Tropical Storm John Forecast Discussion

000
WTPZ45 KNHC 231447
TCDEP5

Tropical Storm John Discussion Number   4
NWS National Hurricane Center Miami FL       EP102024
900 AM CST Mon Sep 23 2024

Satellite images show that John has been strengthening quickly.  A 
large area of cold cloud tops between -70C and -85C is present over 
the tropical cyclone, with tight curved bands wrapping around the 
center.  The latest subjective Dvorak intensity estimates are 
T-3.5/55 kt from TAFB and T-3.0/45 kt from SAB.  Recent objective 
Dvorak intensity estimates from UW-CIMSS have been ranging between 
57 and 76 kt.  A 23/0836 UTC AMSR-2 microwave pass shows that the 
inner core has become much better defined, showing a small eye-like 
feature on microwave imagery, although there are no signs of an eye 
showing up yet in infrared or visible images.  A 23/1011 UTC SSMIS 
pass showed a similar structure to the AMSR-2 pass.  Based on the 
above data and imagery, the initial intensity is raised to 60 kt.

John will continue to be located within an environment of extremely 
warm ocean temperatures, light wind shear and a moist low- to 
mid-level troposphere as it approaches the coast of Mexico.  While 
the hurricane regional and dynamical models show a more limited 
amount of additional strengthening, the statistical models like 
SHIPS and LGEM are at the high end of the guidance suite.  Given 
that the system has already intensified by 30 kt in the past 18 h, 
and given that the SHIPS rapid intensity index is showing greater 
than an 85 percent chance of a 30-kt increase in the next 24 h, the 
NHC intensity forecast will now explicitly show rapid strengthening 
over the next 24 h.  This intensity forecast is at the high end of 
the guidance suite.  It is possible that John could peak even 
higher than shown below between the 24 h forecast point and when it 
is forecast to move inland around 36 hours.  Interests along the 
coast of southern Mexico should closely monitor future forecast 
updates on John.

Based on center fixes over the past 6 to 12 hours, John's motion is 
estimated to be northward, or 0 degrees at 3 kt.  John is embedded 
in very weak steering currents, and the primary steering mechanism 
is likely the southwesterly monsoonal flow that the cyclone is 
embedded in.  The new NHC track forecast has been shifted to the 
west of the previous official forecast and now lies very near the 
latest model consensus.  It should be noted that the track forecast 
and timing of when the center reaches the coast is more uncertain 
than normal.


KEY MESSAGES:

1. John is expected to become a hurricane later today and 
additional strengthening is likely before the center reaches the 
coast of southern Mexico within the Hurricane Warning area on 
Tuesday. Damaging hurricane-force winds and a dangerous storm surge 
are expected within portions of the warning area. 

2. John will bring very heavy rainfall to coastal portions of 
southern Mexico through this 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.


FORECAST POSITIONS AND MAX WINDS

INIT  23/1500Z 14.8N  98.5W   60 KT  70 MPH
 12H  24/0000Z 15.2N  98.3W   75 KT  85 MPH
 24H  24/1200Z 15.7N  97.9W   90 KT 105 MPH
 36H  25/0000Z 16.0N  97.6W   90 KT 105 MPH...INLAND
 48H  25/1200Z 16.2N  97.4W   35 KT  40 MPH...INLAND
 60H  26/0000Z...DISSIPATED

$$
Forecaster Hagen