Tropical Storm John Forecast Discussion

000
WTPZ45 KNHC 260335 CCA
TCDEP5

Tropical Storm John Discussion Number  13...Corrected
NWS National Hurricane Center Miami FL       EP102024
900 PM CST Wed Sep 25 2024

Corrected Hurricane to Tropical Storm John

An Air Force Reserve Hurricane Hunter Aircraft just finished its 
investigation of Tropical Storm John and found that the central 
pressure has dropped to 984 mb.  Dropsonde data and SFMR data 
support an intensity of 55 kt, which is also the average of the 
latest TAFB and SAB intensity estimates.  Satellite imagery shows a 
very large area of very cold cloud tops.  Based on the available 
data, the initial intensity is increased to 55 kt.

Based on fixes from the Air Force plane, the initial motion is 
estimated to be northwestward, or 325/4 kt.  The model guidance has 
made a big shift to the west on this cycle, which makes sense since 
John has been moving significantly to the west of the previous NHC 
forecast track.  The new NHC track forecast has been shifted well 
to the west, and currently lies slightly to the east of the latest 
track consensus aids.  The westward shift to the track has 
necessitated the Hurricane Warnings to be shifted farther west 
along the coast of Mexico.  The new forecast shows landfall 
occurring about 40 to 50 miles farther to the west compared to the 
previous official forecast, with the forecast landfall now well to 
the west of Lazaro Cardenas later Thursday night.  It should be 
noted that there is still quite a bit of uncertainty in the track 
forecast.

Tropical Storm John is currently located over 31C sea-surface 
temperatures, which is extremely warm.  The tropical storm is also 
in a moist environment and appears to also have a favorable 
upper-level wind environment.  Both the GFS-SHIPS and the 
ECMWF-SHIPS Rapid-Intensity-Index shows a very high chance of rapid 
intensification over the next 24 hours.  The new NHC intensity 
forecast has been increased significantly from the previous official 
forecast,  and lies near the high end of the latest intensity 
guidance suite.  Given the extremely favorable environmental 
conditions, it wouldn't be surprising if further increases to the 
intensity forecast are needed on subsequent forecasts.

Although this forecast shows dissipation by hour 60, there is a 
chance that John could survive a bit longer, moving 
west-northwestward near the coast of Mexico.

It cannot be emphasized enough that catastrophic life-threatening 
flash flooding and mudslides are expected to continue through the 
end of the week for portions of the Mexican States of Guerrero,
Oaxaca and Michoacan.


KEY MESSAGES:

1. Tropical Storm John will bring additional very heavy rainfall to
coastal portions of southwest Mexico through Friday.  The additional
rainfall totals will be falling across areas that have received very
heavy rainfall amounts over the past few days.  This heavy rainfall
will likely cause significant and catastrophic life-threatening
flash flooding and mudslides to the Mexican States of Guerrero,
Oaxaca and Michoacan.

2. John is forecast to strengthen to a hurricane before landfall,
and the Hurricane Warning has been extended westward along the 
coastline of southwestern Mexico, where tropical storm conditions
could begin within the next few hours.  Tropical Storm Warnings are 
also in effect for portions of the area.


FORECAST POSITIONS AND MAX WINDS

INIT  26/0300Z 17.0N 102.1W   55 KT  65 MPH
 12H  26/1200Z 17.3N 102.3W   65 KT  75 MPH
 24H  27/0000Z 17.7N 102.5W   80 KT  90 MPH
 36H  27/1200Z 18.1N 102.7W   70 KT  80 MPH...INLAND
 48H  28/0000Z 18.4N 103.2W   30 KT  35 MPH...INLAND
 60H  28/1200Z...DISSIPATED

$$
Forecaster Hagen