Central Pacific Basin Tropical Cyclones

Tropical Storm Calvin Forecast Discussion

WTPA41 PHFO 190856

Tropical Storm Calvin Discussion Number  31
NWS Central Pacific Hurricane Center Honolulu HI   EP032023
1100 PM HST Tue Jul 18 2023

Deep convection has decreased considerably near the center of 
Calvin since late this afternoon, but has begun to increase farther 
to the north along windward shores and slopes of the Big Island of 
Hawaii. The low level circulation center (LLCC) is rapidly becoming 
unveiled again. Instruments aboard the US Air Force Reserve's 
Hurricane Hunter aircraft showed maximum flight level winds of 50 
kt, reduced to about 45 kt at the surface. Objective Dvorak 
satellite analyses from PHFO and JTWC gave current intensity 
numbers of 2.5 or 35 kt. Based on these data, the initial intensity 
for this advisory has been decreased to 45 kt. 

The initial motion for this advisory is 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.
After passing south of the Big Island tonight, the forecast is
essentially the same as the previous forecast track but slightly 
to the south to account for a small adjustment in initial position, 
thanks to the newly revealed LLCC. The forecast track lies within, 
but on the southern side, of the guidance envelope due to the 
initial point adjustment. Although the center of Calvin is passing 
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 imminent or occurring.

The forecast calls for Calvin to pass south of the Big Island, then 
southwest of the rest of the main island chain, as a weakening 
tropical storm. Vertical shear affecting the tropical cyclone is 
expected to be moderate through tonight and into Wednesday, then 
strong by Wednesday night. The strong shear should result in a 
weakening to post-tropical/remnant low status on Friday.


1. Calvin has begun to pass south of Hawaii County. Expect periods
of flash flooding, dangerous surf and damaging winds. Calvin will
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  19/0900Z 17.7N 155.2W   45 KT  50 MPH
 12H  19/1800Z 17.8N 158.1W   45 KT  50 MPH
 24H  20/0600Z 18.1N 161.9W   40 KT  45 MPH
 36H  20/1800Z 18.3N 165.8W   35 KT  40 MPH
 48H  21/0600Z 18.3N 169.7W   30 KT  35 MPH
 60H  21/1800Z 18.0N 173.4W   30 KT  35 MPH...POST-TROP/REMNT LOW
 72H  22/0600Z...DISSIPATED

Forecaster Powell

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