TROPICAL STORM NIALA DISCUSSION NUMBER 7...CORRECTED 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 FOR NOW. AN AMSU PASS AT 1245 UTC AND AN SSMI PASS AT 1307 UTC STILL INDICATED A PARTIAL EYEWALL FEATURE...BUT THERE ARE INDICATIONS THAT THE CENTER IS BECOMING INCREASINGLY VERTICALLY TILTED OVER TIME. 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...WITH 3.5 FROM SAB. THE 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 COMPLETELY SEPERATE 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 WILL 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 TIMEFRAME. 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