Advisories

WDPN31 PGTW 260300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 17W (SAUDEL) WARNING NR 031//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 27.8N 127.3E
   INITIAL INTENSITY: 75 KTS
   GEOGRAPHIC REFERENCE: 82 NM NORTH-NORTHWEST OF KADENA AB
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 45 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TYPHOON (TY)
17W (SAUDEL) AS HAVING AN ASYMMETRIC CLOUD SHIELD WITH SPARCE CLOUD
COVERAGE IN SEVERAL PLACES. THE DRY MOAT THAT HAD PREVIOUSLY
DEGRADED THE STORM STRUCTURE HAS ABATED SLIGHTLY, AS HAS THE
MID-LEVEL VERTICAL WIND SHEAR, THOUGH DRY AIR REMAINS A PERSISTENT
PROBLEM. THE 260108Z ASCAT DATA REVEALS AN ASYMMETRIC WIND FIELD,
WITH THE LARGEST EXTENT OF STORM- AND TYPHOON-FORCE WINDS FOUND
WITHIN THE NORTHERN SEMICIRCLE AND A SMALLER BELT OF INTENSE WINDS
SURROUNDING THE CORE TO THE SOUTH. ADDITIONALLY, THE 252203Z SSMIS
PRODUCT SUITE REVEALS A PINHOLE MICROWAVE EYE FEATURE IN THE LOWER
LEVELS, BUT AN ASYMMETRIC AND BROAD RING OF DEEP CONVECTION IN THE
UPPER LEVELS WITH THE DRY MOAT STILL EVIDENT. ENVIRONMENTAL
ANALYSIS REVEALS A MARGINALLY FAVORABLE ENVIRONMENT, CHARACTERIZED
BY LOW (10-15 KTS) VERTICAL WIND SHEAR (VWS), WARM (27-28 C) SEA
SURFACE TEMPERATURES (SST), AND DECENT OUTFLOW, OFFSET BY THE
SUBSTANTIAL DRY AIR ENTRAINMENT. THE INITIAL INTENSITY OF 75 KTS IS
ASSESSED WITH HIGH CONFIDENCE BASED ON THE AFOREMENTIONED 260108Z
ASCAT DATA AND EXTRAPOLATION FROM THE 252203Z SSMIS MICROWAVE DATA.
THE INITIAL INTENSITY OF 75 KTS IS ASSESSED WITH HIGH CONFIDENCE
BASED ON THE 260108Z ASCAT.

INITIAL WIND RADII BASIS: CONFIRMED WIND RADII USING THE 260108Z
ASCAT SCATTEROMETER DATA

CURRENT STEERING MECHANISM: THE NORTHWESTERN EXTENSION OF THE
SUBTROPICAL RIDGE (STR) CENTERED TO THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.5 - 77 KTS
   RJTD: T4.5 - 77 KTS
   RCTP: T4.0 - 65 KTS
   KNES: T5.0 - 90 KTS
   CIMSS SATCON: 70 KTS AT 260030Z
   CIMSS ADT: 49 KTS AT 260100Z
   CIMSS AIDT: 63 KTS AT 260100Z
   CIMSS D-MINT: 77 KTS AT 252203Z
   CIMSS D-PRINT: 78 KTS AT 260030Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TY 17W IS FORECAST TO CONTINUE TRACKING
NORTHWESTWARD FOR THE NEXT 12 HOURS WHILE IT REMAINS UNDER THE
STEERING INFLUENCE OF THE STR CENTERED TO THE NORTHEAST OF THE
SYSTEM. BEGINNING IMMINENTLY, 17W WILL ENCOUNTER A WEAKNESS IN THE
RIDGE EXTENSION AND SLOW ITS SPEED OF ADVANCE. SIMULTANEOUSLY, A
STR CENTERED OVER EASTERN CHINA WILL BUILD AND ASSUME STEERING
AUTHORITY OF 17W AT BETWEEN TAU 12-24 AND INDUCE A
WEST-SOUTHWESTWARD TRACK THROUGH TAU 96. WHILE THE STR BUILDS
BETWEEN TAU 12-36, SAUDEL'S TRACK SPEEDS WILL BE RELATIVELY SLOWER,
BUT IT IS EXPECTED TO ACCELERATE THE STR STRENGTHENS AND DRIVES 17W
WEST-SOUTHWESTWARD TOWARDS THE COAST. LANDFALL IS FORECAST TO OCCUR
JUST AFTER TAU 48 WITH DISSIPATED EXPECTED OVERLAND NO LATER THAN
TAU 96. REGARDING INTENSITY, 17W IS FORECAST TO GRADUALLY WEAKEN AS
THE
ENVIRONMENT CONTINUES TO DEGRADE FOR THE NEXT 48 HOURS. THE
ENVIRONMENT IS DEFINED BY MODERATE TO HIGH (20-30 KTS) VWS AND
SIGNIFICANT DRY AIR ENTRAINMENT, BUT WEAKENING IS EXPECTED TO BE
SLIGHTLY OFFSET BY THE FAVORABLE SSTS AND GOOD OUTFLOW. RAPID
WEAKENING IS EXPECTED AFTER LANDFALL WITH DISSIPATION ULTIMATELY
OCCURRING BY TAU 96 DUE TO TERRAIN INTERACTION.


MODEL DISCUSSION: DETERMINISTIC MODEL TRACK GUIDANCE IS IN GOOD
AGREEMENT THROUGH LANDFALL, WITH GSM, NAVGEM, AND THE ECMWF
EXPERIMENTAL AI MODEL AS THE EQUATORWARD OUTLIERS. DISREGARDING
THE OUTLIERS, MODEL TRACK GUIDANCE IS IN EXCELLENT AGREEMENT UNTIL
TAU 48-60, SUPPORTING THE JTWS SHORT-TERM FORECAST WITH HIGH
CONFIDENCE. POST-LANDFALL, THE GUIDANCE ENVELOPE OPENS AS MODELS
DISAGREE ON THE IMPACTS OF LAND INTERACTION. OVERALL, ALL MODELS
DEPICT THE STORM TRACKING SOUTHWESTWARD OVER MAINLAND CHINA AFTER
LANDFALL. DUE TO THE DISCREPANCY, THE LONG-TERM JTWC FORECAST IS
PLACED WITH MEDIUM CONFIDENCE. RELIABLE INTENSITY AIDS ARE IN
EXCELLENT AGREEMENT IN A GRADUAL WEAKENING TREND FOR THROUGH TAU
48, FOLLOWED BY RAPID DECAY AFTER LANDFALL. AS A RESULT, THE JTWC
INTENSITY FORECAST IS PLACED WITH OVERALL HIGH CONFIDENCE.


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