769
WTPA41 PHFO 311439
TCDCP1
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
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page