Central Pacific Basin Tropical Cyclones

Tropical Storm Calvin Forecast Discussion

WTPA41 PHFO 182039

Tropical Storm Calvin Discussion Number  29
NWS Central Pacific Hurricane Center Honolulu HI   EP032023
1100 AM HST Tue Jul 18 2023

A large area of deep convection developed overnight in the northern 
semicircle of Calvin's circulation. This convection has persisted 
through this morning and has obscured the low level circulation 
center that was so clearly visible yesterday. Fortunately, the US 
Air Force Reserve has a WC-130J flying a mission into the system 
today that has helped better identify the system's center and 
surrounding wind field. SFMR data indicated 46 kt in the NE 
quadrant and a large area of winds at or above 34 kt. As a result, 
the initial intensity for this advisory has been increased slightly 
to 45 kt. The 34 kt radius in the NE quadrant has also been 
increased as well.

The initial motion for this advisory is 280/19 kt as Calvin 
continues to move south of a 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 track forecast is essentially the same 
as the previous advisory and is on the northern side of the 
guidance envelope, but still south of the Big Island.

The overall environment around Calvin remains dry and stable. Calvin 
is currently passing over SSTs of 25-26C. Along the forecast track, 
SSTs will be gradually increasing and will be 26-27C as Calvin 
passes the Big Island. Vertical shear affecting the tropical cyclone 
is expected to be moderate through tonight. The lack of strong 
vertical shear, and the warmer SSTs should allow Calvin to maintain 
winds at tropical storm intensities through Wednesday. Shear levels 
should significantly increase Wednesday night onward, and should 
result in a weakening to post-tropical/remnant low status on 


1. Calvin is forecast to pass 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  18/2100Z 17.8N 151.9W   45 KT  50 MPH
 12H  19/0600Z 18.2N 154.3W   45 KT  50 MPH
 24H  19/1800Z 18.8N 158.1W   40 KT  45 MPH
 36H  20/0600Z 19.2N 161.8W   35 KT  40 MPH
 48H  20/1800Z 19.5N 165.6W   30 KT  35 MPH
 60H  21/0600Z 19.8N 169.3W   25 KT  30 MPH...POST-TROP/REMNT LOW
 72H  21/1800Z...DISSIPATED

Forecaster Kodama

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