Advisories

WDPN32 PGTW 010900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 17W (SAUDEL) WARNING NR 
040//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.0N 114.4E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 138 NM SOUTH OF HONG KONG
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 12 KTS
   SIGNIFICANT WAVE HEIGHT: 18 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION: 
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL STORM 
17W (SAUDEL) HAVING RECONSOLIDATED SOUTH OF HONG KONG AFTER A LONG 
TREK ACROSS EASTERN CHINA. THE SYSTEM NOW CONSISTS OF A FULLY-EXPOSED 
LOW-LEVEL CIRCULATION CENTER (LLCC) THAT IS POSITIONED ALONG THE 
NORTHERN SIDE OF THE STRONG MONSOONAL SOUTHWESTERLIES. AS A RESULT, 
THE WIND FIELD IS HIGHLY ASYMMETRIC WITH THE STRONGEST WINDS CONFINED 
TO THE SOUTHEASTERN QUADRANT. THE INITIAL POSITION IS PLACED WITH HIGH 
CONFIDENCE BASED ON THE WELL-DEFINED LLCC IN ANIMATED MSI. THE INITIAL 
INTENSITY OF 35 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON NEARBY 
SHIP OBSERVATIONS IN THE SOUTH CHINA SEA AND AGENCY DVORAK FIXES OF
AROUND T2.0-2.5. ENVIRONMENTAL ANALYSIS INDICATES THAT 17W IS LOCATED 
WITHIN A MARGINALLY FAVORABLE ENVIRONMENT CHARACTERIZED BY MODERATE 
EQUATORWARD OUTFLOW ALOFT, MODERATE TO HIGH (20-25 KTS) EASTERLY 
VERTICAL WIND SHEAR, AND WARM (29-30 C) SEA SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: THE NORTHERN PERIPHERY OF A NEAR
EQUATORIAL RIDGE (NER) CENTERED OVER MINDANAO

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T1.5 - 25 KTS
   RCTP: T2.5 - 35 KTS
   KNES: T2.5 - 35 KTS
   CIMSS D-MINT: 28 KTS AT 010535Z
   CIMSS D-PRINT: 32 KTS AT 010600Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS PROGNOSTIC REASONING MESSAGE
ESTABLISHES THE FORECAST PHILOSOPHY FOR THE REGENERATED SYSTEM

FORECAST DISCUSSION: TS 17W IS FORECAST TO TRACK NORTHEASTWARD, ALONG 
THE NORTHERN PERIPHERY OF THE NER TO THE SOUTH THROUGH TAU 36. AROUND 
TAU 36, THE SYSTEM WILL ENTER A WEAK STEERING ENVIRONMENT AS THE NER 
MOVES OFF TO THE WEST, CAUSING TRACK SPEEDS TO SLOW. NEAR TAU 48, A 
BUILDING RIDGE OVER EASTERN CHINA IS EXPECTED TO GAIN CONTROL AS THE 
PRIMARY STEERING MECHANISM, TURNING 17W RATHER SHARPLY WESTWARD. 17W 
WILL THEN CONTINUE ON A WESTWARD TRACK THROUGH TAU 72, SKIRTING THE 
COAST OF CHINA, NEAR HONG KONG. IN TERMS OF INTENSITY, 17W IS FORECAST 
TO MAINTAIN ITS CURRENT INTENSITY THROUGH TAU 48 AS THE ENVIRONMENT 
REMAINS MARGINALLY FAVORABLE. MODELS SHOW THE VORTEX REMAINING SHALLOW 
THROUGHOUT ITS LIFE AND NEVER BECOMING VERTICALLY STACKED. THE MID-
LEVEL VORTEX IS SHOWN TO REMAIN DISPLACED FAR TO THE WEST, NEVER 
ALLOWING FOR THE SYSTEM TO INTENSIFY. AFTER TAU 48, TERRAIN 
INTERACTION AND INCREASING EASTERLY SHEAR WILL CAUSE 17W TO WEAKEN 
THROUGH TAU 72. DISSIPATION IS CURRENTLY FORECAST TO OCCUR AROUND TAU 
72, NEAR HONG KONG AS THE REMNANT VORTEX TRACKS OFF TO THE WEST-
SOUTHWEST. IF THE SYSTEM IS ABLE TO STAY OVER WATER RATHER THAN GOING 
INLAND, THERE IS A POTENTIAL FOR MARGINAL INTENSIFICATION.

MODEL DISCUSSION: MODEL TRACK GUIDANCE IS IN GOOD AGREEMENT ONLY 
THROUGH TAU 48, WITH ALL MODELS AGREEING ON A NORTHEASTWARD TRACK. 
AFTER TAU 48, GUIDANCE BEGINS TO SPREAD SIGNIFICANTLY. SOME MODELS
SHOW A MORE QUASISTATIONARY MOVEMENT EAST OF HONG KONG WITH A SHARPER 
TURN TO THE SOUTHWEST, KEEPING THE SYSTEM OVER WATER FOR LONGER. THE 
ECMWF SHOWS THE SHARPEST TURN WHILE THE EC-AIFS TAKES THE WIDEST TURN. 
THE JTWC TRACK FORECAST IS PLACED NEAR THE MULTI-MODEL CONSENSUS 
THROUGH TAU 48 AND THEN CLOSER TO THE EC-AIFS AND EXPERIMENTAL GOOGLE 
DEEPMIND ENSEMBLE MEAN THROUGH TAU 72 WITH OVERALL LOW CONFIDENCE. 
INTENSITY GUIDANCE GENERALLY SHOWS A STEADY INTENSITY THROUGH TAU 24-
36 FOLLOWED BY WEAKENING. THE HAFS-A IS THE QUICKEST TO WEAKEN THE 
SYSTEM, SHOWING THE SYSTEM QUICKLY DROPPING BELOW 30 KTS. THE JTWC 
INTENSITY FORECAST IS PLACED SLIGHTLY HIGHER THAN CONSENSUS THROUGH 
TAU 36 AND THEN FOLLOWS THE CONSENSUS THROUGH TAU 72. THE CONFIDENCE 
IN THE INTENSITY FORECAST IS PLACED WITH LOW CONFIDENCE DUE TO SMALL 
CHANGES IN THE TRACK POTENTIALLY CAUSING SIGNIFICANT CHANGES IN THE 
INTENSITY OF THE SYSTEM.

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