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Tropical Storm Hilary Discussion Number   4
NWS National Hurricane Center Miami FL       EP092023
300 AM MDT Thu Aug 17 2023
 
Hilary is a large tropical storm.  An elongated band of deep 
convection curves around the southern and eastern side of the 
circulation, and a Central Dense Overcast has begun to develop due 
to new convection near Hilary's center, with possibly the formative 
stages of an eye.  Subjective Dvorak estimates from TAFB and SAB 
have jumped to a consensus T3.5/55 kt, and given the improved 
satellite presentation since 06 UTC, the initial intensity is set at 
60 kt.
 
Hilary is moving toward the west-northwest (300/11 kt), to the south 
of a mid-tropospheric high stretching from the U.S. Rockies 
southward into northern Mexico.  The ridge is expected to shift 
eastward to the Central Plains during the next 48 hours, while a 
mid- to upper-level low moves eastward to the California coast. This 
steering pattern evolution should cause Hilary to recurve toward the 
northwest and then north during the next 3 days.  Model guidance is 
in generally good agreement on Hilary's future path, with a fairly 
tightly packed guidance envelope for much of the forecast period.  
Therefore, the new NHC track forecast is very close to a blend of 
the previous forecast with the HFIP Corrected Consensus aid (HCCA) 
and the TVCE consensus.  Despite the high confidence in the track, 
Hilary's oblique angle of approach to the west coast of the Baja 
California peninsula does make it nearly impossible to know at this 
point if the center will remain just offshore or move over the 
peninsula before reaching the southwestern United States.
 
Negligible shear, very warm waters of 30 degrees Celsius, and 
plentiful atmospheric moisture mean the environment is ripe for 
Hilary to rapidly intensify during the next couple of days.  In 
fact, many of the SHIPS Rapid Intensification (RI) indices are 
showing a 100 percent chance of RI during the next 48 hours.  The 
NHC intensity forecast follows this thinking and shows Hilary 
becoming a hurricane very soon and then a major hurricane by tonight 
or early Friday, more or less a blend of the HCCA aid and the SHIPS 
solutions.  Hilary should reach waters colder than 26 degrees 
Celsius soon after 72 hours, and fast weakening is indicated after 
that time, especially if the storm crosses over the Baja California 
peninsula.  The surface circulation is likely to be dissipated by 
day 5, but a day 5 forecast point (as a remnant low) is still 
provided to maintain a forecast track north of the Baja California 
peninsula.
 
 
KEY MESSAGES:
 
1. Hilary has the potential to bring significant impacts to the
Baja California Peninsula and portions of the southwestern United
States this weekend and early next week, including after it becomes
post-tropical.  Although it is too soon to determine the location
and magnitude of wind impacts, interests in these areas should
monitor the progress of Hilary and updates to the forecast.  A
Tropical Storm Watch is now in effect for southern portions of Baja
California Sur, and additional watches or warnings will likely be
required later today.
 
2. Heavy rainfall associated with Hilary may produce areas of flash 
flooding and result in landslides over portions of the Baja 
California peninsula from late Friday into late Sunday.  Rainfall 
impacts from Hilary within the Southwestern United States are 
expected to peak on Sunday and Monday.
 
3. 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  17/0900Z 14.6N 106.3W   60 KT  70 MPH
 12H  17/1800Z 15.5N 108.0W   75 KT  85 MPH
 24H  18/0600Z 16.7N 110.1W   95 KT 110 MPH
 36H  18/1800Z 18.1N 111.7W  110 KT 125 MPH
 48H  19/0600Z 19.9N 113.0W  115 KT 130 MPH
 60H  19/1800Z 22.0N 113.8W  110 KT 125 MPH
 72H  20/0600Z 24.6N 114.3W   95 KT 110 MPH
 96H  21/0600Z 31.6N 116.0W   50 KT  60 MPH...INLAND
120H  22/0600Z 39.1N 118.6W   25 KT  30 MPH...POST-TROP/INLAND
 
$$
Forecaster Berg
 
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