WDPN31 PGTW 221500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (PRAPIROON) WARNING 
NR 012//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.5N 108.2E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 132 NM EAST-SOUTHEAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS
TROPICAL STORM 04W (PRAPIROON) AS HAVING PERSISTENT DEEP BURSTS OF
CONVECTION OF THE CENTER BUT IS QUICKLY ENTERING A HIGHER SHEAR
ENVIRONMENT. THE PEAK INTENSITY OF 04W SEEMS TO HAVE OCCURRED RIGHT
AROUND 221200Z. CONVECTION NOW APPEARS TO BE SHEARING OFF TO THE
WEST AS OF 221300Z. ENVIRONMENTAL ANALYSIS INDICATES THAT 04W IS
CURRENTLY IN A MARGINAL ENVIRONMENT FOR INTENSIFICATION CHARACTERIZED 
BY MODERATE EQUATORWARD OUTFLOW ALOFT AND VERY WARM (30-31 C) SEA 
SURFACE TEMPERATURES OFFSET BY INCREASING EASTERLY VERTICAL WIND 
SHEAR AND TERRAIN INTERACTION. THE INITIAL POSITION IS PLACED WITH 
HIGH CONFIDENCE BASED ON A 221045Z SSMIS 91 GHZ SSMIS MICROWAVE IMAGE 
SHOWING A MICROWAVE EYE FEATURE. THE INITIAL INTENSITY OF 60 KTS IS 
ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE OBJECTIVE AND SUBJECTIVE 
INTENSITY ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   RJTD: T3.5 - 55 KTS
   RCTP: T3.0 - 45 KTS
   KNES: T3.5 - 55 KTS
   CIMSS SATCON: 75 KTS AT 221200Z
   CIMSS DMINT: 57 KTS AT 221045Z
   CIMSS DPRINT: 60 KTS AT 221300Z
   

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TS 04W IS FORECAST TO CONTINUE TRACKING ALONG
THE SOUTHWESTERN PERIPHERY OF THE STR FOR THE REMAINDER OF THE
FORECAST PERIOD. LANDFALL IS FORECAST TO OCCUR NEAR TAU 12 ABOUT
100 NM EAST OF HANOI, VIETNAM. AFTER MAKING LANDFALL, 04W IS
EXPECTED TO QUICKLY WEAKEN, AND AROUND TAU 36, MAKE A TURN BACK
TOWARDS THE GULF OF TONKIN AS A DISSIPATING VORTEX. DISSIPATION IS
FORECASTED TO OCCUR AROUND TAU 48 OVER NORTHERN VIETNAM. THE
CURRENT INTENSITY IS FORECAST TO NEARLY MAINTAIN FOR THE NEXT 12
HOURS AS THE VERY WARM AND HIGH OHC IN THE AREA PROVIDE AMPLE
ENERGY TO THE SYSTEM BUT IF SHEAR PERSISTS OR WORSENS A MORE RAPID
WEAKENING COULD ENSUE.  

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT FOR
THE SHORT TERM, WITH THE EXCEPTION OF GALWEM, WHICH TRACKS THE
SYSTEM NORTHEASTWARD VICE NORTHWESTWARD. AT TAU 24, THERE IS A 70
NM CROSS-TRACK SPREAD WHICH QUICKLY WIDENS THOUGH. MANY MODELS
BECOME ERRATIC AFTERWARDS AS THE VORTEX WEAKENS, BUT MOST CALL FOR
A WESTWARD TURN AND THEN A DRIFT BACK TOWARDS THE GULF OF TONKIN
UNDER THE EFFECTS OF THE LOW-LEVEL STEERING. AS A RESULT, THE JTWC
TRACK FORECAST IS PLACED WITH MEDIUM CONFIDENCE. THE INTENSITY
GUIDANCE IS ALSO IN GOOD AGREEMENT, HAVING 04W BEGIN A WEAKENING
TREND TO A SIMILAR DEGREE AS TO THE JTWC FORECAST AFTER TAU 12.
SOME GUIDANCE SUGGESTS A MORE RAPID WEAKENING THAN THE JTWC
FORECAST, BUT THE WARM WATERS OF THE GULF OF TONKIN ARE EXPECTED TO
KEEP THE SYSTEM FROM WEAKENING MORE QUICKLY BEFORE LANDFALL.

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