Advisories

WDPN32 PGTW 012100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 17W (SAUDEL) 
WARNING NR 042//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.4N 116.3E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 129 NM EAST-SOUTHEAST OF HONG KONG
   MOVEMENT PAST 6 HOURS: NORTHEASTWARD AT 09 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
DEPRESSION (TD) 17W WITH A PARTIALLY EXPOSED LOW-LEVEL CIRCULATION 
CENTER (LLCC) SURROUNDED BY FLARING CONVECTION THAT REMAINS MOST 
PERSISTENT IN THE SOUTHWEST QUADRANT. WEAK EQUATORWARD OUTFLOW CAN BE 
SEEN IN ANIMATED WATER VAPOR IMAGERY. A 011400Z METOP-B ASCAT WIND 
SPEED IMAGE REVEALED POOR SYMMETRY IN THE LLCC AND A HIGHLY ASYMMETRIC 
WIND FIELD WITH THE STRONGEST WINDS ALONG THE SOUTHEASTERN PERIPHERY, 
ENHANCED BY THE SOUTHWEST MONSOONAL FLOW. THE INITIAL POSITION IS 
PLACED WITH MEDIUM CONFIDENCE BASED ON THE PARTIALLY EXPOSED LLCC IN 
EIR AND EXTRAPOLATION FROM THE ASSESSED CENTER OF THE ASCAT IMAGE. THE 
INITIAL INTENSITY OF 30 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED 
ON A CORRELATION OF THE AGENCY DVORAK FIXES AND THE ASCAT WIND SPEEDS. 
THE ENVIRONMENT IS CHARACTERIZED AS MARGINALLY FAVORABLE WITH WARM SEA 
SURFACE TEMPERATURES BEING OFFSET BY LOW TO MODERATE VERTICAL WIND 
SHEAR (VWS) AND WEAK OUTFLOW ALOFT.

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

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: T2.0 - 30 KTS
   CIMSS ADT: 37 KTS AT 011740Z
   CIMSS AIDT: 39 KTS AT 011740Z
   CIMSS D-MINT: 29 KTS AT 011804Z
   CIMSS D-PRINT: 30 KTS AT 011710Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: WEAK EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: MEDIUM
   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: TD 17W HAS CONTINUED TRACKING NORTHEASTWARD UNDER 
THE STEERING INFLUENCE OF THE NER TO THE SOUTHEAST. THE SYSTEM IS 
FORECAST TO CONTINUE ALONG A NORTHWESTWARD TRACK FOR THE NEXT 12 HOURS 
BEFORE TURNING POLEWARD THEN WESTWARD BETWEEN TAU 24-36 AS IT ENTERS A 
WEAKER STEERING ENVIRONMENT. A RIDGE BUILDING OVER EASTERN CHINA IS 
EXPECTED TO ASSERT SOME CONTROL AROUND TAU 36 AND STEER THE SYSTEM 
SOUTHWESTWARD ALONG THE COAST OF CHINA FOR THE REMAINDER OF THE 
FORECAST PERIOD. THERE IS SOME UNCERTAINTY ABOUT HOW SHARPLY THE TRACK 
WILL TURN SOUTHWESTWARD AND WHETHER THE VORTEX WILL BE JUST ONSHORE OR 
JUST OFFSHORE OF SOUTHEASTERN CHINA. THE CURRENT FORECAST FAVORS THE 
FORMER SCENARIO WHICH IS EXPECTED TO RESULT IN A STEADY INTENSITY 
THROUGH LANDFALL JUST AFTER TAU 36, THEN WEAKENING AND DISSIPATION 
OVER LAND BY TAU 72.

MODEL DISCUSSION: MODEL TRACK GUIDANCE IS IN GENERALLY GOOD AGREEMENT 
ABOUT THE SHORT TERM NORTHEASTWARD TRACK FOLLOWED BY A TURN 
SOUTHWESTWARD AT VARYING DEGREES. BECAUSE OF THE TIMING OF THE 
SOUTHWESTWARD TURN AND UNCERTAINTY IN THE STEERING ENVIRONMENT AFTER 
ABOUT TAU 24, THE JTWC TRACK IS PLACED WITH LOW CONFIDENCE CLOSER TO 
THE GROUPING OF AI GUIDANCE THAT FAVORS LANDFALL. THE INTENSITY 
GUIDANCE HAS MEDIUM CONFIDENCE FOR THE SAME REASON, AS ANY TRACK 
REMAINING OVER WATER GIVES THE POTENTIAL FOR SOME INTENSIFICATION IN 
THE LATER TAUS, HOWEVER, THE JTWC INTENSITY FORECAST REMAINS ON THE 
HIGHER END OF AVAILABLE GUIDANCE.

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