WDXS31 PGTW 301500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 11S (ELEVEN) WARNING NR 
005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.6S 69.9E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 590 NM SOUTH-SOUTHWEST OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: WEST-SOUTHWESTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 17 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 11S (ELEVEN) IS STRUGGLING TO MAINTAIN
TROPICAL STORM INTENSITY, UNDER THE PERNICIOUS INFLUENCE OF
PERSISTENT EASTERLY SHEAR AND DRY AIR INTRUSION. ANIMATED
MULTISPECTRAL SATELLITE IMAGERY (MSI) SHOWS THAT THE STRONG
CONVECTIVE ACTIVITY THAT WAS PREVIOUSLY OVER THE LOW LEVEL
CIRCULATION CENTER (LLCC) HAS SINCE BEEN BLOWN OFF TO THE WEST BY
20-25 KNOTS OF EASTERLY SHEAR, LEAVING BEHIND A FULLY EXPOSED LLCC.
A 301147Z SSMIS 91GHZ MICROWAVE IMAGE REVEALED THE WELL-DEFINED
NATURE OF THE BANDING FEATURES WRAPPING INTO THE LLCC, ALBEIT
COMPLETELY LIMITED TO THE LOWER-LEVELS OF THE ATMOSPHERE.
ADDITIONALLY, THE MICROWAVE IMAGERY REVEALED A DEEP WEDGE OF DRY
AIR EXTENDING ACROSS THE ENTIRE NORTHERN PERIMETER OF THE
CIRCULATION AND STARTING TO WRAP INTO THE EASTERN PERIMETER. THE
INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE
WELL-DEFINED LLCC IN THE MICROWAVE IMAGERY DISCUSSED ABOVE. THE
INITIAL INTENSITY IS ASSESSED WITH MEDIUM CONFIDENCE, BASED ON A
BLEND OF THE OBJECTIVE AND SUBJECTIVE INTENSITY ESTIMATES OUTLINED
ABOVE. THE ENVIRONMENT IS MARGINAL, WITH WARM SSTS AND MODERATE
POLEWARD AND EQUATORWARD OUTFLOW BEING OFFSET BY STRONG EASTERLY
SHEAR AND DRY AIR INTRUSION.

INITIAL WIND RADII BASIS: PERSISTENCE 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   DEMS: T2.5 - 35 KTS
   FMEE: T2.5 - 35 KTS
   CIMSS SATCON: 40 KTS AT 301100Z
   CIMSS ADT: 37 KTS AT 301200Z
   CIMSS AIDT: 38 KTS AT 301200Z
   CIMSS D-MINT: 29 KTS AT 301150Z
   CIMSS D-PRINT: 30 KTS AT 301200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD
   OTHER FACTORS: DRY MID-LEVEL AIR IN THE EASTERN HEMISPHERE OF
THE CIRCULATION. 

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE TRACK HAS SHIFTED NORTH OF THE
MASCARENE ISLANDS WITH THIS FORECAST. 

FORECAST DISCUSSION: TC 11S WILL TRACK GENERALLY WESTWARD ALONG THE
NORTHWESTERN PERIPHERY OF THE DEEP STR FOR THE NEXT 12 TO 18 HOURS.
A WEAKNESS IN THE RIDGING PATTERN TO THE SOUTH WILL RESULT IN
WEAKENING IN THE STEERING GRADIENT, ALLOWING THE SYSTEM TO SLOW
DOWN A FEW KNOTS THROUGH ABOUT TAU 24. A NEW DEEP-LAYER STR CENTER
MOVES EASTWARD TO A POSITION SOUTHEAST OF THE MASCARENES AFTER TAU
24, WHICH WILL TIGHTEN UP THE GRADIENT, ALLOWING TC 11S TO ONCE AGAIN 
PICK UP SPEED. AFTER TAU 36, AS THE RIDGE MOVES TO THE SOUTH
OF TC 11S, IT WILL TURN ONTO A MORE SOUTHWESTWARD TRACK AND
MAINTAIN THIS MOTION VECTOR THROUGH ABOUT TAU 72. THE SYSTEM IS
FORECAST TO TRACK JUST NORTH OF MAURITIUS AROUND 72, THEN TURN
WESTWARD ONCE AGAIN AS IT PASSES NORTH OF LA REUNION BY TAU 96. IN
TERMS OF INTENSITY, THE SYSTEM WILL CONTINUE TO STRUGGLE AGAINST
THE EASTERLY SHEAR AND DRY AIR, WHICH ARE EXPECTED TO PERSIST FOR
THE NEXT 36 TO 48 HOURS. MOMENTUM CONSERVATION AND BRIEF BURSTS OF
CONVECTION ARE EXPECTED TO LEAD TO A STEADY 35 KNOT INTENSITY
DURING THIS PERIOD, THOUGH THERE IS ABOUT AN EQUAL CHANCE THAT THE
SYSTEM WILL WEAKEN TO LESS THAN 35 KNOTS DURING THIS TIMEFRAME.
BOTH THE GLOBAL AND MESOSCALE MODELS PREDICT THE SHEAR TO DROP OFF
SIGNIFICANTLY AFTER TAU 48, COINCIDENT WITH A SLIGHT INCREASE IN
SSTS AND A GENERAL MOISTENING OF THE ENVIRONMENT. IF THE SYSTEM CAN
MAKE IT THROUGH THE NEXT 48 HOURS, IT IS LIKELY TO REINTENSIFY AS IT
PASSES THE MASCARENES AND HEADS TOWARDS MADAGASCAR, WHERE IT WILL
FIND A MUCH MORE FAVORABLE ENVIRONMENT. 

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE SHOWS A WIDE
DISPERSION, WITH THREE DISTINCT GROUPINGS OF TRACKERS. THE GFS AND
NAVGEM COMPRISE THE NORTHERN GROUP, WITH GFS TAKING THE SYSTEM NEAR
NORTHERN MADAGASCAR BEFORE DIVING SOUTHWARD ALONG THE COAST. THE
MIDDLE GROUP IS THE LARGEST, WITH ECMWF, ECEPS, GALWEM, GEFS AND
THE CONSENSUS MEAN. THE THIRD GROUP CONSISTS OF THE UKMET AND
MOGREPS, TAKING THE SYSTEM SHARPLY POLEWARD AFTER PASSING
MAURITIUS, LEADING TO A MORE THAN 700NM CROSS-TRACK SPREAD BY THE
END OF THE FORECAST. ECEPS AND GEFS ARE IN FAIRLY GOOD AGREEMENT IN
TERMS OF THE MEAN, WITH THE MODEST MEMBER DISPERSION IN BOTH. THE
JTWC FORECAST HAS SHIFTED NORTHWARD SIGNIFICANTLY SINCE THE
PREVIOUS FORECAST AND CLOSELY TRACKS THE ECMWF TRACKER THROUGH THE
ENTIRE FORECAST. CONFIDENCE THROUGH TAU 72 IS MEDIUM, AND LOW
THEREAFTER DUE TO THE EXTREME DISPERSION IN THE GUIDANCE. INTENSITY
GUIDANCE SHOWS A SIMILAR AMOUNT OF UNCERTAINTY, WITH THE SHIPS
GUIDANCE STEADILY INTENSIFYING THE SYSTEM UP TO A PEAK OF 75 KNOTS
IN THE NAVGEM VERSION. THE REMAINDER OF THE GUIDANCE PACKAGE SHOWS
EITHER A FLAT INTENSITY OR A WEAKENING THROUGH ABOUT TAU 72,
FOLLOWED BY INTENSIFICATION THEREAFTER. THE JTWC FORECAST IS ROUGHLY
CONSISTENT WITH THE CONSENSUS MEAN WITH LOW CONFIDENCE. 

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