WTPA41 PHFO 261504

TCDCP1



TROPICAL STORM NIALA DISCUSSION NUMBER   7

NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI   CP062015

500 AM HST SAT SEP 26 2015



LIKE MANY OF THE TROPICAL CYCLONES IN THE CENTRAL PACIFIC BASIN

THIS SEASON...NIALA IS DOING A PRETTY GOOD JOB OF SHRUGGING OFF 25

TO 35 KNOTS OF DEEP LAYER SHEAR. AN 0724 UTC AMSU PASS INDICATED

THERE WAS STILL A WEAK EYEWALL FEATURE...AND CONVENTIONAL SATELLITE

IMAGERY HAS SHOWN PERSISTENT VIGOROUS CONVECTION REMAINING NEAR THE

CENTER OVER THE PAST SEVERAL HOURS. SUBJECTIVE SATELLITE INTENSITY

ESTIMATES WERE A STEADY 3.0 FROM PHFO AND JTWC...WHILE SAB WENT UP

TO 3.5. ADT NUMBERS WERE 3.9 FROM CIMSS AND 3.4 FROM SAB. BASED ON

A CONSENSUS OF THESE DATA...HAVE RAISED THE ADVISORY INTENSITY TO

50 KT...AND THIS MAY BE A BIT CONSERVATIVE.



THE INITIAL MOTION ESTIMATE IS 315/6. NIALA IS STILL GAINING

LATITUDE AS IT FEELS THE INFLUENCE OF A LARGE MID-TO-UPPER LEVEL

TROUGH EXTENDING FAR TO THE NORTHEAST OF HAWAII. THERE IS ALSO SOME

WESTWARD COMPONENT TO THE MOTION OWING TO STRONG HIGH PRESSURE

ROUGHLY 1500 MILES NORTH OF HAWAII. THE FARTHER NORTH THAT NIALA

GETS...THE STRONGER THE SHEAR WILL BECOME...AND SOONER OR LATER THE

DEEP CONVECTION WILL SEPARATE FROM THE LOW LEVEL CIRCULATION

CENTER. THIS WILL CAUSE NIALA TO RATHER ABRUPTLY BECOME STEERED

TOWARD THE WEST-SOUTHWEST BY THE LOW LEVEL FLOW. IT IS DIFFICULT TO

SAY EXACTLY WHEN THIS WILL HAPPEN...LEADING TO A FAIR AMOUNT OF

UNCERTAINTY IN THE FORECAST AND THE PRIMARY REASON FOR MAINTAINING

THE TROPICAL STORM WATCH FOR THE BIG ISLAND. OUR FORECAST DEPICTS

A MORE GENTLE TURN OVER THE NEXT 36-48 HOURS THAN DEPICTED BY MOST

OF THE GUIDANCE WHICH IS LIKELY TOO QUICK TO DECOUPLE NIALA. AFTER

48 HOURS...THE FORECAST TRACK MORE CLOSELY FALLS IN LINE WITH THE

CONSENSUS TRACKS. IT SHOULD BE NOTED THERE IS CONSIDERABLE SPREAD IN

THE GUIDANCE REGARDING THE SPEED OF THE SYSTEM IN THE 72-120 HOUR

TIME FRAME.



IT SEEMS UNLIKELY THAT NIALA WOULD STRENGTHEN MUCH MORE GIVEN THE

INCREASINGLY HOSTILE CONDITIONS THE CYCLONE IS GOING TO ENCOUNTER.

HAVE INDICATED A GRADUAL WEAKENING THROUGH THE FIRST 48 HOURS...

WITH MORE RAPID WEAKENING TO REMNANT LOW BY 96 HOURS IN THE

FACE OF RELENTLESS SHEAR. THIS WEAKENING TREND IS A LITTLE SLOWER

WHAT MOST OF THE GUIDANCE INDICATES...AND IS MORE IN LINE WITH THE

ECMWF FOR THE FIRST 48 HOURS...THEN WEAKENS MORE QUICKLY IN THE DAYS

3-5 TIME FRAME.





FORECAST POSITIONS AND MAX WINDS



INIT  26/1500Z 17.3N 151.7W   50 KT  60 MPH

 12H  27/0000Z 17.7N 152.5W   45 KT  50 MPH

 24H  27/1200Z 17.8N 153.5W   45 KT  50 MPH

 36H  28/0000Z 17.6N 154.6W   45 KT  50 MPH

 48H  28/1200Z 17.3N 155.6W   40 KT  45 MPH

 72H  29/1200Z 16.6N 157.4W   30 KT  35 MPH

 96H  30/1200Z 15.9N 159.4W   25 KT  30 MPH...POST-TROP/REMNT LOW

120H  01/1200Z 15.2N 161.5W   25 KT  30 MPH...POST-TROP/REMNT LOW



$$

FORECASTER R BALLARD