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Tropical Storm Hilary Discussion Number   3
NWS National Hurricane Center Miami FL       EP092023
1000 PM CDT Wed Aug 16 2023
 
This evening, Hilary appears to be getting its act together, with 
its structure quickly improving. Satellite imagery shows a 
well-defined curved band wrapping around the storm's southern 
semicircle, with deep convection blossoming near the estimated 
center. Earlier microwave imagery also suggested that a tighter 
center might have reformed a bit further to the south, more 
embedded in the deep convection. The latest subjective Dvorak 
estimates form TAFB and SAB were 35 kt at 0000 UTC. However, the 
objective estimates have risen more quickly, with the most recent 
ADT estimate at 49 kt, and SATCON at 48 kt. The initial intensity 
this advisory is set at 45 kt, closer to these higher end 
estimates. 

While Hilary may have earlier reformed a bit closer to the 
convection, its estimated motion still appears to be 
west-northwestward, just a tad slower than earlier at 290/10 kt. 
Initially, Hilary is being steered west-northwestward by a prominent 
mid-level ridge draped between northwest Mexico and the four corners 
region of the US. However, this ridging is expected to shift 
eastward, allowing a large weakness to appear to the south of a 
deep-layer cutoff low that remains parked just offshore of central 
California. The net impact of this synoptic pattern evolution is 
that Hilary will begin to turn to the northwest and north between 
48-72 hours with some gradual acceleration as the tropical cyclone 
is steered between the cutoff low to the west, and the mid-level 
ridge to the east. While the track guidance this cycle shifted a bit 
to the west in the short term, this is more a reflection of an 
initial position adjustment, and the NHC track forecast was not 
modified much from the prior cycle. This track forecast opts to 
remain on the east side of the track guidance envelope, between the 
consensus aids and the GFS forecast track. On this forecast track, 
Hilary could pass near or over the Baja California peninsula 
sometime over the weekend. 
 
Assuming the storm is in the initial stages of developing an inner 
core, Hilary appears poised to intensify, likely rapidly, over the 
next couple of days. SHIPS-RII predictors are basically as high as 
they can go, thanks in part to low shear, very warm sea-surface 
temperatures, and high mid-level environmental moisture. In fact, 
the latest SHIPS-RII cycle shows a 100 percent chance of a 45 kt 
increase in intensity over the next 36 hours. That is verbatim what 
the latest NHC intensity forecast shows, and Hilary is forecast to 
become a major hurricane in a couple of days. After that time, the 
large wind field of Hilary may begin to upwell some cooler waters, 
with sea-surface temperatures dropping under 26 C between 72-96 h. 
Possible land interaction may also begin to disrupt the core 
structure of the storm during that time as it nears the Baja 
California peninsula. A combination of these factors should 
ultimately cause Hilary to become a post-tropical cyclone by the end 
of the forecast period. The NHC intensity forecast is in best
agreement with the HFIP Corrected Consensus Approach (HCCA). 

As stated previously, 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. Due to the forecast 
parallel angle of approach to the Baja California peninsula, small 
changes in the cross track position may result in large changes to 
when and where the system ultimately approaches the coast.  
Regardless, the large size of the cyclone means wind and rainfall 
impacts are likely to extend far from the center of Hilary.
 
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. 
Watches may be required for parts of the Baja California Peninsula 
as soon as tomorrow morning. 
 
2. Large swells from Hilary will spread northward along the coast
of southwestern Mexico and the Baja California Peninsula during the
next several days. 
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  17/0300Z 13.4N 105.1W   45 KT  50 MPH
 12H  17/1200Z 14.3N 106.9W   60 KT  70 MPH
 24H  18/0000Z 15.5N 109.1W   75 KT  85 MPH
 36H  18/1200Z 16.8N 111.0W   90 KT 105 MPH
 48H  19/0000Z 18.5N 112.5W  100 KT 115 MPH
 60H  19/1200Z 20.5N 113.4W  105 KT 120 MPH
 72H  20/0000Z 22.6N 114.0W   95 KT 110 MPH
 96H  21/0000Z 28.3N 115.4W   60 KT  70 MPH
120H  22/0000Z 34.3N 117.7W   35 KT  40 MPH...POST-TROP/INLAND
 
$$
Forecaster Papin
 
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