WDXS31 PGTW 062100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 12S (JENNA) WARNING NR 
005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.7S 93.6E
   INITIAL INTENSITY: 80 KTS
   GEOGRAPHIC REFERENCE: 331 NM SOUTHWEST OF THE COCOS ISLANDS
   MOVEMENT PAST 6 HOURS: SOUTHWESTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 30 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL CYCLONE (TC) 12S (JENNA) PEAKED IN INTENSITY BETWEEN 0600Z
AND 1200Z LAST NIGHT OF APPROXIMATELY 90 KNOTS. SUBSEQUENTLY, THE 
SYSTEM HAS BEGUN TO WEAKEN; THE CURRENT RATE OF WEAKENING IS GRADUAL, 
ALTHOUGH A MORE RAPID DECLINE IN INTENSITY IS IMMINENT. ANIMATED
ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A SYMMETRICAL
CENTRAL DENSE OVERCAST (CDO) OBSCURING A SUBSTANTIAL PORTION OF THE 
UNDERLYING LOWER-LEVEL STRUCTURE. CLOUD TOP TEMPERATURES WITHIN THE 
CDO HAVE WARMED SIGNIFICANTLY OVER THE PAST SIX HOURS AND AN EYE IS 
NO LONGER DISCERNIBLE. ANALYSIS OF THE OUTERMOST BANDING FEATURES IN
ANIMATED SHORTWAVE INFRARED (SWIR) IMAGERY SUGGESTS INDICATES THE 
ONSET OF VORTEX DECOUPLING, AS TRACING OF THE BANDING FEATURES
INDICATES THE LOW LEVEL CIRCULATION CENTER (LLCC) IS POSITIONED ON
THE NORTHERN EDGE OF THE CDO RATHER THAN BEING CENTRALLY EMBEDDED. A 
061559Z ASCAT-C PASS REVEALED AN INCREASINGLY ASYMMETRIC WIND FIELD, 
WITH INCREASING NORTHERLY SHEAR BEGINNING TO IMPINGE ON THE SYSTEM, 
AND DECREASING THE EXTENT OF THE WIND FIELD ON THE NORTHERN 
PERIPHERY, WHILE EXPANDING IT TO THE SOUTH. THE INITIAL POSITION IS 
ASSESSED WITH MEDIUM CONFIDENCE DUE TO THE OBSCURED LLCC AND A LACK OF
RECENT MICROWAVE IMAGERY. THE INITIAL INTENSITY IS ASSESSED WITH
MEDIUM CONFIDENCE BASED ON A BLEND OF THE SUBJECTIVE AND OBJECTIVE
INTENSITY ESTIMATES NOTED BELOW. ENVIRONMENTAL CONDITIONS ARE
MARGINALLY FAVORABLE, CHARACTERIZED BY MARGINAL (26-27C) SSTS,
STRONG POLEWARD OUTFLOW ALOFT AND LOW VWS, OFFSET BY HIGH MID-LEVEL
SHEAR AND MID-LEVEL DRY AIR ENTRAINMENT.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.0 - 90 KTS
   DEMS: T5.0 - 90 KTS
   APRF: T4.5 - 77 KTS
   CIMSS SATCON: 71 KTS AT 061827Z
   CIMSS ADT: 87 KTS AT 061730Z
   CIMSS AIDT: 77 KTS AT 061730Z
   CIMSS D-PRINT: 85 KTS AT 061830Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 26-27 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: STRONG MID-LEVEL SHEAR, DRY MID-LEVEL AIR TO THE
NORTHEAST. 

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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: TC 12S HAS TRACKED SOUTHWESTWARD OVER THE PAST 
12 HOURS, ALONG THE WESTERN FLANK OF A DEEP-LAYER STR CENTERED TO THE 
EAST. THIS RIDGE IS SLOWLY RECEDING TOWARD THE SOUTHEAST, WEAKENING 
THE STEERING GRADIENT AND RESULTING IN A SLIGHT DECELERATION, THOUGH 
NO SIGNIFICANT CHANGE IN THE TRACK DIRECTION IS EXPECTED FOR THE NEXT 
12 TO 24 HOURS. AS NOTED ABOVE, THE SYSTEM HAS ALREADY BEGUN TO 
WEAKEN AS INCREASING SHEAR AND DRY AIR ENTRAINMENT BEGIN TO EXERT 
THEIR EFFECTS. OVER THE NEXT 12 TO 24 HOURS, BOTH OF THESE ADVERSE 
PARAMETERS ARE EXPECTED TO INCREASE SHARPLY, WITH SHEAR FORECAST TO 
EXCEED 35 KNOTS BY TAU 24, WHILE DRY AIR WILL COMPLETELY ENGULF THE 
SYSTEM FROM THE NORTHWEST BY THE SAME FORECAST PERIOD. THE VORTEX IS 
EXPECTED TO FULLY DECOUPLE BY TAU 24, AND AS ITS VERTICAL DEPTH 
DECREASES, THE REMNANT LLCC WILL COME UNDER THE STEERING INFLUENCE OF 
THE EASTERLY LOW-LEVEL FLOW ALONG THE NORTHERN PERIPHERY OF A STRONG 
RIDGE TO THE SOUTH. AS THE SYSTEM TRACKS WESTWARD, IT WILL RAPIDLY 
WEAKEN, WITH DISSIPATION EXPECTED NO LATER THAN TAU 72, AND POSSIBLY 
AS EARLY AS TAU 60. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE EXHIBITS A HIGH DEGREE 
OF CONSENSUS ON THE TRACK SCENARIO. ALL MEMBERS OF THE CONSENSUS 
PACKAGE, AS WELL AS THE ENSEMBLES AND AI MODELS, ARE CONFINED TO A 
GRADUALLY EXPANDING ENVELOPE WHICH INCREASES TO APPROXIMATELY 125NM 
BY TAU 72. THE NAVGEM IS POSITIONED ON THE NORTHERN EXTENT OF THE 
ENVELOPE, AND ECMWF ON THE SOUTHERN EXTENT. CONSISTENT WITH THE PAST 
SEVERAL MODEL RUNS, ALONG-TRACK SPREAD INCREASES AFTER THE SYSTEM 
TURNS WESTWARD; HOWEVER, THE SPREAD HAS DECREASED SOMEWHAT, DOWN TO 
APPROXIMATELY 140NM BETWEEN THE FAST GEFS MEAN AND THE SLOW ECMWF. 
THE JTWC FORECAST IS POSITIONED IN CLOSE AGREEMENT WITH THE MULTI-
MODEL CONSENSUS MEAN AND THE GDM FGN3 TRACKER WITH HIGH CONFIDENCE. 
INTENSITY GUIDANCE CONCURS ON A RAPID WEAKENING TREND THROUGH TAU 48, 
FOLLOWED BY A MORE GRADUAL WEAKENING THEREAFTER AS THE SYSTEM 
PERSISTS AT LOW INTENSITY BEFORE FINALLY DISSIPATING. THE JTWC 
FORECAST IS POSITIONED NEAR THE CONSENSUS MEAN WITH HIGH CONFIDENCE, 
WITH THE ACKNOWLEDGMENT THAT THE SYSTEM IS LIKELY TO DISSIPATE PRIOR 
TO THE TAU 72 FORECAST POINT.

FORECAST CONFIDENCE:
   TRACK 00-72 HR: HIGH
   INTENSITY 00-72 HR: HIGH//
NNNN