TROPICAL DEPRESSION SIX-C DISCUSSION NUMBER 1
NWS CENTRAL PACIFIC HURRICANE CENTER HONOLULU HI CP062015
500 PM HST THU SEP 24 2015
RECENT SATELLITE IMAGES SHOWED THAT A TROPICAL DISTURBANCE SOUTHEAST
OF THE MAIN HAWAIIAN ISLANDS HAS DEVELOPED DEEP CONVECTION WITHIN A
CURVED BAND NEAR ITS CENTER. IN ADDITION...A 1953 UTC ASCAT PASS
SHOWED WINDS AT 30 KT WITHIN 80 NM OF THE DISTURBANCE/S EASTERN
SEMICIRCLE. SUBJECTIVE DVORAK FIXES UNANIMOUSLY CAME IN AT 2.0/30
KT. AS A RESULT OF THESE DATA...ADVISORIES ARE BEING INITIATED ON
TROPICAL DEPRESSION SIX-C. THIS IS THE ELEVENTH TROPICAL CYCLONE IN
THE CENTRAL NORTH PACIFIC BASIN FOR THE 2015 SEASON AND TIES THE
RECORD FOR THE MOST ACTIVE SEASON WHICH WAS SET IN 1992 AND
REPEATED IN 1994.
THE DEPRESSION IS MOVING AT 320/7 KT TOWARDS A WEAKNESS IN THE
SUBTROPICAL RIDGE. THIS MOTION IS EXPECTED TO CONTINUE OVER THE NEXT
COUPLE OF DAYS. A LARGE MIDDLE AND UPPER LEVEL LOW PRESSURE SYSTEM
CURRENTLY FAR NORTHEAST OF THE HAWAIIAN ISLANDS IS FORECAST TO MOVE
TOWARD THE SOUTHEAST. AN ASSOCIATED TROUGH EXTENDING SOUTHWEST FROM
THE LOW WILL SHIFT SOUTHWARD...RESULTING IN AN INCREASE IN UPPER
LEVEL SOUTHWESTERLIES...AND VERTICAL WIND SHEAR...THAT WILL IMPACT
THE TROPICAL CYCLONE. GLOBAL MODELS SHOW THAT THE VERTICAL SHEAR
WILL DECOUPLE THE SYSTEM BEYOND 48 HOURS...WHICH MEANS THE
CYCLONE WILL BECOME SHALLOW AND MOVE WITH THE LOWER LEVEL STEERING
CURRENT. THE RELIABLE DYNAMICAL MODELS ARE IN GENERAL AGREEMENT WITH
THIS SCENARIO AND ARE FAIRLY CLOSE TO EACH OTHER WITH THE MAIN
DIFFERENCES BEING THE FORWARD SPEED AFTER DECOUPLING OCCURS. THE
TRACK FORECAST FOR THIS ADVISORY CLOSELY FOLLOWS THE DYNAMIC
CONSENSUS AND IS BETWEEN THE GFS TO THE SOUTH AND THE ECMWF TO THE
NORTH.
TROPICAL DEPRESSION SIX-C IS PASSING OVER WARM WATER...28C TO 29C
BASED ON THE LATEST ANALYSIS. MODERATE SHEAR IS ALREADY AFFECTING
THE SYSTEM ACCORDING TO THE 0000 UTC CIMSS ANALYSIS AND IS FORECAST
TO INCREASE...ESPECIALLY BEYOND 48 HOURS. BEFORE THE STRONGER SHEAR
SETS IN AND DECOUPLES THE CYCLONE...THERE APPEARS TO BE A WINDOW OF
OPPORTUNITY FOR STRENGTHENING. THUS...THE FORECAST CALLS FOR SIX-C
TO BECOME A TROPICAL STORM TONIGHT FOLLOWED BY STEADY WEAKENING
AFTER 36 HOURS. IF IT DOES BECOME A TROPICAL STORM IT WILL BE NAMED
NIALA...PRONOUNCED NEE-AH-LA.
FORECAST POSITIONS AND MAX WINDS
INIT 25/0300Z 14.5N 149.3W 30 KT 35 MPH
12H 25/1200Z 15.1N 150.0W 35 KT 40 MPH
24H 26/0000Z 16.2N 151.0W 40 KT 45 MPH
36H 26/1200Z 17.0N 152.2W 40 KT 45 MPH
48H 27/0000Z 17.4N 153.5W 35 KT 40 MPH
72H 28/0000Z 17.5N 156.0W 30 KT 35 MPH
96H 29/0000Z 17.0N 157.5W 30 KT 35 MPH
120H 30/0000Z 16.5N 159.0W 25 KT 30 MPH
