Tropical Storm John Forecast Discussion

000
WTPZ45 KNHC 252052
TCDEP5

Tropical Storm John Discussion Number  12
NWS National Hurricane Center Miami FL       EP102024
300 PM CST Wed Sep 25 2024

John is becoming more organized this afternoon.  Satellite imagery
has shown deep bursts of convection forming near the center with
curved banding around the majority of the circulation. Subjective
Dvorak from TAFB and SAB were 3.5/55 kt and 2.5/35 kt, respectively,
and the initial intensity for this advisory is set to 45 kt to
represent a blend of these estimates.  An Air Force Hurricane
Reconnaissance aircraft is en route to investigate John and gather
more information about the intensity, location, and structure.  The
tropical-storm-force winds in the southeast quadrant have been
adjusted outward significantly based on an earlier partial ASCAT
pass.

Little has changed in the intensity forecast reasoning.  Oceanic and 
atmospheric conditions are conducive for further intensification, as 
long as the storm remains over water.  SHIPS-RII, a rapid 
intensification indicator, shows about a 70 percent chance of rapid 
intensification in the next 24 hours.  The official intensity 
forecast now shows John becoming a hurricane in 24 hours as it nears 
the coast of southwestern Mexico.  This forecast is near the top of 
the guidance, closest to HCCA, but could still be conservative.

John is moving with an estimated motion of 355/3 kt.  The track
forecast is rather uncertain.  Model guidance has shifted westward,
and is showing John either nearly stationary or slowly following the
coastline northwestward.  This motion seems to depend on the
strength of the ridge to the north-northwest, which is centered over
the southwestern United States.  However, the current motion
suggests John should move closer to the coast with the center
likely to move inland on Thursday afternoon or evening, though
this could occur sooner.  Future adjustments to the track forecast
may be necessary, and there remains a large spread in the aids.

A Hurricane Warning has been issued by the government of Mexico for
portions of the coast of southwestern Mexico, and interests there
should closely monitor future forecasts for updates.  The Tropical
Storm Warning and Hurricane Watch have also been extended westward.

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 a Hurricane Warning has been issued for a portion of the
coastline of southwestern Mexico, where tropical storm conditions
could begin later today.  The Tropical Storm Warning and Hurricane
Watch have also been extended westward.


FORECAST POSITIONS AND MAX WINDS

INIT  25/2100Z 16.8N 101.5W   45 KT  50 MPH
 12H  26/0600Z 17.1N 101.5W   60 KT  70 MPH
 24H  26/1800Z 17.5N 101.5W   65 KT  75 MPH
 36H  27/0600Z 18.0N 101.6W   40 KT  45 MPH...INLAND
 48H  27/1800Z 18.2N 102.1W   25 KT  30 MPH...INLAND
 60H  28/0600Z...DISSIPATED

$$
Forecaster Bucci/R. Zelinsky