WDXS32 PGTW 011500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 09S (GRANT) WARNING NR 
031//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.0S 68.2E
   INITIAL INTENSITY: 50 KTS
   GEOGRAPHIC REFERENCE: 591 NM SOUTH-SOUTHWEST OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: WESTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 21 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) PAINTS A
SIGNIFICANTLY DIFFERENT PICTURE THAN IT DID 24 HOURS AGO. OVER THE
PAST 12 HOURS, PERSISTENT, STRONG EASTERLY SHEAR HAS TAKEN ITS
TOLL, AND THE LOW LEVEL CIRCULATION CENTER (LLCC) IS NOW FULLY
EXPOSED TO THE EAST OF SMALL REGION OF DETERIORATING CONVECTION. IN
ADDITION TO THE IMPACT OF THE SHEAR, THE SYSTEM IS NEARLY ENCIRCLED
BY DRY AIR, PARTICULARLY NOTABLE ON THE EASTERN AND NORTHERN
PERIPHERY AND DEPICTED WELL IN A 010851Z NOAA-20 ATMS 89GHZ IMAGE.
THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE
EXPOSED LLCC IN THE ANIMATED MSI. THE INITIAL INTENSITY IS ASSESSED
WITH MEDIUM CONFIDENCE, BASED ON THE RAPID DETERIORATION OF THE
CONVECTION AND VORTEX DECOUPLING. WHILE DVORAK CURRENT INTENSITY
(CI) ESTIMATES REMAIN ELEVATED, THE FINAL-T NUMBERS ARE DROPPING
INTO THE T3.0 TO T3.5 (45-55 KNOTS) RANGE, MORE IN LINE WITH THE
OBJECTIVE ESTIMATES NOTED BELOW. THE ENVIRONMENT IS MARGINAL AT
BEST, WITH MODERATE TO HIGH EASTERLY SHEAR, WEAK OUTFLOW ALOFT,
SIGNIFICANT DRY AIR OVER THE EASTERN HALF OF THE SYSTEM AND
MODERATELY WARM SSTS.

INITIAL WIND RADII BASIS: PERSISTENCE 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.5 - 77 KTS
   DEMS: T5.5 - 102 KTS
   FMEE: T3.5 - 55 KTS
   CIMSS SATCON: 51 KTS AT 010853Z
   CIMSS ADT: 59 KTS AT 011200Z
   CIMSS AIDT: 52 KTS AT 011230Z
   CIMSS D-MINT: 47 KTS AT 010853Z
   CIMSS D-PRINT: 46 KTS AT 011200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: WEAK POLEWARD
   OTHER FACTORS: DRY MID-LEVEL AIR ALONG THE EASTERN AND NORTHERN
FLANKS.

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: OVER THE PAST 12 HOURS, TC 09S HAS TAKEN A
WEST-NORTHWESTWARD TRAJECTORY, ALONG THE NORTHWESTERN SIDE OF THE
STR FAR TO THE SOUTHEAST. TRACK SPEEDS HAVE SLOWED AS THE SYSTEM
HAS RUN INTO A BUILDING RIDGE OVER NORTHERN MADAGASCAR. TC 09S WILL
CONTINUE ITS WEST-NORTHWESTWARD TRACK OVER THE NEXT 36 HOURS, AS
THE STR TO THE EAST BUILDS TOWARDS THE WEST-SOUTHWEST, AND THE
RIDGE OVER MADAGASCAR ELONGATES AND PINCHES INTO A NARROW
NORTHWEST-SOUTHEAST ORIENTED RIDGE AXIS. AFTER TAU 36, THE SYSTEM
TURNS FIRST WEST, THEN SOUTHWEST THROUGH TAU 120 AS THE STR SLIDES
BY TO THE SOUTH AND REPOSITIONS TO THE SOUTHEAST OF TC 09S BY THE
END OF THE FORECAST PERIOD. THE FORECAST INTENSITY SCENARIO IS A
BIT MORE COMPLICATED AND UNCERTAIN. ALL AVAILABLE MODEL GUIDANCE
INDICATES THE SYSTEM WILL CONTINUE TO FACE ELEVATED EASTERLY SHEAR
AND SIGNIFICANT DRY AIR INTRUSION OVER THE NEXT 24 HOURS OR SO. THE
SYSTEM IS UNLIKELY TO BE ABLE TO ACHIEVE VORTEX ALIGNMENT IN THE
NEAR-TERM IN LIGHT OF THE SHEAR AND DRY AIR, AND INTENSITY IS
LIKELY TO DROP PRECIPITOUSLY. THE FORECAST CALLS FOR ANOTHER 15
KNOTS OR SO OF WEAKENING, BEFORE FLATTENING OUT AND CONDITIONS
STABILIZE AFTER TAU 24. THE MAJORITY OF THE MODELS SHOW A GRADUAL
MOISTENING OF THE ENVIRONMENT AND DECREASED SHEAR STARTING BETWEEN
TAU 36 AND TAU 48, WHICH WILL ALLOW THE SYSTEM TO REINTENSIFY
BEYOND TAU 48. HOWEVER, THE PEAK INTENSITY REACHED IN THE LONG-TERM
FORECAST WILL DEPEND ON WHETHER THE SYSTEM SURVIVES AS A TROPICAL
CYCLONE JUST HOW LOW THE INTENSITY GETS IN THE NEAR-TERM. 

MODEL DISCUSSION: DETERMINISTIC, ENSEMBLE AND ARTIFICIAL
INTELLIGENCE (AI) MODELS ARE GENERALLY GOOD AGREEMENT ON THE
OVERALL TRACK SCENARIO, BUT DO SHOW A FAIR AMOUNT OF BOTH CROSS-
AND ALONG-TRACK SPREAD, PARTICULARLY IN THE LONG-TERM FORECAST. AT
TAU 72, THE GUIDANCE ENVELOPE EXHIBITS A CROSS-TRACK SPREAD BETWEEN
THE ECMWF TO THE NORTH AND THE GEFS MEAN TO THE SOUTH, OF 90NM.
ALONG-TRACK SPREAD BETWEEN THE SLOW ECMWF AND FAST GEFS MEAN IS
APPROXIMATELY 155NM AT THE SAME TIME. BY TAU 120, THE SPREADS OPEN
UP TO 165NM AND 170NM RESPECTIVELY, BETWEEN THE SAME MODELS. THE
JTWC FORECAST IS POSITIONED CLOSE TO THE AI CONSENSUS AND
MULTI-MODEL CONSENSUS MEANS WITH MEDIUM CONFIDENCE. INTENSITY
GUIDANCE IS IN GOOD AGREEMENT, WITH ALL MODELS DEPICTING A
WEAKENING IN THE NEAR-TERM, TO A LOW-POINT BETWEEN 30-35 KNOTS,
FOLLOWED BY A REINTENSIFICATION PHASE WHICH PEAKS AROUND TAU 96
BETWEEN 45-60 KNOTS. THE JTWC FORECAST IS PLACED CLOSE TO THE
CONSENSUS MEAN AND COAMPS-TC (GFS) THROUGH THE FORECAST PERIOD WITH
MEDIUM CONFIDENCE. 

FORECAST CONFIDENCE:
   TRACK 00-72 HR: HIGH
   TRACK 72-120 HR: MEDIUM
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: LOW//
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