WDXS32 PGTW 082100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 25S (TWENTYFIVE) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 14.5S 45.6E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 285 NM EAST OF NACALA, MOZAMBIQUE
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
CYCLONE 25S WITH PROMINENT CONVECTIVE BANDING ALONG THE WESTERN
PORTION OF A QUICKLY CONSOLIDATING LOW-LEVEL CIRCULATION CENTER
(LLCC). AFTER MAKING LANDFALL IN NORTHERN MADAGASCAR, THE SYSTEM
HAS RAPIDLY IMPROVED IN APPEARANCE AS THE WARM WATERS WITHIN THE
MOZAMBIQUE CHANNEL AID IN DEVELOPMENT. ENVIRONMENTAL ANALYSIS
INDICATES THAT 25S IS IN A HIGHLY FAVORABLE ENVIRONMENT
CHARACTERIZED BY STRONG POLEWARD AND EQUATORWARD OUTFLOW, LOW (5-10 
KTS) VERTICAL WIND SHEAR, AND WARM (29-30 C) SEA SURFACE
TEMPERATURES. THE INITIAL POSITION IS PLACED WITH MEDIUM CONFIDENCE
BASED ON THE ANIMATED EIR IMAGERY. THE INITIAL INTENSITY OF 35 KTS
IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY DVORAK FIXES
ALL AGREEING ON A T2.5 ESTIMATE.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: NORTHERN PERIPHERY OF A SUBTROPICAL
RIDGE (STR) POSITIONED TO THE SOUTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   FMEE: T2.5 - 35 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD AND EQUATORWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TC 25S IS FORECAST TO CONTINUE TRACKING
WESTWARD, ALONG THE NORTHERN PERIPHERY OF THE STR THROUGH TAU 48.
LANDFALL IS CURRENTLY FORECAST TO OCCUR AROUND TAU 36, ABOUT 30 NM
SOUTH OF NACALA, MOZAMBIQUE. AFTER TAU 48, THE RIDGE TO THE SOUTH
WILL WEAKEN AND A NEAR EQUATORIAL RIDGE (NER) WILL BUILD IN AND
TAKE OVER AS THE PRIMARY STEERING MECHANISM. THE SYSTEM WILL RIDE
ALONG THE SOUTHWESTERN PERIPHERY OF THE RIDGE AND ACCELERATE TOWARD
SOUTHERN MADAGASCAR. REGARDING INTENSITY, TC 25S IS FORECAST TO
RAPIDLY INTENSIFY THROUGH TAU 36, TO A PEAK OF AROUND 80 KTS, AS
THE ENVIRONMENT REMAINS HIGHLY CONDUCIVE (VERY WARM SST, LOW SHEAR,
AND STRONG OUTFLOW). ONCE THE SYSTEM MAKES LANDFALL, TERRAIN
INTERACTION WILL CAUSE THE VORTEX TO QUICKLY ERODE. THERE IS A
POSSIBILITY THAT 25S WILL WEAKEN BELOW THE 35 KNOT THRESHOLD
BETWEEN TAUS 48 AND 72. HOWEVER, ONCE THE VORTEX RE-EMERGES WITHIN
THE MOZAMBIQUE CHANNEL, A REINTENSIFICATION PHASE WILL ENSUE.

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN MODERATE AGREEMENT
REGARDING THE TRACK OF 25S. NEAR THE TIME OF LANDFALL, THERE IS A
67 NM CROSS-TRACK SPREAD BETWEEN MODELS. ECMWF AND ITS ENSEMBLE
MEAN ARE THE NORTHERNMOST MEMBERS WHILE NAVGEM IS THE SOUTHERNMOST
MEMBER. CROSS-TRACK SPREAD THEN OPENS UP TO 110 NM AT TAU 72, WITH
ECMWF TAKING THE WIDEST TURN, AND GFS THE SHARPEST TURN, AS THE
STEERING PATTERN TRANSITIONS. AFTER TAU 72, ALONG-TRACK SPREAD
INCREASES DUE TO DIFFERING AMOUNTS OF LAND INTERACTION. THE JTWC
TRACK FORECAST IS PLACED WITH MEDIUM CONFIDENCE THROUGH TAU 72 AND
LOW CONFIDENCE THEREAFTER. INTENSITY GUIDANCE IS IN GOOD AGREEMENT
THROUGH TAU 36, WITH PEAK INTENSITIES RANGING FROM 70 TO 80 KTS.
HOWEVER, GUIDANCE DIVERGES AFTERWARD DUE TO THE VARYING TIMELINES
FOR LAND INTERACTION. AS A RESULT, THE JTWC INTENSITY FORECAST IS
ALSO PLACED WITH MEDIUM CONFIDENCE THROUGH TAU 72 AND LOW
CONFIDENCE AFTERWARD.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   TRACK 72-120 HR: LOW
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: LOW//
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