Advisories

WDPN31 PGTW 250900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 17W (SAUDEL) WARNING NR 028//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 26.8N 130.1E
   INITIAL INTENSITY: 80 KTS
   GEOGRAPHIC REFERENCE: 125 NM EAST OF KADENA AB
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 42 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A WEAKENING
TYPHOON (TY) ON APPROACH TO THE RYUKYU ISLANDS. WHILE THE CENTRAL
DENSE OVERCAST REMAINS LARGELY INTACT, A DRY SLOT IS WEDGED INTO
THE WESTERN SEMICIRCLE. A LARGE CONVECTIVE BAND SPANS THE NORTHERN
PERIPHERY, BUT THE UPPER-LEVEL OUTFLOW IS RESTRICTED ON THE WESTERN
SIDE. A 250412Z AMSR2 PASS INDICATES AN EROSION OF THE WESTERN SIDE
OF THE INNER CORE AND A SLIGHT EASTWARD TILT OF THE VORTEX WITH
HEIGHT. THIS DETERIORATION IN STRUCTURE IS CAUSED BY UNFAVORABLE,
MODERATE-TO-HIGH WESTERLY VERTICAL WIND SHEAR (VWS) OF 20-25 KTS.
THE OCEANIC PARAMETERS ARE MORE SUPPORTIVE, CHARACTERIZED BY SEA
SURFACE TEMPERATURES (SST) OF 27-28 DEGREES CELSIUS AND
LOW-TO-MODERATE OCEAN HEAT CONTENT (OHC) OF 30-40 KJ PER CM2. THE
INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON A
COMBINATION OF RADAR IMAGERY AND THE MSI. THE INITIAL INTENSITY HAS
BEEN LOWERED TO 80 KTS BASED ON THE SLOW DEGRADATION OF STORM
STRUCTURE, CONSISTENT WITH THE FALLING SUBJECTIVE DVORAK FIXES OF
T4.5-5.0 AND THE OBJECTIVE INTENSITY ESTIMATES RANGING FROM 65 KTS
TO 86 KTS.

INITIAL WIND RADII BASIS: SAR DATA 

CURRENT STEERING MECHANISM: SOUTHWESTERN FLANK OF A SUBTROPICAL
RIDGE (STR) POSITIONED OVER HONSHU

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.5 - 77 KTS
   RJTD: T5.0 - 90 KTS
   RCTP: T4.5 - 77 KTS
   KNES: T4.5 - 77 KTS
   CIMSS SATCON: 86 KTS AT 250530Z
   CIMSS ADT: 65 KTS AT 250530Z
   CIMSS AIDT: 77 KTS AT 250530Z
   CIMSS D-MINT: 80 KTS AT 250427Z
   CIMSS D-PRINT: 76 KTS AT 250530Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: WEAK RADIAL

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE FORECAST TRACK HAS BEEN SHIFTED
NORTHWARD BY 20 NM, AND THE FORECAST INTENSITY HAS BEEN LOWERED BY
APPROXIMATELY 10 KTS.

FORECAST DISCUSSION: TY 17W HAS BEGUN TO SLOW AS IT APPROACHES THE
RYUKYU ISLANDS. A LARGE DEEP-LAYER RIDGE POSITIONED OVER CENTRAL
HONSHU IS MIGRATING WESTWARD, WHICH IS STEERING THE SYSTEM ON A
NORTHWESTWARD TRAJECTORY THAT WILL TAKE THE CENTER OF THE SYSTEM
BETWEEN THE ISLANDS OF OKINAWA AND AMAMI. OVER THE NEXT 12 HOURS,
TY 17W WILL TURN WESTWARD UNDER THE STEERING INFLUENCE OF THE STR
TO THE NORTH. THE STR TO THE NORTH WILL WEAKEN OVER THE NEXT 24
HOURS, CAUSING THE STORM'S FORWARD SPEED TO DECREASE TO
APPROXIMATELY 06 KTS. THIS SLOW MOTION WILL PERSIST UNTIL A
SEPARATE, STRONG ANTICYCLONE OVER NORTHERN CHINA CAPTURES THE
VORTEX AND ACCELERATES IT TOWARD AN EVENTUAL LANDFALL SOUTH OF
WENZHOU. THIS MOTION WILL CONTINUE UNTIL DISSIPATION. TY 17W WILL
GRADUALLY WEAKEN OVER THE NEXT 24 HOURS, PRIMARILY DUE TO
MODERATE-TO-HIGH VWS, WHICH WILL EXACERBATE THE EFFECTS OF DRY AIR
ENTRAINMENT INTO THE VORTEX. THE SYSTEM WILL ENCOUNTER A BRIEF
REPRIEVE FROM THE WEAKENING TREND BETWEEN TAU 24 AND TAU 36 AS THE
VWS BRIEFLY SUBSIDES. HOWEVER, INCREASING SHEAR WILL RE-INITIATE A
TERMINAL WEAKENING TREND AFTER TAU 36. THE SYSTEM WILL RAPIDLY
DECAY AFTER TAU 60 DUE TO CONTINUING SHEAR AND THE FRICTIONAL AND
DRYING EFFECTS OF LAND, WITH DISSIPATION PREDICTED BY TAU 96.

MODEL DISCUSSION: WITH THE EXCEPTION OF NAVGEM, WHICH IS THE MOST
EQUATORWARD OUTLIER, THE TRACK GUIDANCE IS IN GOOD AGREEMENT. THE
EXPERIMENTAL GOOGLE DEEPMIND SOLUTION IS NOTABLY THE MOST POLEWARD,
SHOWING A LANDFALL POINT NEAR WENZHOU. AFTER LANDFALL, THE
CROSS-TRACK AND ALONG-TRACK SPREAD INCREASES, OWING TO
UNCERTAINTIES ASSOCIATED WITH THE STRENGTH OF THE CONTINENTAL
ANTICYCLONE. THE GFS AND JGSM MODELS ARE THE FASTEST AND MOST
SOUTHWARD AFTER LANDFALL. THE OVERALL GUIDANCE ENVELOPE HAS SHIFTED
NORTHWARD THIS CYCLE, NECESSITATING A POLEWARD ADJUSTMENT TO THE
JTWC FORECAST TRACK, WHICH IS POSITIONED NEAR THE AI CONSENSUS
THROUGH TAU 96. THE INTENSITY AIDS DEPICT STEADY WEAKENING THROUGH
TAU 24, FOLLOWED BY A FLATTENING OF THE DOWNWARD TREND BETWEEN TAU
24 AND TAU 36 AND RAPID DECAY AFTER TAU 60. THE DEEPMIND SOLUTION
IS THE MOST AGGRESSIVE IN WEAKENING THE SYSTEM THROUGH THE ENTIRE
FORECAST PERIOD. THE JTWC INTENSITY FORECAST IS SET WITH MEDIUM
CONFIDENCE CLOSEST TO THE COAMPS-TC, GFS, AND HAFS-A MODELS, WHICH
IS SLIGHTLY BELOW THE MULTI-MODEL CONSENSUS BUT HIGHER THAN
DEEPMIND.

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