WTPA41 PHFO 260253

TCDCP1



TROPICAL STORM NIALA DISCUSSION NUMBER   5

NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI   CP062015

500 PM HST FRI SEP 25 2015



NIALA PRODUCED BURSTS OF CONVECTION AROUND THE CENTER THROUGH EARLY

AFTERNOON WITH BRIEF EYE WALL STRUCTURES APPARENT ON VISIBLE

IMAGERY. A DISTINCT OUTFLOW CHANNEL PERSISTS TO THE NORTHEAST...

WHILE OUTFLOW REMAINS SOMEWHAT DISRUPTED WITHIN THE WESTERN QUADRANT

BY WESTERLY WINDS ALOFT. DVORAK CURRENT INTENSITY ESTIMATES CAME IN

AT 3.0/45 KT FROM ALL FIX AGENCIES...AND CIMSS ADT SUGGESTED A

SLIGHTLY HIGHER VALUE. THUS...THE INITIAL INTENSITY IS 45 KT FOR

THIS ADVISORY.



THE INITIAL MOTION REMAINS NEARLY UNCHANGED TOWARD THE NORTHWEST...

OR 320 DEGREES...AT 6 KT. THE MAIN STEERING MECHANISMS ARE A LOW

LEVEL RIDGE CENTERED FAR NORTH OF NIALA...WHICH IS IMPARTING SOME

WESTWARD MOTION...AND A DEEP MID TO UPPER LEVEL TROUGH RUNNING FROM

THE MAIN HAWAIIAN ISLANDS OFF TO THE NORTHEAST...LEADING TO SOME

NORTHWARD CONTRIBUTION TO THE MOTION. THE NET EFFECT OF THESE

FEATURES IS CAUSING THE CYCLONE TO MOVE TOWARD THE NORTHWEST. NIALA

IS EXPECTED TO CONTINUE ON A GENERAL NORTHWESTWARD TRACK THROUGH

TONIGHT...FOLLOWED BY A GRADUAL TURN TOWARD THE WEST ON SATURDAY AND

SUNDAY AS THE LOW LEVEL RIDGE TO THE NORTH BECOMES THE MAIN STEERING

FEATURE OF THE WEAKENING CYCLONE. THE TRACK FORECAST HAS BEEN

CHANGED LITTLE AND RUNS NEAR THE GFEX ALONG THE NORTHERN PORTION OF

THE DYNAMICAL AND CONSENSUS GUIDANCE ENVELOPE. THE GUIDANCE IS

REASONABLY WELL-CLUSTERED BUT STILL CONTAINS SIGNIFICANT DIFFERENCES

IN FORWARD MOTION...MAINLY DUE TO THE GFS...HWRF...AND GFDL

FORECASTING THE TURN TOWARD THE WEST OCCURRING OVER A LONGER PERIOD

OF TIME.



A SMALL WINDOW FOR ADDITIONAL INTENSIFICATION REMAINS BEFORE

INCREASING VERTICAL WIND SHEAR INITIATES A WEAKENING TREND. NIALA

WILL BE MOVING NORTH OF A RIDGE AXIS ALOFT TONIGHT OR SATURDAY...

GRADUALLY EXPOSING THE CYCLONE TO INCREASING WESTERLY WINDS ALOFT

ASSOCIATED WITH THE MID TO UPPER LEVEL TROUGH SITTING OVER THE

HAWAIIAN ISLANDS. WESTERLY VERTICAL WIND SHEAR IS ALREADY AROUND 12

KT ACCORDING TO SHIPS...WILL LIKELY DOUBLE BY 24 HOURS...AND WILL

REMAIN STRONG THROUGH THE REMAINDER OF THE FORECAST PERIOD. THIS

WILL LEAD TO STEADY WEAKENING OF NIALA FROM LATE SATURDAY THROUGH

EARLY NEXT WEEK...WITH THE FORECAST CALLING FOR THE SYSTEM TO BECOME

A REMNANT LOW BY 96 HOURS. THE OFFICIAL FORECAST IS CLOSE TO ICON

AND IS LESS AGGRESSIVE THAN SHIPS IN THE WEAKENING TREND.









FORECAST POSITIONS AND MAX WINDS



INIT  26/0300Z 16.2N 150.7W   45 KT  50 MPH

 12H  26/1200Z 16.8N 151.7W   50 KT  60 MPH

 24H  27/0000Z 17.3N 152.9W   50 KT  60 MPH

 36H  27/1200Z 17.3N 154.0W   45 KT  50 MPH

 48H  28/0000Z 17.2N 154.7W   40 KT  45 MPH

 72H  29/0000Z 16.9N 155.9W   30 KT  35 MPH

 96H  30/0000Z 16.5N 157.4W   25 KT  30 MPH...POST-TROP/REMNT LOW

120H  01/0000Z 16.0N 159.2W   25 KT  30 MPH...POST-TROP/REMNT LOW



$$

FORECASTER WROE