WDXS31 PGTW 240900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 13S (EMNATI) WARNING 
NR 017//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 27.0S 43.0E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 326 NM SOUTH-SOUTHEAST OF EUROPA ISLAND
   MOVEMENT PAST 6 HOURS: SOUTHWARD AT 18 KTS
   SIGNIFICANT WAVE HEIGHT: 18 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS AN
ILL-DEFINED AND BROAD CIRCULATION SOUTHWEST OF MADAGASCAR, WITH AT
LEAST THREE DISTINCT VORTICES ROTATING ABOUT THE LARGER CENTER,
WHICH IS DEVOID OF ANY SIGNIFICANT ORGANIZED CONVECTIVE ACTIVITY.
THE INITIAL POSITION IS ASSESSED WITH MEDIUM CONFIDENCE, PLACED IN
THE CENTROID OF THE TRIO OF VORTICES, AS THERE IS NO CLEAR-CUT
CENTER PER SE. THE INITIAL INTENSITY OF 45 KNOTS IS ASSESSED WITH
HIGH CONFIDENCE, WELL ABOVE THE PGTW AND ADT DVORAK ESTIMATES,
BASED ON A 240236Z SMAP PASS WHICH INDICATED AN ARC OF 45-50 KNOT
WINDS EXTENDING ACROSS MOST OF THE SOUTHERN HEMISPHERE OF THE
CIRCULATION, WITH A RADIUS OF MAXIMUM WINDS OF ROUGHLY 80NM. WHILE
WATER VAPOR IMAGERY SUGGESTS THE SYSTEM REMAINS RELATIVELY ISOLATED
FROM THE DRY AIR ENSCONCED OVER THE MOZAMBIQUE CHANNEL, TOTAL
PRECIPITABLE WATER (TPW) DATA INDICATES A NARROW SWATH OF VERY DRY
AIR IS BEING ENTRAINED FROM MADAGASCAR DIRECTLY INTO THE CORE OF
THE SYSTEM. POLEWARD OUTFLOW REMAINS ROBUST, AND SHEAR RELATIVELY
LOW, BUT THE LACK OF ANY SIGNIFICANT OCEAN HEAT CONTENT IS
INHIBITING INTENSIFICATION.

INITIAL WIND RADII BASIS: PERSISTENCE WITH EARLIER SMAP DATA.  

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED FAR TO THE EAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   CIMSS ADT: 33 KTS AT 240315Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY
UNFAVORABLE
   VWS: 15-20 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: LOW OCEAN HEAT CONTENT AND DRY AIR ENTRAINMENT. 

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TC 13S IS FORECAST TO CONTINUE TRACKING SOUTH
TO SOUTHEASTWARD ALONG THE WESTERN PERIPHERY OF THE DEEP STR TO THE
EAST THROUGH THE FORECAST PERIOD. AFTER THE END OF THE FORECAST
PERIOD, THE REMAINING SUBTROPICAL LOW WILL LIKELY TURN POLEWARD
ONCE MORE AS IT MOVES TOWARDS A WEAKNESS THAT DEVELOPS IN THE STR
AHEAD OF AN APPROACHING MID-LATITUDE TROUGH. OVERALL, NO
SIGNIFICANT CHANGE IN THE INTENSITY IS FORECAST THROUGH THE
FORECAST PERIOD, WITH THE CONTINUED ROBUST POLEWARD OUTFLOW BEING
OFFSET BY VERY LOW OHC, DECREASING SEA SURFACE TEMPERATURES,
PROGRESSIVELY INCREASING VERTICAL WIND SHEAR AND MORE SIGNIFICANT
DRY AIR ENTRAINMENT. THE SYSTEM WILL BEGIN LOSING ITS TROPICAL
CHARACTERISTICS BY TAU 12 AND IS FORECAST TO COMPLETE TRANSITION TO
A STRONG GALE-FORCE SUBTROPICAL LOW NO LATER THAN TAU 36. 

MODEL DISCUSSION: DETERMINISTIC AND ENSEMBLE TRACK GUIDANCE IS IN
TIGHT AGREEMENT ON THE FORECAST TRACK, WITH VERY LITTLE SPREAD
THROUGH THE PERIOD OF THE FORECAST, LENDING HIGH CONFIDENCE TO THE
JTWC TRACK. INTENSITY GUIDANCE IS MIXED, WITH THE HWRF AND
COAMPS-TC SUGGESTING SOME MINOR INTENSIFICATION TO ABOUT 50 KNOTS
BY TAU 24, WHILE THE REMAINDER OF THE GUIDANCE INDICATES STEADY OR
WEAKENING INTENSITY FROM TAU 00. THE JTWC FORECAST LIES SLIGHTLY
BELOW THE INTENSITY CONSENSUS MEAN WITH HIGH CONFIDENCE. 

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