Central Pacific Basin Tropical Cyclones

Tropical Storm Calvin Forecast Discussion

WTPA41 PHFO 190240

Tropical Storm Calvin Discussion Number  30
NWS Central Pacific Hurricane Center Honolulu HI   EP032023
500 PM HST Tue Jul 18 2023

A burst of deep convection north of the center today strengthened 
Calvin, and a couple of passes through the circulation by 
the US Air Force Reserve's Hurricane Hunters showed an increase in 
the winds. SFMR showed some peaks at around 55 kt, though these may 
have had some rain contamination. The on-board weather officer 
indicated 48-50 kt may be more reasonable. Reduction of flight level 
winds showed 45-50 kt in portions of the northern semicircle. Based 
on these data, the initial intensity for this advisory has been 
increased to 50 kt. The aircraft data also resulted in an increase 
in the 34 kt wind radii. The recent strengthening may have been a 
short term trend as the deep convection has eased since this morning 
with cloud top temperatures warming through the afternoon. 

The initial motion for this advisory is 275/18 kt as Calvin 
continues to move south of a large subtropical ridge. No significant 
changes in this steering is expected over the next several days. 
The main objective aids have the center of Calvin passing south of 
the Big Island tonight. The forecast for this advisory is 
essentially on the previous forecast track but slightly slower as a 
reflection of the slower initial motion. The forecast track also 
remains on the northern side of the guidance envelope. Although the 
center of Calvin is expected to pass south of the Big Island, most 
of the island is well within the 34 kt radius, and impacts from 
strong winds, heavy rainfall, and high surf are expected.

Although Calvin strengthened today, the dynamical models suggest 
that this was a short term change and the forecast still calls for 
Calvin to pass south of the Big Island as a weakening tropical 
storm. Vertical shear affecting the tropical cyclone is expected to 
be moderate through tonight and into Wednesday, then will quickly 
become strong by Wednesday night. The strong shear should result in 
a weakening to post-tropical/remnant low status on Thursday.


1. Calvin is forecast to pass south of Hawaii County tonight,
bringing a period of flash flooding, dangerous surf and damaging 
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.


INIT  19/0300Z 17.8N 153.3W   50 KT  60 MPH
 12H  19/1200Z 18.4N 156.0W   45 KT  50 MPH
 24H  20/0000Z 18.9N 159.8W   40 KT  45 MPH
 36H  20/1200Z 19.4N 163.6W   35 KT  40 MPH
 48H  21/0000Z 19.7N 167.5W   30 KT  35 MPH
 60H  21/1200Z 19.9N 171.2W   30 KT  35 MPH...POST-TROP/REMNT LOW
 72H  22/0000Z...DISSIPATED

Forecaster Kodama

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