Hawaii County on a Tropical Storm Watch for Calvin's approach

Here is the NWS Honolulu 11:00am update for 7.17.23 NWS-CPHC — At 1100 AM HST (2100 UTC), the center of Tropical Storm Calvin was located near latitude 17.3 North, longitude 143.3 West. Calvin is moving toward the west near 21 mph (33 km/h) and this motion is expected to continue over the next several days. On the forecast track, Calvin is expected to approach the main Hawaiian Islands on Tuesday, with tropical storm conditions possible over some areas starting Tuesday night. Maximum sustained winds are near 50 mph (85 km/h) with higher gusts. Calvin is forecast to gradually weaken during the next several days. Tropical-storm-force winds extend outward up to 125 miles (205 km) from the center. DISCUSSION: 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.