WDPS33 PGTW 081500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 12P (TWELVE) WARNING 
NR 003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.0S 164.2E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 257 NM WEST-NORTHWEST OF PORT VILA,
VANUATU
   MOVEMENT PAST 6 HOURS: EAST-SOUTHEASTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 19 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 12P (TWELVE) CONTINUES TO STRUGGLE AGAINST
PERSISTENT WEST-NORTHWESTERLY SHEAR AND SO FAR HAS FAILED TO BUILD
A VERTICALLY ALIGNED VORTEX. THE LATEST ANIMATED ENHANCED INFRARED
(EIR) SATELLITE IMAGERY DEPICTS A SHEARED SYSTEM, WITH AN ELONGATED
AND PARTIALLY EXPOSED LOW LEVEL CIRCULATION CENTER (LLCC),
DISPLACED TO THE NORTHWEST OF PERSISTENT DEEP CONVECTION. IF
ANYTHING, THE LLCC HAS BECOME MORE ELONGATED AND STRETCHED OUT OVER
THE PAST SIX HOURS. HOT OFF THE PRESS, A 081248Z GMI MICROWAVE
PASS SUGGESTS THERE MAY BE TWO CENTERS OF ROTATION OR VORTICITY
WITHIN THE ELONGATED CIRCULATION CENTER. FOR NOW, THE INITIAL
POSITION IS ASSESSED AS LOW CONFIDENCE AND REANALYSIS BASED ON THE
MICROWAVE IMAGERY IS LIKELY. THE INITIAL INTENSITY IS SET AT A
GENEROUS 40 KNOTS BASED PRIMARILY ON PERSISTENCE AND THE SUBJECTIVE
AND OBJECTIVE FIXES NOTED BELOW. THE LATEST CIMSS ANALYSIS
REVEALS 20 KNOTS OF NORTHWEST SHEAR, AND THE GFS MODEL SOUNDINGS
SUGGEST EVEN HIGHER VALUES IN THE MID-LEVELS. HOWEVER THE MOST
RECENT AMV PICTURE SHOWS THE UPPER-LEVEL FLOW SPLITTING AS IT
APPROACHES THE SYSTEM, PROVIDING A HINT OF A POSSIBLE CHANGE IN THE
UPPER-LEVEL PATTERN. DRY MID-LEVEL AIR REMAINS ENTRENCHED TO THE
WEST OF THE SYSTEM AS EVIDENCED BY THE LACK OF MID-LEVEL CLOUDS ON
THE WESTERN SIDE OF THE CIRCULATION. OVERALL, THE ENVIRONMENT CAN
BE CHARACTERIZED AS MARGINALLY FAVORABLE AT BEST, WITH THE STRONG
POLEWARD OUTFLOW AND WARM SSTS ONLY WEAKLY OFFSETTING THE OTHER
NEGATIVE FACTORS NOTED ABOVE. THE SYSTEM REMAINS MOVING SOUTHWEST
ALONG THE SOUTHERN SIDE OF A STRONG NER TO THE NORTH.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF A DEEP-LAYER NER
TO THE NORTH. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   KNES: T3.0 - 45 KTS
   PHFO: T3.0 - 45 KTS
   CIMSS ADT: 49 KTS AT 081130Z
   CIMSS SATCON: 46 KNOTS AT 081230Z
   CIMSS DPRINT: 41 KNOTS AT 081200Z
   CIMSS DMINT:  41 KNOTS AT 081243Z    

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: HIGHER MID-LEVEL SHEAR AND DRY MID-LEVEL AIR TO
THE WEST. 

ANALYSIS CONFIDENCE:
   INITIAL POSITION: LOW
   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: TC 12P WILL CONTINUE TO TRACK EAST-SOUTHEASTWARD 
THROUGH THE MAJORITY OF THE FORECAST PERIOD, ALONG THE SOUTHERN SIDE 
OF THE NER ENTRENCHED TO THE NORTH. THE SYSTEM WILL BEGIN TO SLOW DOWN 
AFTER TAU 72 AS THE NER RECEDES EASTWARD AND THE SYSTEM MOVES INTO A 
WEAK STEERING ENVIRONMENT, TRAPPED BETWEEN COMPETING STEERING 
INFLUENCES TO THE NORTH AND SOUTH. IN THE NEAR-TERM, AS LONG AS THE 
VORTEX REMAINS MISALIGNED AND THE LLCC REMAINS HIGHLY ELONGATED, THERE 
IS LITTLE CHANCE OF INTENSIFICATION AND THE SYSTEM MAY IN FACT WEAKEN 
SLIGHTLY IN THE NEXT 12 HOURS. THE FORECAST CALLS FOR THE LOW-LEVEL 
WIND FIELD TO MAINTAIN AT 40 KNOTS THROUGH TAU 24, PRIMARILY DRIVEN BY 
THE INFLUENCE OF A LOW-LEVEL WESTERLY WIND BURST (WWB) WHICH WILL PUSH
ELEVATED WINDS AND VORTICITY INTO THE SYSTEM FROM THE NORTH. DUE TO
THE INFLUX OF DRY AIR AND PERSISTENT NORTHWEST SHEAR, THE VORTEX IS
LIKELY TO REMAIN MISALIGNED AND CONVECTION WILL REMAIN SHEARED TO
THE SOUTHEAST FOR THE MOST PART. AFTER TAU 24, SHEAR IS ANTICIPATED
TO WEAKEN SLIGHTLY, WHICH SHOULD ALLOW FOR A BRIEF PERIOD OF
INTENSIFICATION TO 45 KNOTS, BEFORE SHEAR PICKS UP ONCE AGAIN AND
DRIVES A SLOW BUT STEADY WEAKENING TREND THROUGH THE REMAINDER OF
THE FORECAST PERIOD. AS IT WEAKENS TO MINIMAL TROPICAL STORM
STRENGTH BY TAU 120, TC 12P BECOMES QUASI-STATIONARY BY TAU 120
UNDER THE COMPETING LOW-LEVEL STEERING INFLUENCES. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN FAIRLY GOOD
AGREEMENT THROUGH TAU 72, WITH CROSS-TRACK SPREAD INCREASING
STEADILY TO 140NM. THE LATEST RUN SHOWS ECMWF AND NAVGEM MARKING
THE SOUTHERN SIDE OF THE ENVELOPE, WHILE GFS CONTINUES TO FLATTEN
OUT AFTER TAU 48 AND MARK THE NORTHERN EDGE OF THE GUIDANCE
PACKAGE. ALONG-TRACK SPREAD PICKS UP QUICKLY AFTER TAU 48, WITH THE
GALWEM FAR OUTPACING ALL THE OTHER MEMBERS, REACHING FIJI BY TAU
72, WHILE ECMWF HOLDS BACK AND BARELY CROSSES THE VANUATU ISLANDS.
THE JTWC FORECAST TRACK LIES JUST SOUTH OF THE CONSENSUS MEAN
THROUGH TAU 72, WITH MEDIUM CONFIDENCE. AFTER TAU 72, THE MODELS
CONTINUE TO DIVERGE DRASTICALLY, WITH ROUGHLY A 50-50 SPLIT IN THE
MEMBERS TURNING NORTH OR TURNING SOUTH. THE GFS, GFS ENSEMBLE, AND
THE CONSENSUS MEAN TURN NORTHWARD WHILE THE ECMWF, ECMWF ENSEMBLE,
NAVGEM, AND UKMET MODELS TURN SOUTHWARD. BY TAU 120, ALL OF THE
GUIDANCE EXCEPT GALWEM SHOWS THE LLCC TRACKING GENERALLY WESTWARD.
IN LIGHT OF THE EXTREME UNCERTAINTY AND HIGH SPREAD IN THE MODEL
PACKAGE, THERE IS LOW CONFIDENCE IN THE JTWC FORECAST AFTER TAU 72.
INTENSITY GUIDANCE IS, WITH THE EXCEPTION OF SHIPS (NAVGEM), IN
GENERALLY GOOD AGREEMENT, SHOWING A SINUSOIDAL INTENSITY TREND,
MOVING ABOVE AND BELOW THE MEAN OVER TIME. THE JTWC FORECAST IS
BASED ON A BLEND OF ALL THE GUIDANCE, GENERALLY CONSISTENT WITH BUT
BELOW, 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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