Thirtyone

WDPN31 PGTW 061500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 31W (THIRTYONE) 
WARNING NR 002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.9N 147.5E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 293 NM NORTH-NORTHEAST OF ANDERSEN AFB
   MOVEMENT PAST 6 HOURS: WESTWARD AT 14 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED SHORTWAVE INFRARED (SWIR) SATELLITE IMAGERY DEPICTS A 
VERTICALLY TILTED, ASYMMETRIC SYSTEM, WITH THE MID-LEVEL VORTEX 
DISPLACED WESTWARD (DOWNSHEAR) OF A PARTIALLY EXPOSED LOW-LEVEL 
CIRCULATION CENTER (LLCC). A 061113Z ASCAT PARTIAL PASS REVEALED A 
COMPACT LLCC ON THE EDGE OF THE OBSERVATIONAL SWATH, WITH WINDS OF 
25-30 KNOTS ON THE NORTHERN FLANK OF THE LLCC, 20-KNOT WINDS 
BEGINNING TO WRAP AROUND THE WESTERN SEMICIRCLE, AND LIGHTER WINDS ON 
THE SOUTHERN FLANK. A 061059Z GMI COLOR-ENHANCED 89 GHZ MICROWAVE 
IMAGE DEPICTED SHALLOW CONVECTIVE BANDING FEATURES WRAPPING INTO A 
WELL-DEFINED, BUT EXPOSED LLCC LOCATED TO THE EAST OF AN AREA OF 
DISORGANIZED DEEP CONVECTION, WHICH REMAINS UNABLE TO WRAP UPSHEAR 
DUE TO MODERATE EASTERLY VERTICAL WIND SHEAR (VWS). UPPER-LEVEL 
ATMOSPHERIC MOTION VECTORS (AMVS) DEPICT DEEP, STRONG EASTERLY FLOW 
DIRECTLY OVERLYING TD 31W, THOUGH INITIAL SIGNS OF DIFLUENCE AND A 
SPLIT IN THE UPSTREAM FLOW PATTERN ARE BEGINNING TO EMERGE IN THE AMV 
DATA. THE 061200Z UPPER-AIR SOUNDING FROM PGUM INDICATES DEEP 
EASTERLIES EXTENDING UP TO APPROXIMATELY 400 MB, BEFORE VEERING 
SHARPLY TO SOUTHWESTERLIES BETWEEN 400 MB AND 350 MB AND RESUMING 
EASTERLY FLOW ABOVE THAT LAYER, HIGHLIGHTING THE PRIMARY MECHANISM 
DRIVING THE ENHANCED MID-LEVEL SHEAR. THE INITIAL POSITION IS 
ASSESSED WITH HIGH CONFIDENCE BASED ON THE WELL-DEFINED LLCC IN THE 
SWIR IMAGERY AND THE ASCAT DATA. THE INITIAL INTENSITY IS ASSESSED 
WITH HIGH CONFIDENCE BASED ON THE ASCAT DATA, CONFIRMING THE 
OBJECTIVE INTENSITY ESTIMATES OUTLINED BELOW. ENVIRONMENTAL 
CONDITIONS ARE GENERALLY FAVORABLE, CHARACTERIZED BY LOW TO MODERATE 
DEEP-LAYER SHEAR, HIGH SEA SURFACE TEMPERATURES (SSTS), AND STRONG 
DIVERGENT OUTFLOW ALOFT, THOUGH TEMPERED BY THE VERTICALLY DECOUPLED 
VORTEX.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   RJTD: T1.5 - 25 KTS
   CIMSS ADT: 28 KTS AT 061130Z
   CIMSS AIDT: 33 KTS AT 061130Z
   CIMSS D-MINT: 30 KTS AT 061102Z
   CIMSS D-PRINT: 30 KTS AT 061130Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG WESTWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

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

FORECAST DISCUSSION: OVER THE PAST SIX HOURS, TROPICAL DEPRESSION 
(TD) 31W HAS TRANSITIONED TO A DUE WESTWARD TRAJECTORY, STEERED BY 
THE MORE ZONALLY ORIENTED LOWER-LEVEL FLOW AXIS OF THE STR TO THE 
NORTHEAST DUE TO SHALLOW VORTEX DEPTH. GLOBAL MODELS AGREE THAT OVER 
THE NEXT 6-12 HOURS, THE RIDGE AXIS WILL AMPLIFY INTO A MORE 
NORTHWEST-SOUTHEAST ORIENTATION, ALLOWING TD 31W TO RESUME A 
NORTHWESTWARD TRAJECTORY; HOWEVER, A MORE ZONAL WESTWARD NEAR-TERM 
TRAJECTORY REMAINS POSSIBLE IF THE VORTEX REMAINS DECOUPLED. 
TRANSLATIONAL SPEEDS ARE FORECAST TO REMAIN RELATIVELY STABLE THROUGH 
TAU 24 BEFORE GRADUALLY DECELERATING AS THE STR CENTER PROPAGATES 
EASTWARD AND THE STEERING GRADIENT RELAXES. AFTER TAU 36, THE PASSAGE 
OF A MID-LATITUDE SHORTWAVE TROUGH TO THE NORTH WILL BEGIN TO ERODE 
THE RIDGE, WITH A COL EXPECTED TO DEVELOP SOUTH OF JAPAN. BY TAU 60, 
TD 31W WILL APPROACH THIS COL AND RAPIDLY DECELERATE AS IT ENTERS THE 
WEAK STEERING ENVIRONMENT. SIMULTANEOUSLY, A STRONG LOW-LEVEL 
NORTHEASTERLY COLD SURGE WILL ADVECT OUT OF THE EAST CHINA SEA AND 
INTO THE PHILIPPINE SEA. BY TAU 72, THE LEADING EDGE OF THE SURGE 
FLOW WILL IMPINGE UPON TD 31W, WHILE ALOFT THE SYSTEM REMAINS 
POSITIONED WITHIN THE COL-INDUCED WEAK STEERING PATTERN, INDUCING A 
GRADUAL POLEWARD DRIFT. CONCURRENTLY, IN THE DEEP-LAYER FLOW PATTERN, 
AN EXTENSION OF THE STR WILL BIFURCATE INTO A SEPARATE NEAR-
EQUATORIAL RIDGE (NER) SOUTHWEST OF GUAM, IMPARTING A WESTERLY DEEP-
LAYER STEERING INFLUENCE ON TD 31W. THE COMBINATION OF LOW-LEVEL 
CONVERGENCE ALONG THE COLD FRONTAL BOUNDARY AND THE DEEP-LAYER 
WESTERLY STEERING FLOW WILL ACCELERATE TD 31W EAST-NORTHEASTWARD FROM 
TAU 72 THROUGH TAU 120. REGARDING INTENSITY, AS LONG AS DEEP EASTERLY 
SHEAR PERSISTS, THE SYSTEM WILL INTENSIFY GRADUALLY AT A 
CLIMATOLOGICAL RATE OF APPROXIMATELY ONE DVORAK T-NUMBER PER DAY. 
VERTICAL WIND SHEAR IS FORECAST TO ABATE AROUND TAU 36, WHICH SHOULD 
FACILITATE RAPID VERTICAL VORTEX ALIGNMENT AND SUBSEQUENT ACCELERATED 
INTENSIFICATION, THOUGH THE OFFICIAL FORECAST REMAINS JUST BELOW 
EXPLICIT RAPID INTENSIFICATION (RI) THRESHOLDS. FAVORABLE 
THERMODYNAMIC AND DYNAMIC CONDITIONS PERSIST THROUGH TAU 72, 
SUPPORTING A PEAK INTENSITY OF 100 KNOTS, BEFORE RAPIDLY INCREASING 
MID-LATITUDE WESTERLY SHEAR, DRY AIR ENTRAINMENT, AND DECREASING SSTS 
INDUCE A SHARP WEAKENING TREND AFTER TAU 96.

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN FAIR AGREEMENT 
THROUGH TAU 72, WITH SIGNIFICANT TRACK DIVERGENCE EMERGING AT TAU 72. 
CROSS-TRACK SPREAD IS LOW THROUGH TAU 60, BEFORE RAPIDLY INCREASING 
TO 210 NM BY TAU 72 AS THE SYSTEM REACHES THE SYNOPTIC INFLECTION 
POINT, WHERE ALL CONSENSUS MEMBERS AGREE THE SYSTEM WILL RECURVE 
NORTHEASTWARD. THE GFS IS POSITIONED ON THE SOUTHERN BOUNDARY OF THE 
GUIDANCE ENVELOPE THROUGH TAU 72, WHILE THE EC-AIFS AND ECMWF RESIDE 
ON THE NORTHERN BOUNDARY. AFTER TAU 72, CROSS-TRACK SPREAD REMAINS 
RELATIVELY CONSTRAINED; HOWEVER, ALONG-TRACK SPREAD INCREASES 
DRAMATICALLY. THE GFS AND NAVGEM DEPICT A VERY SLOW EAST-
SOUTHEASTWARD DRIFT AFTER TAU 72, FOLLOWED CLOSELY BY GALWEM. THE 
ECMWF, GEFS, ECEPS, WINDBORNE EXPERIMENTAL MODEL, AND THE CONSENSUS 
MEAN ADVECT THE SYSTEM NORTHEASTWARD AT A MODERATE PACE, WHILE THE 
DATA-DRIVEN AI MODELS, LED BY THE OPERATIONAL GOOGLE DEEPMIND (GDM) 
AND EC-AIFS, ACCELERATE NORTHEASTWARD AT TRANSLATIONAL SPEEDS 
EXCEEDING 50 KNOTS. THIS GENERATES AN EXTREME ALONG-TRACK SPREAD 
EXCEEDING 1,500 NM BETWEEN THE GFS AND EC-AIFS. ENSEMBLE GUIDANCE IS 
SIMILARLY DISPERSED, DEPICTING SIGNIFICANT INTRA-ENSEMBLE SPREAD EVEN 
THOUGH THE MAJORITY OF ENSEMBLE MEANS AGREE ON THE SYNOPTIC 
TRAJECTORY. INDIVIDUAL MEMBERS OF THE EXPERIMENTAL JTWC SUPER 
ENSEMBLE RANGE IN THEIR TAU 120 POSITIONS FROM NEAR LUZON TO FAR EAST 
OF JAPAN, SPANNING WELL OVER 1,500 NM. THE OFFICIAL JTWC FORECAST IS 
PLACED WITH LOW CONFIDENCE NEAR THE GEFS MEAN, ECEPS MEAN, AND 
WINDBORNE CLUSTER, AND SLIGHTLY SLOWER THAN THE DETERMINISTIC ECMWF. 
INTENSITY GUIDANCE IS IN GENERALLY GOOD AGREEMENT ON THE OVERALL 
TIMING AND EVOLUTION OF THE INTENSITY PROFILE, BUT EXHIBITS 
SIGNIFICANT SPREAD REGARDING PEAK INTENSITY, WHICH RANGES FROM A 
MINIMUM OF 55 KNOTS (COTC AND SHIPS-NAVGEM) TO A MAXIMUM OF 110 KNOTS 
(HAFS-A, HWRF, AND CTCX). THE GDM SOLUTION RESIDES CLOSE TO THE 
CONSENSUS AT 80-90 KNOTS. THE JTWC INTENSITY FORECAST IS PLACED JUST 
BELOW THE CONSENSUS MEAN THROUGH TAU 48, BLENDS TOWARD THE HWRF PEAK 
AT TAU 72, AND SUBSEQUENTLY FOLLOWS THE GRADUAL WEAKENING TREND OF 
THE MESOSCALE DYNAMICAL MODELS AFTER TAU 72. 

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