WDXS31 PGTW 202100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 13S (EMNATI) WARNING NR 
010//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.6S 54.6E
   INITIAL INTENSITY: 110 KTS
   GEOGRAPHIC REFERENCE: 178 NM NORTH-NORTHWEST OF ST DENIS
   MOVEMENT PAST 6 HOURS: WESTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 32 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
WHILE UNDERGOING AN EYEWALL REPLACEMENT CYCLE (ERC), ANIMATED
ENHANCED INFRARED (EIR) SATELLITE IMAGERY INDICATES THE SYSTEM HAS
TRACKED ERRATICALLY WESTWARD WITH A TROCHOIDAL MOTION CLEARLY
EVIDENT OVER THE PAST NINE HOURS. AFTER COMPLETING A QUICK LOOP
EARLIER, TC 13S HAS STARTED ANOTHER LOOP AROUND 202000Z. THE CORE
CONVECTIVE STRUCTURE CONTINUES TO FLUCTUATE RAPIDLY WITH A SLIGHT
WEAKENING TREND AND WARMING CLOUD TOP TEMPERATURES EVIDENT IN EIR
IMAGERY THROUGH ABOUT 201700Z. AFTER 201700Z, THE EYEWALL BEGAN TO
RESTRENGTHEN WITH COOLING CLOUD TOP TEMPERATURES AND IMPROVED
STRUCTURE EVIDENT. CONCENTRIC EYEWALLS ARE BEST SEEN IN A 201434Z
SSMIS 91GHZ COLOR COMPOSITE MICROWAVE IMAGE SHOWING A VERY SMALL
INNER EYE WALL SURROUNDED BY A MOAT AND OUTER EYEWALL. THE INITIAL
POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON A 17NM EYE IN EIR
IMAGERY. A FORTUITOUS 201440Z RADARSAT2 SAR IMAGE WAS USED TO
REFINE THE INITIAL 34-, 50- AND 64-KNOT WIND RADIUSES BUT SHOWED
MAXIMUM SUSTAINED WINDS (3KM) OF ONLY 90-92 KNOTS IN THE NORTHWESTERN
SECTOR. THIS APPEARS TO BE TOO LOW SINCE ERC TYPICALLY BEGINS WITH
INTENSITIES NEAR 120 KNOTS. THUS, THE INITIAL INTENSITY OF 110 KTS
IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE DVORAK CURRENT
INTENSITY ESTIMATES RANGING FROM 102-115 KNOTS.

INITIAL WIND RADII BASIS: DETAILED ANALYSIS OF 201440Z SAR IMAGE.
ALSO SUPPORTED BY 201821Z ASCAT-B SCATTEROMETER IMAGE. 

CURRENT STEERING MECHANISM: NORTHWEST PERIPHERY OF A DEEP-LAYERED
SUBTROPICAL RIDGE (STR) CENTERED TO THE SOUTHEAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.0 - 90 KTS / CI 6.0 - 115 KTS
   FMEE: T5.5 - 102 KTS / CI 5.5 - 102 KTS
   FIMP: T5.5 - 102 KTS / CI 5.5 - 102 KTS
   CIMSS SATCON: 117 KTS AT 201622Z
   CIMSS ADT: 119 KTS AT 201815Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG POLEWARD AND EQUATORWARD
   OTHER FACTORS: UPWELLING OF COOLER WATERS

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: THE JTWC FORECAST TRACK REMAINS HIGHLY
CONSISTENT WITH THE PREVIOUS FORECAST TRACKS. TC 13S IS TRACKING
ALONG THE NORTHWEST PERIPHERY OF A STR POSITIONED TO THE SOUTHEAST
AND EAST, AND IS FORECAST TO TRACK WEST-SOUTHWESTWARD TO SOUTHWESTWARD 
THROUGH TAU 48. THERE IS SIGNIFICANT UNCERTAINTY IN THE INTENSITY 
FORECAST OVER THE NEXT 12 HOURS CONSIDERING THE ONGOING ERC EVENT AND 
POSSIBLE UPWELLING OF COOLER WATER WITH SLIGHT WEAKENING POSSIBLE 
WITHIN THE NEXT 12 HOURS. AFTER THE ERC COMPLETES NEAR TAU 12, STRONG 
OUTFLOW AND LOW VERTICAL WIND SHEAR (VWS) SHOULD ALLOW THE SYSTEM TO 
REINTENSIFY QUICKLY TO A PEAK OF 120 KNOTS BY TAU 24. AFTER TAU 24, TC 
13S WILL STEADILY WEAKEN WITH INCREASING INTERACTION WITH LAND AS THE 
SYSTEM APPROACHES MADAGASCAR. AFTER MAKING LANDFALL NEAR TAU 48, THE 
SYSTEM WILL TURN SOUTHWESTWARD TOWARD A WEAKNESS IN THE STR PRODUCED 
BY AN APPROACHING DEEP MIDLATITUDE UPPER-LEVEL TROUGH AND WEAKEN 
RAPIDLY AS IT TRACKS OVER THE MOUNTAINOUS TERRAIN OF MADAGASCAR.
AFTER TRACKING BACK OVER WATER NEAR TAU 72, SOUTHWEST OF MADAGASCAR, 
TC 13S WILL COMMENCE SUBTROPICAL TRANSITION AS THE SYSTEM INTERACTS 
WITH AN UPPER-LEVEL SUBTROPICAL TROUGH. BY TAU 120, TC 13S WILL 
COMPLETE SUBTROPICAL TRANSITION AS IT IS BECOMES EMBEDDED IN
THE UPPER LEVEL TROUGH WITH STRONG VWS (30-40 KNOTS) AND COOL SST 
(24C). 

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE SUPPORTS THE JTWC
FORECAST TRACK WITH HIGH CONFIDENCE THROUGH TAU 72. AFTER TAU 72, THE 
TRACK GUIDANCE DIVERGES WITH A SPREAD IN SOLUTIONS RANGING FROM 210NM 
AT TAU 72 TO 400NM AT TAU 120 THUS MEDIUM TRACK CONFIDENCE. THERE IS 
SIGNIFICANT UNCERTAINTY IN THE INITIAL INTENSITY AND THE JTWC 
INTENSITY FORECAST THROUGH TAU 48. HWRF INTENSITY GUIDANCE INDICATES 
REINTENSIFICATION TO A PEAK OF 113 KNOTS WHILE THE MAJORITY OF 
GUIDANCE INDICATES A GRADUAL WEAKENING TREND. AFTER THE PERIOD OF 
RAPID WEAKENING OVER LAND, INTENSITY GUIDANCE INDICATES A LARGE SPREAD 
OF INTENSITY FORECASTS RANGING FROM 20 KNOTS TO 57 KNOTS. ENSEMBLE 
INTENSITY GUIDANCE SHOWS NO POTENTIAL FOR RAPID INTENSIFICATION OVER 
THE NEXT 48 HOURS, HOWEVER, THE GFS ENSEMBLE (201200Z) INDICATES 
MODERATE INTENSIFICATION IS LIKELY WITHIN THE NEXT 24 HOURS. 
ADDITIONALLY, NONE OF THE RAPID INTENSIFICATION GUIDANCE HAS 
TRIGGERED.      

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