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Tropical Storm Hilary Discussion Number   1
NWS National Hurricane Center Miami FL       EP092023
1000 AM CDT Wed Aug 16 2023

The disturbance south of the southern coast of Mexico has increased 
in organization this morning.  Satellite imagery indicates that it 
has a well-defined circulation, with convective banding features 
that are becoming better defined.  Subjective Dvorak estimates from 
TAFB and SAB are at T1.5 and T2.0, respectively, however the 
T-number from TAFB was constrained.  First-light visible satellite 
imagery confirms the organizing trend and, based on the improved 
appearance of the cloud pattern over the past few hours, the system 
is being upgraded to Tropical Storm Hilary.

Hilary is moving west-northwestward at 12 kt.  A building ridge over 
the United States and a trough off the coast of California are the 
main steering features affecting the storm.  The tropical cyclone 
is forecast to make a gradual turn to the northwest in 24 hours, 
and then turn more northward between the trough and the western 
periphery of the ridge by day 3.  Global model guidance is 
relatively well-clustered for the first 3 days, with some larger 
cross-track differences by day 4 and beyond.  The official track 
forecast is closest to the simple and corrected consensus aids.   

The environmental and oceanic conditions appear ripe for further 
strengthening.  Sea surface temperatures are at or near a warm 30 
degrees C through 72 h, vertical wind shear is expected to be 
weak, and the near-storm relative humidities are moist.  
Statistical intensity guidance show high probabilities of rapid 
intensification in the next couple of days.  Therefore, the NHC 
intensity prediction explicitly shows rapid intensification in the 
first 24 hours and Hilary is expected to become a hurricane on 
Thursday.  However, the system is broad and it could take slightly 
longer to consolidate and strengthen.  The storm is likely to reach 
its peak intensity in 60-72 hours.  By day 4, the vertical wind 
shear is forecast to increase and the track forecast moves Hilary 
over waters cooler than 26 degree C.  Convection will likely 
collapse and the system is expected to become post-tropical by day 
5.

It is important to remind users to not focus on the exact forecast 
track and intensity of Hilary, especially in the latter parts of 
the forecast period. 

KEY MESSAGES:

1. Hilary has the potential to bring impacts to the Baja California 
Peninsula and portions of the southwestern United States this 
weekend.  Although it is too soon to determine the location and 
magnitude of rainfall and wind impacts, interests in these areas 
should monitor the progress of Hilary and updates to the forecast.

2. Large swells from Hilary will spread northward along the coast 
of southwestern Mexico and the Baja California Peninsula during the 
next days.
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  16/1500Z 12.4N 102.6W   35 KT  40 MPH
 12H  17/0000Z 13.1N 104.3W   50 KT  60 MPH
 24H  17/1200Z 14.1N 106.5W   65 KT  75 MPH
 36H  18/0000Z 15.4N 108.7W   75 KT  85 MPH
 48H  18/1200Z 16.7N 110.7W   90 KT 105 MPH
 60H  19/0000Z 18.5N 112.3W  105 KT 120 MPH
 72H  19/1200Z 20.4N 113.6W  105 KT 120 MPH
 96H  20/1200Z 25.7N 114.8W   80 KT  90 MPH
120H  21/1200Z 31.5N 116.8W   50 KT  60 MPH...POST-TROPICAL
 
$$
Forecaster Bucci/Camposano
 
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