Central Pacific Basin Tropical Cyclones

Tropical Storm Calvin Forecast Discussion

WTPA41 PHFO 181459

Tropical Storm Calvin Discussion Number  28
NWS Central Pacific Hurricane Center Honolulu HI   EP032023
500 AM HST Tue Jul 18 2023

A large area of thunderstorms continues to expand in coverage 
near the partially exposed low-level circulation center (LLCC) of 
Tropical Storm Calvin. An aircraft from the 53rd Weather 
Reconnaissance Squadron "Hurricane Hunters" completed sampling 
Calvin a few hours ago. They found winds at flight-level and from
dropwindsondes that suggested the peak surface wind speeds were 
close to 40 kt. The most recent subjective Dvorak satellite 
intensity estimates were 35 kt from PHFO and JTWC. The latest CIMSS 
SATCON intensity estimate indicates 45 kt. The initial intensity 
for this advisory is maintained at 40 kt based on a blend of these 

Calvin's initial motion for this advisory remains 275/19 kt as the
cyclone continues to be steered rapidly westward south of a large
subtropical ridge. This steering is expected to continue over the
next 3 to 4 days. The latest model guidance, including the HCCA, 
indicates that the center of Calvin will pass over, or just south 
of the Big Island tonight. The latest track forecast is close to 
the previous forecast.

The surrounding environment around Calvin continues to be somewhat 
dry and stable. The SSTs are gradually increasing to around 25C, 
and we expect additional warming of the SSTs to 26-27C as it moves 
closer to the main Hawaiian Islands. The higher SSTs will likely
support additional deep convection spreading around the northern 
semicircle of Calvin at least into Wednesday. This is expected to 
keep the cyclone at tropical storm intensity during the next 36  
hours or so. As a result, a Tropical Storm Warning remains in 
effect for Hawaii County. After it passes southwest of the main 
Hawaiian Islands, we expect Calvin to move near an upper 
tropospheric trough by day 2. This feature aloft will likely 
increase the vertical wind shear over the system, and will 
eventually result in it becoming a post-tropical/remnant low by 
late Thursday. Finally, another "Hurricane Hunter" aircraft is 
scheduled to fly a sampling mission into Calvin a few hours after 
daybreak this morning. They will monitor any significant changes 
in intensity and size as the system moves toward the Big Island.


1. Calvin is forecast to pass over, or very close to Hawaii County
tonight, bringing a period of heavy rain, high surf and locally 
strong winds. Calvin is expected to weaken as it moves westward to 
the south of the other Hawaiian Islands Wednesday and Wednesday 
night, bringing the potential for some peripheral impacts.

2. It is important to not focus on the exact forecast track and
intensity when preparing for Calvin. Persons in Hawaii County
should prepare during daylight hours for impacts prior to the 
onset of tropical storm conditions, which are expected to start 
this evening. These impacts could include flash flooding, damaging 
winds, and large and dangerous surf.


INIT  18/1500Z 17.5N 149.5W   40 KT  45 MPH
 12H  19/0000Z 17.9N 152.4W   40 KT  45 MPH
 24H  19/1200Z 18.6N 156.1W   35 KT  40 MPH
 36H  20/0000Z 19.1N 159.9W   35 KT  40 MPH
 48H  20/1200Z 19.5N 163.6W   30 KT  35 MPH
 60H  21/0000Z 19.6N 167.4W   25 KT  30 MPH
 72H  21/1200Z 20.0N 171.0W   20 KT  25 MPH...POST-TROP/REMNT LOW
 96H  22/1200Z...DISSIPATED

Forecaster Houston

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