WDPN32 PGTW 250900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NARRA) WARNING NR 
019//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.4N 108.2E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 161 NM SOUTHEAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: EASTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS THAT A
STRONG BURST OF CONVECTION FLARED UP OVER THE WESTERN SEMICIRCLE OF
A COMPACT LOW-LEVEL CIRCULATION CENTER (LLCC) BEGINNING AT 240000Z.
THIS FEATURE PERSISTED FOR THE NEXT 04 HOURS, FIGHTING AGAINST
30-40 KTS OF VERTICAL WIND SHEAR (VWS). BY 250600Z, THE LLCC BECAME
FULLY EXPOSED, SWIFTLY JOGGING EASTWARD AS IT DETACHED FROM THE
CONVECTION. THE SYSTEM HAS RESUMED A SLOW TRANSLATION SPEED TOWARD
THE NORTHEAST. A 250132Z ASCAT-B PASS REVEALED 35-40 KT WINDS ALONG
THE SOUTHERN SEMICIRCLE. GIVEN THE SUSTAINED CONVECTION SINCE THE
TIME OF THAT PASS, THE INITIAL INTENSITY IS SET AT 40 KTS WITH HIGH
CONFIDENCE, WHICH IS HIGHER THAN THE SUBJECTIVE DVORAK FIXES AND ON
THE HIGH END OF THE OBJECTIVE ESTIMATES FROM CIMSS. THE INITIAL
POSITION IS ALSO ASSESSED WITH HIGH CONFIDENCE BASED ON THE EXPOSED
LLCC. THE ENVIRONMENT IS HIGHLY UNFAVORABLE DUE TO THE VERY HIGH
VWS.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: LOW- TO MID-LEVEL SOUTHWESTERLY FLOW

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T2.0 - 30 KTS
   KNES: T2.0 - 30 KTS
   CIMSS SATCON: 39 KTS AT 250610Z
   CIMSS ADT: 34 KTS AT 250610Z
   CIMSS AIDT: 34 KTS AT 250610Z
   CIMSS D-MINT: 31 KTS AT 250606Z
   CIMSS D-PRINT: 32 KTS AT 250610Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY UNFAVORABLE
   VWS: 30+ KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK EQUATORWARD
   OTHER FACTORS: LAND INTERACTION

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: THE SHALLOW VORTEX OF TS 18W IS EMBEDDED IN
SOUTHWESTERLY FLOW, WHICH EXTENDS UPWARD TO 600 MB. THIS FLOW,
ENHANCED BY SUBTROPICAL LOWER-LEVEL RIDGE OVER SOUTHERN CHINA, WILL
DRIVE THE CIRCULATION NORTH-NORTHWESTWARD INTO LAND. WHILE SEA
SURFACE TEMPERATURES ARE SUFFICIENTLY WARM AT 29-30 DEGREES
CELSIUS, THE POTENT VWS WILL CAUSE THE DEMISE OF THE SYSTEM. WITH
THE CONVECTION NOW COMPLETELY SHEARED OFF THE CIRCULATION, THE
COMPACT, FRAGILE LLCC WILL RAPIDLY SPIN DOWN AS THE SOUTHERN
SEMICIRCLE CLIPS HAINAN. THE CIRCULATION WILL BEGIN TO OPEN UP INTO
A TROUGH AFTER TAU 12. WHILE THE FORECAST DEPICTS DISSIPATION
OCCURRING BY TAU 36, IT COULD HAPPEN MUCH SOONER DUE TO THE HIGH
VWS AND FRICTIONAL EFFECTS OF NEARBY LAND.

MODEL DISCUSSION: THE MODEL TRACKERS LARGELY DEPICT A NORTHWESTWARD
TRACK THROUGH TAU 12, FOLLOWED BY A MORE POLEWARD TURN INTO
MAINLAND CHINA. THE JTWC FORECAST TRACK IS POSITIONED ALONG THE
SOUTHERN SIDE OF THE GUIDANCE ENVELOPE, SPURRED BY THE OBSERVED
TRAVEL PATTERN IN THE MSI. THIS IS CONSISTENT WITH THE EXPERIMENTAL
GOOGLE DEEPMIND SOLUTION, WHICH IS AMONG THE MOST EQUATORWARD
MODELS. AFTER TAU 12, THE FORECAST TRACK IS SET CLOSE TO THE AI
CONSENSUS WITH MEDIUM CONFIDENCE. THE INTENSITY AIDS UNANIMOUSLY
PREDICT RAPID WEAKENING, WITH COMPLETE DISSIPATION OCCURRING NO
LATER THAN TAU 36. THE JTWC INTENSITY FORECAST ALIGNS WITH THIS
PHILOSOPHY WITH HIGH CONFIDENCE.

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