WDIO31 PGTW 251500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 04B (FOUR) WARNING NR 
001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 4.7N 99.0E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 872 NM SOUTHWEST OF DA NANG, VIETNAM
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 03 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE 04B WITH A PARTIALLY EXPOSED LOW-LEVEL CIRCULATION CENTER
(LLCC) AND DEEP CONVECTION SHEARED OFF TO THE WEST. THE EXPOSED
EASTERN PORTION OF THE LLCC SHOWS THAT THE LLCC IS WELL-DEFINED AND
HIGHLY COMPACT. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE
BASED ON THE PARTIALLY EXPOSED LLCC IN ANIMATED EIR IMAGERY. THE
INITIAL INTENSITY OF 35 KTS IS ASSESSED WITH MEDIUM CONFIDENCE
BASED ON THE HIGHER SIDE OF THE AGENCY DVORAK FIXES, CLOSER TO THE
CIMSS OBJECTIVE ESTIMATES NOTED BELOW. ENVIRONMENTAL ANALYSIS
INDICATES THAT 04B IS IN A MARGINALLY FAVORABLE ENVIRONMENT
CHARACTERIZED BY WEAK OUTFLOW ALOFT, MODERATE (15-20 KTS) EASTERLY
VERTICAL WIND SHEAR, AND WARM (28-29 C) SEA SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF A SUBTROPICAL
RIDGE POSITIONED TO THE NORTH

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   KNES: T1.5 - 25 KTS
   DEMS: T1.5 - 25 KTS
   CIMSS D-MINT: 43 KTS AT 251157Z
   CIMSS D-PRINT: 38 KTS AT 251200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: WEAK POLEWARD AND EQUATORWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: 04B IS FORECAST TO TRACK GENERALLY WESTWARD,
ALONG THE SOUTHERN PERIPHERY OF THE SUBTROPICAL RIDGE TO THE NORTH,
THROUGH TAU 24. AFTER TAU 24, 04B BECOMES PLACED WITHIN A COMPETING
STEERING ENVIRONMENT AS ENHANCED LOW-LEVEL WESTERLY FLOW BEGINS TO
INTERACT WITH THE VORTEX. THE COMBINATION OF THESE TWO FACTORS IS
EXPECTED TO RESULT IN VERY SLOW AND ERRATIC MOVEMENT THROUGH TAU
48. THE JTWC TRACK FORECAST CALLS FOR A CHECKMARK BACK TO THE EAST
AFTER TAU 24 AS THE VORTEX SHALLOWS OVER THE MOUNTAINOUS TERRAIN OF
NORTHERN SUMATRA AND THE LOW-LEVEL STEERING BECOMES THE PRIMARY
MECHANISM. REGARDING INTENSITY, 04B IS FORECAST TO INTENSIFY OVER
THE NEXT 12 HOURS AS IT REMAINS OVER WATER WITH LOW TO MODERATE
SHEAR. AFTER TAU 12, TERRAIN INTERACTION WILL CAUSE 04B TO WEAKEN.
AS THE VORTEX STALLS OVER NORTHERN SUMATRA, THE VORTEX WILL
CONTINUE TO ERODE OVER LAND WITH DISSIPATION FORECAST TO OCCUR
AROUND TAU 48. 

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN VERY POOR
AGREEMENT THROUGHOUT THE SHORT FORECAST PERIOD. GFS TAKES THE
SYSTEM FURTHER NORTHWARD, KEEPING OVER WATER FOR LONGER. THE
REMAINDER OF GUIDANCE HAS A LARGE SPREAD, BUT GENERALLY AGREES ON A
SHARP EASTWARD TURN AFTER TAU 24. THE JTWC TRACK FORECAST IS PLACED
CLOSE TO THE ECMWF AND GFS ENSEMBLE MEANS WITH VERY LOW CONFIDENCE.
INTENSITY GUIDANCE IS IN BETTER AGREEMENT, WITH ALL MODELS
SUGGESTING WEAKENING FROM TAU 12 ONWARD. THE JTWC INTENSITY FORECAST
IS PLACED SLIGHTLY LOWER THAN THE CONSENSUS THROUGHOUT THE FORECAST
PERIOD WITH MEDIUM CONFIDENCE. 

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