ZCZC HFOTCDCP1 ALL
TTAA00 PHFO DDHHMM
 
Tropical Storm Iona Discussion Number  18
NWS Central Pacific Hurricane Center Honolulu HI   CP012025
Issued by NWS National Hurricane Center Miami FL
500 AM HST Thu Jul 31 2025
 
Iona continues to generate a strong burst of convection over the 
low-level center, although there are no recent microwave overpasses 
to shows if there has been an increase in the system's organization. 
The various subjective and objective satellite intensity estimates 
range from 30-50 kt, and based on a blend of these and the current 
convection, the initial intensity is held at 45 kt. This intensity 
is supported by a recent ASCAT overpass with 35-40 kt winds, and 
the scatterometer data has also been used to revise the wind radii.
 
The initial motion is 280/19 kt. Iona is expected to continue 
moving westward over the next day or so along the southern
periphery of a low- to mid-level subtropical ridge located to the
north. By 48 to 60 hours, a gradual turn toward the west-northwest
and a decrease in forward speed are anticipated as the system
approaches a weakness in the ridge.  How much of a turn may be 
dependent on the strength of Iona. The GFS and UKMET have a weaker 
cyclone and show a more westerly track, while the ECMWF and Canadian 
models have a stronger system and a more northward motion. The new 
forecast track splits the difference between these extremes and lies 
near the various consensus models. 
 
Iona continues to be affected by 25 kt of northwesterly vertical 
wind shear, and due to this it is uncertain how long the current 
convective burst will persist.  The intensity guidance shows some 
weakening during the next 24-36 h while the shear persists, and the 
official forecast follows that trend.  After that, the shear is 
forecast to decrease, and there may be a day or two where the 
environment becomes favorable enough for Iona to at least maintain 
its intensity. By the end of the forecast period, the global models 
diverge on the forecast environment near the cyclone, with the GFS 
forecast strong subsidence that would suppress the convection 
while the ECMWF forecasts a less hostile environment. The new 
forecast will lean toward the GFS and the previous forecast in 
showing gradual weakening, with Iona forecast to decay to a remnant 
low by 120 h.
 
 
FORECAST POSITIONS AND MAX WINDS
 
INIT  31/1500Z 12.7N 168.7W   45 KT  50 MPH
 12H  01/0000Z 13.3N 171.5W   40 KT  45 MPH
 24H  01/1200Z 14.0N 174.9W   35 KT  40 MPH
 36H  02/0000Z 14.7N 178.0W   35 KT  40 MPH
 48H  02/1200Z 15.5N 179.2E   35 KT  40 MPH
 60H  03/0000Z 16.4N 176.5E   35 KT  40 MPH
 72H  03/1200Z 17.2N 174.2E   35 KT  40 MPH
 96H  04/1200Z 19.0N 169.7E   30 KT  35 MPH
120H  05/1200Z 21.4N 165.9E   30 KT  35 MPH...POST-TROP/REMNT LOW
 
$$
Forecaster Beven
 
NNNN