Advisories

WDPN33 PGTW 280900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W (BANG-LANG) WARNING 
NR 009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.8N 165.3E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 80 NM WEST-SOUTHWEST OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: 
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL STORM 
20W (BANG-LANG) WITH VIGOROUS BURSTS OF DEEP CONVECTION OBSCURING THE 
EXTREMELY COMPACT LOW-LEVEL CIRCULATION CENTER (LLCC). THE CENTRAL 
DENSE OVERCAST (CDO) HAS BECOME SLIGHTLY MORE IRREGULAR OVER THE PAST 
6 HOURS ALONG WITH THE RECENT DEVELOPMENT OF A CONVECTIVE BAND IN THE 
EASTERN SEMICIRCLE. THE LACK OF RECENT HIGH RESOLUTION MICROWAVE 
IMAGERY CAUSES THERE TO BE SOME UNCERTAINTY IN THE CURRENT STRUCTURE 
OF THE CORE. THE CIRCULATION IS BEGINNING TO INTERACT WITH A BAND OF 
ENHANCED SOUTHWESTERLIES TO THE EAST OF THE SYSTEM. SURFACE 
OBSERVATIONS FROM WAKE ISLAND BRIEFLY JUMPED UP TO 33 KTS FROM THE 
SOUTH AT 280600Z, BUT HAVE SINCE LEVELED OUT CLOSER TO 25 KTS FROM THE 
SOUTHEAST. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED 
ON CLOUD TRACING IN THE ANIMATED MSI. THE INITIAL INTENSITY OF 60 KTS 
IS ASSESSED WITH MEDIUM CONFIDENCE, PLACED HIGHER THAN THE AGENCY 
DVORAK AND CIMSS ESTIMATES DUE TO THE HIGHLY COMPACT NATURE OF THE 
SYSTEM. ENVIRONMENTAL ANALYSIS INDICATES THAT 20W REMAINS PLACED 
WITHIN A FAVORABLE ENVIRONMENT CHARACTERIZED BY MODERATE POLEWARD 
OUTFLOW ALOFT, LOW (5-10 KTS) VERTICAL WIND SHEAR, AND WARM (29-30 C) 
SEA SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: PERSISTENCE WITH SLIGHT ADJUSTMENTS MADE
BASED ON THE OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: SOUTHWESTWARD EXTENSION OF A
SUBTROPICAL RIDGE (STR) POSITIONED TO THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T2.5 - 35 KTS
   KNES: T3.0 - 45 KTS
   PHFO: T3.5 - 55 KTS
   CIMSS ADT: 53 KTS AT 280610Z
   CIMSS AIDT: 44 KTS AT 280610Z
   CIMSS D-PRINT: 43 KTS AT 280610Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: MEDIUM

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO THE 
FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TS 20W (BANG-LANG) IS FORECAST TO TRACK NORTH-
NORTHEASTWARD ALONG THE WESTERN PERIPHERY OF THE SUBTROPICAL RIDGE 
(STR) POSITIONED TO THE NORTHEAST THROUGH TAU 48. NEAR TAU 48, AN 
INCOMING UPPER-LEVEL TROUGH IS EXPECTED TO BREAK DOWN THE STEERING 
RIDGE, CAUSING TRACK SPEEDS TO DECREASE. THE SYSTEM IS ALSO EXPECTED 
TO BEGIN INTERACTING WITH TS 21W (ETAU) STARTING AROUND TAU 60, WHICH 
WILL TUG IT BRIEFLY NORTHWESTWARD THROUGH TAU 72. AFTER TAU 72, THE 
FRONTAL BOUNDARY TO THE NORTH WILL PICK UP 20W AND PUSH IT OFF TO THE 
NORTHEAST THROUGH TAU 96. SUBTROPICAL TRANSITION IS FORECAST TO BEGIN 
AROUND TAU 72 WITH COMPLETION BY TAU 96 AS THE SYSTEM CROSSES THE 26C 
SST ISOTHERM AND INTERACTS WITH THE MIDLATITUDE THROUGH. REGARDING 
INTENSITY, 20W IS FORECAST TO INTENSIFY THROUGH TAU 24 AS VERTICAL 
WIND SHEAR REMAINS LOW (LESS THAN 10 KTS) AND OCEAN HEAT CONTENT 
REMAINS RELATIVELY HIGH. STARTING AROUND TAU 36, WESTERLY SHEAR IS 
EXPECTED TO INCREASE AND OCEAN HEAT CONTENT WILL RAPIDLY DROP, CAUSING 
THE SYSTEM TO WEAKEN THROUGH THE REMAINDER OF THE FORECAST PERIOD. THE 
HIGHLY COMPACT NATURE OF THE SYSTEM MAY CAUSE THE INTENSITY TO BE 
VOLATILE AS SMALL CHANGES IN THE ENVIRONMENT COULD HAVE A LARGE IMPACT 
ON THE VORTEX.

MODEL DISCUSSION: MODEL TRACK GUIDANCE IS IN MODERATE AGREEMENT 
THROUGH TAU 72 WITH THE MAIN OUTLIERS BEING GALWEM AND NAVGEM, WHICH 
CONTINUE TO TRACK THE SYSTEM MUCH FURTHER WEST. DISCOUNTING THE 
OUTLIERS, CROSS-TRACK SPREAD AT TAU 72 IS MEASURED TO BE ABOUT 310 NM. 
AFTER TAU 72, MODELS ARE IN VERY POOR AGREEMENT WITH SOME MODELS, SUCH 
AS GFS, SHOWING A RAPID TURN TO THE NORTHEAST WHILE OTHERS SLOW DOWN 
THE SYSTEM AS IT INTERACTS WITH 21W. AS A RESULT, THE JTWC TRACK 
FORECAST IS PLACED NEAR THE MODIFIED CONSENSUS THROUGH TAU 72 WITH 
MEDIUM CONFIDENCE, AND THEN CLOSER TO THE ECMWF ENSEMBLE MEAN THROUGH 
TAU 96 WITH LOW CONFIDENCE. INTENSITY GUIDANCE REMAINS IN VERY POOR 
AGREEMENT WITH MANY RAPID INTENSIFICATION (RI) AIDS CONTINUING TO 
TRIGGER. PEAK INTENSITIES HAVE A SPREAD FROM 70 KTS (GOOGLE DEEPMIND) 
TO 115 KTS (MOST AGGRESSIVE RI AIDS). THE JTWC INTENSITY FORECAST 
FOLLOWS THE HAFS-A THROUGH TAU 24, CALLING FOR A PEAK OF 85 KTS, 
FOLLOWED BY STEADY WEAKENING THEREAFTER WITH LOW CONFIDENCE.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: MEDIUM
   TRACK 72-120 HR: LOW
   INTENSITY 00-72 HR: LOW
   INTENSITY 72-120 HR: LOW//
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