Central Pacific Basin Tropical Cyclones

Tropical Storm Calvin Forecast Discussion

WTPA41 PHFO 172059

Tropical Storm Calvin Discussion Number  25
NWS Central Pacific Hurricane Center Honolulu HI   EP032023
1100 AM HST Mon Jul 17 2023

Satellite data showed an exposed low-level circulation center with 
sparse deep convection only in the outer rainband well north 
of the center. A US Air Force Reserve WC-130J flew its initial 
mission into Calvin this morning. Based on a combination of 
the flight level winds and dropsonde data, maximum winds appear 
to be around 45 kt in the NE quad and will be used as the initial 
intensity for this advisory. The SFMR data appeared to be too low. 

There is little change in the steering environment. Calvin 
continues to move westward to the south of a subtropical ridge. The 
latest dynamical aids indicated that this steering will continue 
over the next several days. There is still some spread in the 
guidance for the track near the main Hawaiian Islands with HAFS-A  
on the north end and CMC to the south. The track forecast is close 
to the previous advisory, but with a slight increase in forward 
speed. The forecast is also very close to HCCA. This puts the center 
of Calvin near or over the Big Island on Wednesday, with tropical 
storm force winds arriving Tuesday night.

The surrounding environment around Calvin has been dry and stable 
with SSTs around 24-25C. As Calvin moves westward, the SSTs will 
gradually increase, with values of 26-27C around the main Hawaiian 
Islands.  As a result, deep convection may become better developed, 
which is suggested by model-simulated satellite imagery from some 
of the dynamical models. This is expected to keep Calvin at 
tropical storm intensity by the time it reaches the Big Island. 
Calvin will also be affected by an upper tropospheric trough in 
about 24-48 hrs. The trough will increase the vertical shear over 
the tropical cyclone and result in its ultimate demise.


1. Calvin is forecast to pass very close to the Big Island Tuesday
night and Wednesday, 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 for impacts today and Tuesday prior to the onset of 
tropical storm force winds. These impacts could include flash 
flooding, damaging winds, and large and dangerous surf.


INIT  17/2100Z 17.3N 143.3W   45 KT  50 MPH
 12H  18/0600Z 17.4N 146.2W   40 KT  45 MPH
 24H  18/1800Z 17.9N 149.9W   40 KT  45 MPH
 36H  19/0600Z 18.3N 153.7W   40 KT  45 MPH
 48H  19/1800Z 19.0N 157.5W   35 KT  40 MPH
 60H  20/0600Z 19.5N 161.2W   30 KT  35 MPH
 72H  20/1800Z 20.0N 165.0W   25 KT  30 MPH...POST-TROP/REMNT LOW
 96H  21/1800Z...DISSIPATED

Forecaster Kodama

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