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 THORUGH 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 SORT OF 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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