WDPN31 PGTW 212100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (PRAPIROON) WARNING 
NR 009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.8N 110.2E
   INITIAL INTENSITY: 50 KTS
   GEOGRAPHIC REFERENCE: 276 NM EAST-SOUTHEAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY, ANIMATED RADAR
DATA AND SURFACE OBSERVATIONS INDICATE THAT TROPICAL STORM (TS) 04W
(PRAPIROON) MADE LANDFALL BETWEEN LINGSHUI AND WANNING, CHINA
BETWEEN 1600Z AND 1700Z. THE EIR DEPICTS A CENTRAL DENSE OVERCAST
(CDO) FEATURE OBSCURING THE LOW LEVEL CIRCULATION CENTER (LLCC).
HOWEVER, AN EARLIER 211040Z SAR PASS REVEALED A WELL-DEFINED CENTER
UNDER THE HOOD, AND A SERIES OF MICROWAVE IMAGES INCLUDING A
HIGH-RESOLUTION GMI COLOR-ENHANCED 37GHZ IMAGE AT 211558Z DEPICTED
A NASCENT MICROWAVE EYE WITH STRONG CONVECTIVE BANDING FEATURES
ACROSS THE SOUTHERN HEMISPHERE, AS THE SYSTEM APPROACHED HAINAN.
TAKEN TOGETHER, THE DATA ABOVE PROVIDED HIGH CONFIDENCE TO THE
INITIAL POSITION. THE INITIAL INTENSITY IS INCREASED TO 50 KNOTS
FOR THE 1800Z ANALYSIS, THOUGH A PEAK OF 55 KNOTS JUST PRIOR TO
LANDFALL IS POSSIBLE BASED ON THE MICROWAVE EYE AND RADAR
DEPICTION. NOW OVER LAND, THE SYSTEM WILL START TO WEAKEN DUE TO
TERRAIN INTERACTIONS, BUT OTHERWISE THE ENVIRONMENT REMAINS
FAVORABLE WITH LOW NORTHEASTERLY VWS AND GOOD EQUATORWARD OUTFLOW.

INITIAL WIND RADII BASIS: SAR DATA 

CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) TO THE NORTHEAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: NO CURRENT ESTIMATE
   RJTD: T2.5 - 35 KTS
   RCTP: T2.5 - 35 KTS
   CIMSS SATCON: 43 KTS AT 211730Z
   CIMSS ADT: 51 KTS AT 211730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD
   OTHER FACTORS: TERRAIN INTERACTION OVER HAINAN

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: TS 04W WILL CONTINUE TO TRACK RAPIDLY ACROSS
THE ISLAND OF HAINAN AND MOVE INTO THE GULF OF TONKIN WITHIN THE
NEXT 12 HOURS. AS THE SYSTEM APPROACHES THE COAST OF SOUTHWESTERN
CHINA, THE STEERING GRADIENT IS EXPECTED TO LOOSEN, WHICH WILL
RESULT IN THE SYSTEM SLOWING DOWN FAIRLY DRAMATICALLY AFTER TAU 24.
THIS IS A GOOD THING, SINCE THE OHC IN THE GULF OF TONKIN IS
EXTREMELY HIGH, SO LIMITING THE TIME OVER THIS SOURCE OF ENERGY
WILL KEEP A LID ON POTENTIAL REINTENSIFICATION. THE OTHER FACTOR
THAT WILL LIMIT THE INTENSIFICATION POTENTIAL IN THE GULF OF TONKIN
WILL BE THE FORECASTED SHARP INCREASE IN NORTHEASTERLY SHEAR,
EXPECTED TO KICK IN BY TAU 24. HOWEVER, SOME OF THE MODEL GUIDANCE,
PARTICULARLY THE HAFS-A DEPICT A SHARP INTENSIFICATION AS THE
SYSTEM TRACKS MORE SLOWLY ACROSS THE GULF OF TONKIN. THE STRONGER
THE SYSTEM IS WHEN IT REEMERGES OVER WATER, THE FASTER IT WILL MOVE
ACROSS THE GULF AND HENCE THE LOWER THE INTENSITY WILL BE. THE
FORECAST CALLS FOR THE SYSTEM TO COME OFF NORTHWEST HAINAN AT 40
KNOTS, MAINTAIN THAT INTENSITY FOR 12 HOURS THEN WEAKEN AFTER
MAKING LANDFALL IN FAR SOUTHWESTERN CHINA BEFORE TAU 36. ONCE OVER
LAND, THE COMBINATION OF FRICTIONAL EFFECTS OF THE LAND, AND THE
STRONG NORTHEASTERLY SHEAR, WILL RESULT IN RAPID WEAKENING THROUGH
THE END OF THE FORECAST, WITH DISSIPATION OVER THE CHINA-VIETNAM
BORDER REGION NO LATER THAN TAU 72. THE SYSTEM IS EXPECTED TO TURN
BACK TOWARDS THE SOUTHWEST AS IT DISSIPATES UNDER THE INFLUENCE OF
THE LOW-LEVEL NORTHERLY FLOW, AND COULD MOVE BACK OVER WATER AFTER
THE END OF THE FORECAST PERIOD. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN GOOD AGREEMENT
THROUGH TAU 36, BUT THEREAFTER SPREAD OUT DRASTICALLY, WITH ROUGHLY
HALF OF THE CONSENSUS TAKING THE SYSTEM EAST AND THE OTHER HALF
TAKING IT WEST-SOUTHWEST AS IT DISSIPATES. THE JTWC FORECAST TRACKS
THE MEAN, THEN FOLLOWS THE WESTERN GROUPING OF MODEL TRACKERS.
CONFIDENCE IS HIGH TO TAU 36, THEN RAPIDLY SHIFTS TO LOW AS THE
VORTEX DISSIPATES. INTENSITY GUIDANCE IS SPREAD IN THE NEAR-TERM,
WITH HWRF AND CTCX DEPICTING INTENSIFICATION OVER HAINAN, WHILE THE
REMAINDER OF THE GUIDANCE MAKES A LOT MORE SENSE IN SHOWING A SHARP
WEAKENING OVER LAND, THEN STEADY FOR A WHILE, THEN FALLING OFF
RAPIDLY AFTER LANDFALL. THE JTWC FORECAST TRACKS THE MEAN THROUGH
THE MAJORITY OF THE FORECAST WITH MEDIUM CONFIDENCE. 

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