Central Pacific Basin Tropical Cyclones

Tropical Storm Calvin Forecast Discussion

WTPA41 PHFO 191449

Tropical Storm Calvin Discussion Number  32
NWS Central Pacific Hurricane Center Honolulu HI   EP032023
500 AM HST Wed Jul 19 2023

Deep convection around Calvin collapsed overnight, leaving residual 
deep convection across east and southeast-facing shores and 
slopes of the Big Island and Maui. This is the result of deep 
tropical flow interacting with terrain. Calvin's low level 
circulation center (LLCC) has become completely exposed, allowing 
small corrections in initial position and movement for this 
advisory. Instruments aboard the US Air Force Reserve's Hurricane 
Hunter aircraft showed maximum flight level winds of 47 kt, reduced 
to about 42 kt at sea level. Subjective Dvorak satellite analyses 
from PHFO and JTWC gave current intensity numbers of 2.5, 35 kt and 
2.0, 30 kt, respectively. UW-CIMSS derived an ADT intensity of 
37 kt, while an ASCAT pass from last evening showed maximum winds 
of about 40 kt within Calvin's northeast quadrant. Based on these 
data, the initial intensity for this advisory has been decreased to 
40 kt.

The initial motion for this advisory is unchanged at 275/17 kt as 
Calvin continues to move south of a large subtropical ridge. No 
significant change in this steering is expected over the next 
several days. Calvin is about to complete its passage south of the 
Big Island and the forecast track is essentially the same as the 
previous forecast track, aside from small changes in initial 
position and forward speed from tau 12 and beyond. The total track 
is shorter as this system is forecast to dissipate at tau 60. The 
forecast track remains within, but on the southern side, of the 
guidance envelope. Although the center of Calvin is passing south of 
the Big Island, much of the island remains within the 34 kt radius, 
and impacts from strong winds, heavy rainfall, and high surf are 

The forecast calls for Calvin to finish its passage south of the 
Big Island this morning, then continue moving westward away from 
the main island chain as a weakening tropical storm. Vertical shear 
affecting the tropical cyclone is expected to be moderate today, 
then strong by tonight. The strong shear should result in
weakening to post-tropical/remnant low status late Thursday night.


1. Calvin has almost completed its passage south of Hawaii County. 
Expect periods of flash flooding, dangerous surf and damaging 
winds. Calvin will continue to weaken as it moves westward to the 
south of the other Hawaiian Islands today and tonight, bringing the 
potential for some peripheral impacts.


INIT  19/1500Z 17.8N 156.8W   40 KT  45 MPH
 12H  20/0000Z 17.9N 159.5W   35 KT  40 MPH
 24H  20/1200Z 18.1N 163.1W   35 KT  40 MPH
 36H  21/0000Z 18.2N 166.8W   30 KT  35 MPH
 48H  21/1200Z 18.2N 170.7W   25 KT  30 MPH...POST-TROP/REMNT LOW
 60H  22/0000Z...DISSIPATED

Forecaster Powell

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