WDPN31 PGTW 210300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (FOUR) WARNING NR 
006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.0N 111.3E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 429 NM SOUTHEAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 15 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
REANALYSIS OF ANIMATED ENHANCED INFRARED (EIR) AND MICROWAVE
IMAGERY, ALONG WITH SURFACE OBSERVATIONS FROM XISHA DAO AND SANSHU
DAO, ALONG WITH ANIMATED RADAR DATA, HAS RESULTED IN A
REPOSITIONING OF THE LOW LEVEL CIRCULATION CENTER (LLCC) ASSOCIATED
WITH TROPICAL STORM (TS) 04W (FOUR), AND MORE SPECIFICALLY, AN
ADJUSTMENT TO THE TRACK MOTION. OVER THE PAST 12 HOURS, THE LLCC
HAS TRACKED WEST-SOUTHWESTWARD TO ITS CURRENT POSITION, WHILE
DRAMATICALLY SLOWING DOWN, NOW BECOMING NEARLY STATIONARY. CURRENT
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) AND A 202259Z 91GHZ
SSMIS IMAGE SHOW AN ARC OF DEEP CONVECTION ACROSS THE SOUTHERN
HEMISPHERE OF THE CIRCULATION, WITH A DEVELOPING CONVECTIVE BURST
DEVELOPING VERY NEAR THE LLCC POSITION AS THE SUN HAS COME UP. THE
INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE
TIGHT GROUPING OF AGENCY FIX POSITIONS, RADAR DATA FROM VIETNAM,
AND THE AFOREMENTIONED MICROWAVE IMAGE. ADDITIONALLY, THE SURFACE
WINDS FROM SANSHU DAO HAVE SHIFTED TO SOUTHEASTERLY BY 2100Z AND
REMAIN SOUTHEASTERLY AT 0000Z, SUGGESTING THE LLCC MUST BE TO THE
SOUTHWEST OF THE STATION, PROVIDING ADDITIONAL SUPPORT TO THE HIGH
CONFIDENCE POSITIONING. THE INITIAL INTENSITY IS ASSESSED WITH
MEDIUM CONFIDENCE, HEDGED SLIGHTLY HIGHER THAN THE BULK OF THE
AGENCY FIXES FOR PERSISTENCE. BEST TRACK MEAN SEA LEVEL PRESSURE
FOR THE 1200Z AND 1800Z ANALYZES WAS ADJUSTED UPWARDS BY 7MB AND
NOW BETTER REFLECTS THE SURFACE OBSERVATIONS. THE SYSTEM LIES IN A
GENERALLY FAVORABLE ENVIRONMENT WITH WARM SSTS, MODERATE POLEWARD
OUTFLOW OFFSET BY MODERATE NORTHEASTERLY VWS. THE STEERING PATTERN
IS WEAK, WITH A RIDGE TO THE NORTH AND SOUTH, TRAPPING THE SYSTEM
BETWEEN THEM, IN A EAST-WEST ORIENTED TROUGH EXTENDING TO TS 05W IN
THE PHILIPPINE SEA, RESULTING IN THE ERRATIC TRACK MOTION OVER THE
LAST 12-24 HOURS.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: WEAK STEERING PATTERN. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.0 - 30 KTS
   CIMSS SATCON: 40 KTS AT 201800Z
   CIMSS ADT: 31 KTS AT 210000Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE PREVIOUS 12 HOURS OF TRACK HAVE
BEEN SIGNIFICANTLY ADJUSTED, BOTH TURNING THE TRACK
WEST-SOUTHWESTWARD AND SLOWING CONSIDERABLY. 

FORECAST DISCUSSION: WHILE THE CHANGES TO THE TRACK MOTION WOULD
SUGGEST A MAJOR SHIFT IN THE TRACK FORECAST, BOTH THE GLOBAL MODEL
FIELDS AND THE TRACK CONSENSUS GUIDANCE INDICATE A NEAR-TERM SHIFT,
WITH THE SYSTEM RESUMING A TRACK TOWARDS HAINAN ISLAND. FROM A
MODEL PERSPECTIVE, THE DEEP MONSOONAL SOUTHWESTERLIES OVER THE
SOUTH CHINA SEA ARE EXPECTED TO STRENGTHEN, WHILE THE DEEP RIDGING
OVER CHINA MOVES EASTWARD WHILE AT THE SAME TIME BUILDING. THE
COMBINATION OF EVENTS WILL STRENGTHEN THE STEERING GRADIENT ON THE
EAST SIDE OF THE SYSTEM, PUSHING IT BACK TO THE NORTH. IN THE
IMMEDIATE FUTURE (NEXT 6-12 HOURS), TS 04W WILL LIKELY PERFORM A
LOOP JUST WEST OF SANSHU DAO, BEFORE RESUMING A NORTHWESTWARD
TRACK. AFTER TAU 12, TRACK SPEEDS WILL INCREASE, RESULTING IN AN
ANTICIPATED LANDFALL ON THE SOUTHEAST COAST OF HAINAN AROUND TAU
24. SHEAR IS ANTICIPATED TO SLACKEN A BIT THROUGH TAU 24, WHICH
WILL ALLOW THE SYSTEM TO BRIEFLY INTENSIFY UP TO 45 KNOTS PRIOR TO
MAKING LANDFALL. AFTER CROSSING HAINAN, THE SYSTEM WILL MOVE INTO
THE GULF OF TONKIN AS A MINIMAL TROPICAL STORM, HAVING SUCCUMBED TO
INCREASING SHEAR AND TERRAIN INTERACTION. ONCE IN THE GULF OF
TONKIN, SHEAR IS EXPECTED TO SHARPLY INCREASE WHICH WILL TEAR THE
SYSTEM APART PRIOR TO MAKING A SECOND LANDFALL EAST OF HANOI,
VIETNAM BY TAU 72. TRACK SPEEDS WILL QUICKLY SLOW AS THE VORTEX
SHALLOWS OUT AND IS BLOCKED BY A DEEP RIDGE DEVELOPING OVER CENTRAL
CHINA. BY THE END OF THE FORECAST, THE VORTEX SHOULD START TURNING
BACK OUT OVER WATER, BUT AT THIS POINT WILL BE BELOW TROPICAL
DEPRESSION STRENGTH. THE REMNANT VORTEX MAY MOVE BACK OVER WATER
BUT WILL BE ABSORBED INTO A TAILING TROUGH EXTENDING FROM TS 05W AT
THAT TIME.  

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN VERY GOOD
AGREEMENT, EVEN IN LIGHT OF THE RECENT TRACK CHANGES. ALL OF THE
AVAILABLE GUIDANCE IS CONFINED IN A TIGHT ENVELOPE OF JUST 55NM AT
LANDFALL, OPENING UP TO 155NM AT TAU 72. THE JTWC FORECAST TRACK
CLOSELY TRACKS THE CONSENSUS MEAN, WHICH LIES IN THE CENTER OF THE
NARROW GUIDANCE ENVELOPE. CONFIDENCE IS MEDIUM HOWEVER, DUE TO THE
UNCERTAINTY IN THE NEAR-TERM TRACK MOTION, CHANGES TO WHICH COULD
RESULT IN MAJOR CHANGES IN BOTH TIMING AND LOCATION OF LANDFALL AND
SUBSEQUENT TRACK. INTENSITY GUIDANCE IS IN GOOD AGREEMENT AS WELL,
WITH THE BULK OF THE GUIDANCE SHOWING A PEAK BETWEEN 40-45 KNOTS
AND DISSIPATING TO BELOW WARNING CRITERIA BETWEEN TAU 72 AND TAU
96. 

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM//
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