kt Category 3 
Hurricane. Since that time, the the storm has apparently turned more 
leftward and remains just inland, skirting the southern coast of 
Mexico. While surface observations have been sparse, my best guess 
is that the storm recently passed just to the north of Acapulco, 
where the airport earlier reported tropical-storm-force wind gusts 
and west winds as the pressure dropped to 998 mb. Assuming the 
small core has continued to quickly weaken, John is being 
downgraded to a tropical storm, and the initial intensity is 60 kt 
this advisory.

Following fixes from earlier microwave and more recent geostationary 
satellite imagery, John now appears to be moving northwestward at 
310/7 kt. The track forecast, which has been problematic with John 
from the start, is just as difficult now. John has continued to 
deviate westward of the previous forecast track and is now moving 
roughly parallel to the coast of Mexico. To make matters worse, a 
decent chunk of the of the global models guidance (ECMWF, UKMET) and 
hurricane regional models (HWRF, HMON) suggest the circulation could 
continue turning westward and attempt to re-emerge over the Eastern 
Pacific. Even the GFS and CMC solutions, which appear to dissipate 
the surface circulation inland over Mexico, show the mid-level 
circulation persisting and drifting back offshore, leading to the 
development of a new low-level circulation. Because it is becoming 
distinctly possible that John does not move far enough inland to 
dissipate entirely, and the aforementioned continued leftward 
trends, the latest NHC track forecast is altered quite a bit form 
the prior forecast, and now shows the possibility that John, albeit 
as a tropical depression, moving back offshore as it becomes tangled 
in the larger monsoonal flow. By 72 h, most of the track guidance 
shows the system moving back inland. The updated track forecast is 
roughly in between the HCCA and ECMWF trackers, and is of very low 
confidence given John's track history. It also remains possible the 
system could dissipate later today if it moves further inland.

Intensity-wise, John should continue to rapidly weaken as long as
it remains onshore, and the NHC intensity forecast shows the
tropical cyclone weakening to a tropical depression later today.
However, the new NHC forecast now maintains the system as a
depression and does not show dissipation beyond 24 h given the
latest track forecast taking the system along the coast of Mexico
and not far enough inland to completely dissipate. Some of the
models (notably the ECMWF) attempt to also re-intensify John as it
gets far enough back offshore, but given the large changes made on
this forecast cycle, the NHC intensity will not show that solution
quite yet.

Even if John remains inland, larger-scale moist monsoonal 
southeasterly flow will persist along the southern coast of Mexico, 
likely contributing to catastrophic rainfall both along the coast 
and inland over the up-slope portion of the mountainous terrain. 
This is a very dangerous life-threatening flooding scenario.


KEY MESSAGES:

1. Tropical Storm John could still producing tropical storm 
conditions for the next few hours in the tropical storm warning 
area.

2. Slow-moving John will bring very heavy rainfall to coastal
portions of southwest Mexico through the upcoming week. This heavy
rainfall will likely cause significant and possibly catastrophic,
life-threatening flash flooding and mudslides to the Mexican States
of Chiapas, Oaxaca, and southeast Guerrero, particularly in areas
near the coast and along the up-slope portion of areas of higher
terrain.


FORECAST POSITIONS AND MAX WINDS

INIT  24/0900Z 17.3N 100.0W   60 KT  70 MPH...INLAND
 12H  24/1800Z 17.4N 100.8W   30 KT  35 MPH...INLAND
 24H  25/0600Z 17.0N 101.0W   30 KT  35 MPH...OVER WATER
 36H  25/1800Z 16.7N 100.9W   30 KT  35 MPH
 48H  26/0600Z 16.9N 100.6W   30 KT  35 MPH
 60H  26/1800Z 17.4N 100.7W   25 KT  30 MPH...INLAND
 72H  27/0600Z 18.1N 101.3W   20 KT  25 MPH...POST-TROP/REMNT LOW
 96H  28/0600Z...DISSIPATED

$$
Forecaster Papin