Advisories

WDPN32 PGTW 211500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (EIGHTEEN) WARNING NR 
004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.8N 108.8E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 163 NM EAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 03 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
OVER THE PAST SIX HOURS, A MASS OF VERY DEEP CONVECTION HOOKED ONTO
THE WESTERN FLANK OF TS 18W. THIS HAS VISIBLY CAUSED THE LOW-LEVEL
CIRCULATION CENTER (LLCC) TO SPIN UP. THIS CONVECTION HAS SINCE
WEAKENED AND PROGRESSED INLAND OVER QINZHOU. THE LLCC REMAINS OVER
THE VERY WARM WATERS OF THE GULF OF TONKIN, BUT THE MODERATE TO
HIGH VERTICAL WIND SHEAR (VWS) HAS TAKEN A TOLL ON THE CONVECTIVE
ACTIVITY THAT WAS ONCE OVER THE CENTER. A 211032Z WSFM MWI 37 GHZ
IMAGE REVEALED A RATHER DISORGANIZED LOWER-LEVEL STRUCTURE WITH
HEAVY PRECIPITATION OCCURRING WITHIN THE WESTERN SIDE OF THE
CENTER. DRY AIR IS BEING PULLED NORTHWARD AND IS ENVELOPING THE
CIRCULATION, WHICH IS SUPPRESSING CONVECTIVE ACTIVITY. THE INITIAL
POSITION IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON A PARTIALLY
EXPOSED LLCC IN THE ANIMATED ENHANCED INFRARED (EIR) SATELLITE
IMAGERY. THE INITIAL INTENSITY HAS BEEN RAISED TO 35 KTS WITH
MEDIUM CONFIDENCE BASED ON THE RISING OBJECTIVE INTENSITY ESTIMATES
FROM CIMSS, WHICH RANGED FROM 36 KTS TO 43 KTS.

INITIAL WIND RADII BASIS: WSFM MICROWAVE IMAGERY

CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T1.5 - 25 KTS
   CIMSS SATCON: 40 KTS AT 211140Z
   CIMSS ADT: 43 KTS AT 211210Z
   CIMSS AIDT: 36 KTS AT 211210Z
   CIMSS D-MINT: 39 KTS AT 211030Z
   CIMSS D-PRINT: 37 KTS AT 211140Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK RADIAL

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL STORM (TS) 18W IS LOCATED WITHIN A
WEAK STEERING ENVIRONMENT. A WEAK SUBTROPICAL RIDGE (STR) OVER
SOUTHEAST CHINA TRIES TO DRIVE THE SYSTEM WESTWARD, BUT THE
DEEP-LAYER SOUTHWESTERLIES TO THE SOUTH ATTEMPT TO MOVE IT IN THE
OPPOSITE DIRECTION. THE END RESULT IS A VERY SLOW RATE OF FORWARD
MOVEMENT TOWARD THE NORTHEAST. THE STR WILL WEAKEN OVER THE NEXT 12
HOURS. THEREFORE, THE JTWC FORECAST TRACK PRESUMES THAT THE
STEERING INFLUENCE OF THE SOUTHWESTERLY FLOW WILL DOMINATE AND
CALLS FOR A GENERAL NORTHEASTWARD TO EASTWARD MOTION THROUGH THE
PERIOD. WITH MOST OF THE CONVECTION NOW INLAND AND VERY LITTLE OVER
THE CIRCULATION, THE SYSTEM HAS LIKELY REACHED PEAK INTENSITY. THE
ENVIRONMENT WILL BE PERSISTENTLY UNFAVORABLE FOR DEVELOPMENT,
CHARACTERIZED BY WARM SST STRONGLY OFFSET BY MODERATE TO HIGH VWS
AND DRY AIR. THE JTWC INTENSITY FORECAST ASSUMES THAT CONVECTION
WILL ATTEMPT TO REBUILD OVER THE CIRCULATION WHILE IT REMAINS OVER
WARM WATERS, WHICH WILL HELP MAINTAIN INTENSITY IN THE FACE OF
UNDESIRABLE SHEAR OVER THE NEXT 36 HOURS. ONCE THE VORTEX MAKES
LANDFALL, IT WILL DISSIPATE BY TAU 72.

MODEL DISCUSSION: THE CONFIDENCE IN THE TRACK FORECAST IS VERY LOW.
A SLIGHTLY STRONGER STR COULD BRIEFLY STEER THE SYSTEM WESTWARD,
AND SEVERAL MODEL TRACKERS DEPICT THIS SCENARIO. ALTHOUGH MOST OF
THE GUIDANCE EVENTUALLY BRINGS THE CIRCULATION EASTWARD, THE BRIEF
WESTWARD MOTION WOULD DELAY THE INEVITABLE TRACK TOWARD LAND. AN
ALTERNATE SCENARIO IN WHICH THE SHEAR COMPLETELY OVERWHELMS THE
SYSTEM, LEADING TO DISSIPATION OVER WATER, IS ALSO POSSIBLE.
SUBSEQUENT FORECASTS MAY LEAN TOWARD THE ALTERNATE SCENARIOS, WHICH
WOULD CHANGE THE TEMPORAL EXTENT AND TRACK OF THE FORECAST. FOR
NOW, THE JTWC TRACK MAINTAINS A MORE STRAIGHTFORWARD EASTWARD
PROGRESSION, WHICH ALIGNS WITH NAVGEM AND HAFS-A. THE INTENSITY
AIDS GENERALLY DEPICT A STEADY INTENSITY TREND WHILE THE
CIRCULATION REMAINS OVER WATER. THE JTWC CONCURS WITH THIS
PHILOSOPHY, HOLDING THE INTENSITY AT 35 KTS BEFORE DISSIPATING THE
SYSTEM OVER LAND BY TAU 72.

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