WDXS32 PGTW 090900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 13S (DINGANI) WARNING 
NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.9S 89.1E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 1137 NM EAST-SOUTHEAST OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A PARTIALLY
EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC) WITH THE PRIMARY
VERTICAL HOT TOWERS TO THE SOUTH OF THE LLCC. A 090348Z ASCAT-B
SCATTEROMETRY PASS REVEALS A HIGHLY ASYMMETRIC WINDFIELD WITH
35-40KT WINDS IN THE SOUTHERN SEMICIRCLE WHILE 5-15KT WINDS
DOMINATE THE NORTHERN SEMICIRCLE. THE INITIAL POSITION IS PLACED
WITH HIGH CONFIDENCE BASED ON A PARTIALLY EXPOSED LLCC IN MSI. THE
INITIAL INTENSITY OF 40 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED
ON RECENT BULLSEYE ASCAT-B PASS, FURTHER REINFORCED BY AGENCY
DVORAK INTENSITY ESTIMATES SHOWING T2.5-T3.0.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) CENTERED TO THE
SOUTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T3.0 - 45 KTS
   DEMS: T2.5 - 35 KTS
   FMEE: T3.0 - 45 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: MODERATE POLEWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: DESPITE A LACKLUSTER PERFORMANCE OVER THE PAST
SEVERAL DAYS, TC 13S (DINGANI) HAS BEGUN ITS INTENSIFICATION PHASE.
NOW IN A FAVORABLE ENVIRONMENT CHARACTERIZED BY LOW VERTICAL WIND
SHEAR (VWS), WARM SEA SURFACE TEMPERATURES (SST) AND IMPROVING
UPPER-LEVEL OUTFLOW, TC 13S WILL STEADILY INCREASE IN INTENSITY.
THROUGH TAU 48, AS THE SYSTEM TRACKS GENERALLY WESTWARD UNDER THE
STEERING INFLUENCE OF THE STR TO THE SOUTHEAST, 13S WILL INTENSIFY
TO A PEAK INTENSITY OF 60KTS AND POSSIBLY HIGHER. AFTER TAU 48 THE
COMBINED EFFECTS OF TC 13S AND TC 11S TO ITS EAST, REORIENT AND
BREAKDOWN THE STR, RESULTING IN A NEW SOUTHWESTERLY AND EVENTUALLY
SOUTH-SOUTHWESTERLY TRACK SHIFT. THROUGH TAU 96 AS THE SYSTEM
PROGRESSES FURTHER POLEWARD TOWARDS THE 20TH PARALLEL, THE SYSTEMS
INTENSIFICATION BECOMES INCREASINGLY OFFSET BY DRY AIR ENTRAINMENT,
COOLING SSTS AND INCREASING VWS. BY TAU 120, TRACK SPEED WILL BEGIN
TO FALL AS THE SYSTEM ENTERS A COMPLEX STEERING ENVIRONMENT AND THE
COMBINED EFFECTS OF INCREASED VWS AND DRY AIR ENTRAINMENT WILL
CONSPIRE TO WEAKEN 13S THROUGH THE FORECAST PERIOD. 

MODEL DISCUSSION: NUMERICAL MODELS ARE IN GOOD AGREEMENT THAT 13S
WILL TRACK GENERALLY WESTWARD THROUGH TAU 48, AFTER WHICH THE SYSTEM
TRENDS GENERALLY SOUTHWESTWARD THROUGH THE FORECAST PERIOD. FOR
THIS REASON, THE INITIAL FORECAST TRACK IS PLACED WITH HIGH
CONFIDENCE AND THE EXTENDED TRACK IS PLACED WITH MEDIUM CONFIDENCE.
RELIABLE MODEL INTENSITY GUIDANCE IS IN FAIR AGREEMENT WITH A BULK
OF THE GUIDANCE INDICATING INTENSIFICATION THROUGH TAU 48,
AFTER WHICH A GENERAL STAGNATION PHASE UNTIL EVENTUAL WEAKENING NEAR
TAU 100 AND THROUGH THE FORECAST PERIOD. FOR THESE REASONS THE JTWC
FORECAST INTENSITY IS PLACED WITH MEDIUM CONFIDENCE. 

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