Tropical Storm Ileana Forecast Discussion

000
WTPZ44 KNHC 122038
TCDEP4

Tropical Storm Ileana Discussion Number   2
NWS National Hurricane Center Miami FL       EP092024
200 PM MST Thu Sep 12 2024

The overall cloud pattern as seen in GOES-18 visible and infrared 
imagery does not appear to have changed very much from earlier 
today.  While the RMW still seems to be a bit large, the cyclone 
displays a large area of cold cloud tops with some evidence of 
curved banding.  The latest subjective Dvorak intensity estimates 
are a consensus T-2.5/35 kt from both TAFB and SAB, while recent 
objective intensity estimates from UW-CIMSS range from 33 to 42 kt. 
Based on the subjective and objective intensity estimates, the 
depression is being upgraded to Tropical Storm Ileana with an 
initial intensity estimate of 35 kt. 

The initial motion estimate is north-northwest, or 340/8 kt, which 
is a bit faster than before.  The overall synoptic steering pattern 
is unchanged in the models from earlier.  A deep-layer trough 
located over the western United States will continue to steer the 
cyclone toward the north-northwest through Friday, bringing the 
system near the southern portion of Baja California Sur.  After that 
time, this trough is forecast to weaken, which should cause steering 
currents to weaken.  This evolution should induce a slower motion 
toward the north, likely over the waters of the southern Gulf of 
California.  There were no major changes to the guidance for the 
first 36 to 48 h of the forecast, and very little change was made to 
the previous official forecast.  Thereafter, the guidance is a bit 
farther west, with some of the models keeping the cyclone moving 
north-northwestward over the Gulf of California rather than moving 
inland over mainland Mexico.  The NHC forecast was only nudged 
slightly west of the previous forecast, not as far west as the 
consensus aids during the 48 to 96 h time period.

Ileana is currently located within an environment of warm ocean 
waters, moderate vertical wind shear, and within a fairly moist low- 
to mid-level troposphere.  However, very dry air is evident on water 
vapor imagery to the northwest of the cyclone.  These conditions are 
unlikely to change before the cyclone reaches Baja California Sur on 
Friday, and the NHC forecast shows some strengthening before then.  
Land interaction with the peninsula should cause some weakening.  
When the center reaches the Gulf of California, the sea surface 
temperatures are quite warm.  While some restrengthening is possible 
after the cyclone emerges back over water, Ileana will encounter 
increasing westerly wind shear and drier air by hour 48, so it won't 
have much time to restrengthen.  After that time, weakening is 
expected as Ileana's convection is sheared off, and the cyclone is 
forecast to become a remnant low in about 72 h.  It should be noted 
that the GFS and ECMWF simulated satellite imagery show the cyclone 
becoming a remnant low a bit earlier around hour 60.  Although the 
cyclone will likely dissipate prior to 96 h, a 96 h point is carried 
as a remnant low for continuity.

Key Messages:
1. Tropical Storm Ileana will bring heavy rainfall to portions
of western Mexico and southern Baja California through this weekend.
This heavy rainfall will bring a risk of flash flooding and
mudslides to portions of the area.

2. Tropical storm conditions are expected to begin in portions of
the Baja California Sur on Friday, where Tropical Storm Warnings
are in effect.


FORECAST POSITIONS AND MAX WINDS

INIT  12/2100Z 19.9N 108.0W   35 KT  40 MPH
 12H  13/0600Z 21.1N 108.8W   40 KT  45 MPH
 24H  13/1800Z 22.7N 109.7W   45 KT  50 MPH
 36H  14/0600Z 24.1N 110.1W   40 KT  45 MPH...INLAND
 48H  14/1800Z 25.3N 110.1W   40 KT  45 MPH...OVER WATER
 60H  15/0600Z 26.3N 110.2W   35 KT  40 MPH
 72H  15/1800Z 27.2N 110.5W   30 KT  35 MPH...POST-TROP/REMNT LOW
 96H  16/1800Z 29.4N 111.8W   20 KT  25 MPH...POST-TROP/INLAND
120H  17/1800Z...DISSIPATED

$$
Forecaster Hagen