WDXS31 PGTW 200900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 13S (EMNATI) WARNING 
NR 009//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.2S 55.9E
   INITIAL INTENSITY: 105 KTS
   GEOGRAPHIC REFERENCE: 191 NM NORTH-NORTHWEST OF PORT LOUIS,
MAURITIUS
   MOVEMENT PAST 6 HOURS: WESTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 30 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TC 13S RAPIDLY INTENSIFIED FROM 65 KNOTS TO 105 KNOTS THROUGH
200000Z TODAY BUT HAS SINCE PLATEAUED. ANIMATED MULTISPECTRAL
SATELLITE IMAGERY (MSI) SHOWS THAT SINCE THE 0000Z HOUR, TC 13S HAS
MAINTAINED A ROUGHLY 12NM EYE, BUT THE CONVECTIVE SIGNATURE HAS
BEEN CONTINUALLY CYCLING, WITH THE CLOUD TOPS ALTERNATELY WARMING,
THEN COOLING AGAIN, AND ERODING FIRST ON THE EAST, THEN THE WEST
SIDE OF THE EYE. UNFORTUNATELY THERE HAS BEEN NO BULLSEYE MICROWAVE
PASS SINCE 200050Z BUT TWO PARTIAL PASSES FROM 200125Z AND 200200Z
INDICATE THE PRESENCE OF A VERY SMALL CENTRAL CORE OF CONVECTION
SURROUNDING THE MICROWAVE EYE, AND A MUCH LARGER OUTER BAND OF
CONVECTION WRAPPING INTO THE CENTER, INDICATIVE OF A POSSIBLE
EYEWALL REPLACEMENT CYCLE (ERC). THE INITIAL POSITION IS ASSESSED
WITH HIGH CONFIDENCE BASED ON A BLEND OF THE PGTW FIX ON THE
VISIBLE EYE FEATURE AND THE CENTER INDICATED IN A 200551Z ASCAT-B
NORMALIZED RADAR CROSS SECTION (NRCS) IMAGE. BASED ON THE THE NRCS
DATA, THE LOW LEVEL CIRCULATION CENTER (LLCC) IS DISPLACED ABOUT
6NM NORTH OF THE VISIBLE EYE, WHICH ALIGNS WELL WITH THE SOUTHERLY
SHEAR VECTOR INDICATED BY CIMSS UPPER-LEVEL ANALYSIS. DVORAK
CURRENT INTENSITY ESTIMATES SHOW A LARGE SPREAD, BETWEEN T5.0 FROM
FIMP, TO T6.0 FROM PGTW, WITH THE ADT AND SATCON LYING ROUGHLY AT
THE AVERAGE, HOWEVER DATA-T NUMBERS ARE ROUGHLY ONE HALF T-NUMBER
LOWER ACROSS THE BOARD. THE INITIAL INTENSITY ESTIMATE REMAINS
HIGHLY CHALLENGING DUE TO THE RAPIDLY FLUCTUATING STRUCTURE
CHARACTERISTICS. THE INITIAL INTENSITY IS THUS HELD AT 105 KNOTS
WITH MEDIUM CONFIDENCE.

INITIAL WIND RADII BASIS: SCATTEROMETER AND SMAP DATA 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T6.0 - 115 KTS
   FMEE: T5.5 - 102 KTS
   FIMP: T5.0 - 90 KTS
   CIMSS SATCON: 102 KTS AT 200347Z
   CIMSS ADT: 102 KTS AT 200315Z

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: OVER THE PAST FEW HOURS, TC 13S HAS TRACKED
SLIGHTLY MORE WESTWARD THAN PREVIOUSLY EXPECTED WHILE ALSO SLOWING
A FAIR AMOUNT, LIKELY IN RESPONSE TO A SECONDARY STR CENTER
CURRENTLY SOUTH OF MADAGASCAR WHICH WILL QUICKLY SLIDE BY TO THE
SOUTH OF THE SYSTEM AND TAKE UP RESIDENCE TO THE SOUTHEAST, MERGING
WITH THE LARGER STR ALREADY IN PLACE. SO THE CURRENT WESTWARD TRACK
WILL BE RELATIVELY SHORT-LIVED AND TC 13S IS EXPECTED TO RESUME A
GRADUAL TURN TO A SOUTHWESTWARD TRACK OVER THE REMAINDER OF THE
FORECAST AS IT GRACEFULLY ROUNDS THE RIDGE. SOMEWHAT UNEXPECTEDLY,
TC 13S HAS PROVEN EXCEPTIONALLY RESILIENT AND ABLE TO INTENSIFY IN
THE FACE OF UNFAVORABLE UPWELLING AND LOW OHC, LIKELY IN RESPONSE
TO THE ROBUST OUTFLOW ALOFT AND LOW SHEAR. NO SIGNIFICANT CHANGE TO
THE ENVIRONMENT IS ANTICIPATED PRIOR TO LANDFALL, SO ADDITIONAL
INTENSIFICATION IS EXPECTED, TO A PEAK OF 115 KNOTS BY TAU 36. THE
POTENTIAL FOR AT LEAST ONE ERC AND THE POSSIBILITY OF THE LACK OF
OHC CATCHING UP WITH THE SYSTEM, ARE THE ONLY POTENTIAL LIMITING
FACTORS. THE SYSTEM IS EXPECTED TO MAKE LANDFALL ALONG THE
EAST-CENTRAL COAST OF MADAGASCAR AROUND TAU 60, FOLLOWED BY RAPID
WEAKENING OVER THE MOUNTAINOUS TERRAIN. THE SYSTEM WILL TURN MORE
SHARPLY POLEWARD AS IT MOVES BACK OVER WATER TOWARD A WEAKNESS IN
THE RIDGE INDUCED BY AN APPROACHING MID-LATITUDE TROUGH. ONCE BACK
OVER WATER, SOME INTENSIFICATION IS ANTICIPATED, THOUGH THE 26C
ISOTHERM LIES ROUGHLY ALONG THE 25S LATITUDE LINE, AND THE LACK OF
OHC WILL INHIBIT SIGNIFICANT REDEVELOPMENT. THE SYSTEM WILL BEGIN
SUBTROPICAL TRANSITION BY TAU 120 AS IT MOVES OVER 23-25C WATERS
AND BEGINS TO INTERACT WITH A SUBTROPICAL 500MB LOW AND TROUGH. 

MODEL DISCUSSION: TRACK GUIDANCE CONTINUES TO CONSOLIDATE ONTO A
TIGHTER ENVELOPE, WITH THE GFS AND ECMWF OUTLIERS MOVING IN TOWARDS
ONE ANOTHER. THE GUIDANCE DIVERGES AFTER TAU 24, WITH A 180NM
SPREAD AT TAU 72, INCREASING TO 270NM AT TAU 120. THE JTWC FORECAST
REMAINS CONSISTENT WITH THE PREVIOUS FORECAST, WITH ONLY A SLIGHT
ADJUSTMENT IN THE FIRST 24 HOURS DUE TO THE RECENT WESTWARD TRACK
MOTION, AND A SLIGHTLY SHARPER POLEWARD TURN AFTER TAU 96. AS
DISCUSSED, THERE IS CONSIDERABLE UNCERTAINTY IN THE INTENSITY
FORECAST. THE JTWC FORECAST LIES ON THE EXTREME HIGH EDGE OF THE
DETERMINISTIC GUIDANCE PACKAGE, 15-20 KNOTS ABOVE THE INTENSITY
CONSENSUS MEAN (ICNW), CLOSELY TRACKING THE HWRF SOLUTION, WHICH
PEAKS AT 115 KNOTS. ALL OTHER GUIDANCE SUGGESTS NO ADDITIONAL
INTENSIFICATION, FOLLOWED BY RAPID WEAKENING AFTER TAU 24. ENSEMBLE
GUIDANCE INCLUDING COAMPS, ECMWF AND GEFS INDICATE A LOW TO
MODERATE PROBABILITY OF MODERATE INTENSIFICATION, WITH LITTLE TO NO
PROBABILITY OF RAPID INTENSIFICATION AND CONSISTENT WITH THE
PREVIOUS RUN, NONE OF THE RAPID INTENSIFICATION AIDS HAVE BEEN
TRIGGERED.  

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