948 
 WTPA41 PHFO 261507 CCA
 TCDCP1
 
 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
  
 $$
 FORECASTER R BALLARD