Advisories

WDPN32 PGTW 251500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 18W (NARRA) WARNING 
NR 020//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.5N 108.9E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 191 NM EAST-SOUTHEAST OF HANOI, VIETNAM
   MOVEMENT PAST 6 HOURS: EASTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
THE FULLY EXPOSED LOW-LEVEL CIRCULATION CENTER (LLCC) IS RAPIDLY
UNRAVELING AS IT MAKES LANDFALL OVER HAINAN. PRIOR TO MAKING
LANDFALL, A 251053Z WSF-M MWI 37 GHZ IMAGE DETECTED A VERY COMPACT
LOWER-LEVEL CORE, WHICH SUPPORTED THE 250600Z INTENSITY OF 40 KTS.
SINCE THEN, THE CIRCULATION HAS BEEN DEVOID OF CONVECTION AND HAS
BEEN SPINNING DOWN. THE ENVIRONMENT IS HOSTILE DUE TO 30-35 KTS OF
NORTHEASTERLY VERTICAL WIND SHEAR (VWS). THE INITIAL POSITION IS
ASSESSED WITH HIGH CONFIDENCE BASED ON THE ANIMATED
SHORTWAVE-INFRARED AND RADAR IMAGERY. THE INITIAL INTENSITY HAS
BEEN LOWERED TO 30 KTS WITH MEDIUM CONFIDENCE BASED ON THE
SUBJECTIVE DVORAK ESTIMATES OF T2.0.

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

CURRENT STEERING MECHANISM: LOW- TO MID-LEVEL SOUTHWESTERLY FLOW

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: NO CURRENT ESTIMATE
   RJTD: T2.0 - 30 KTS
   RCTP: T2.0 - 30 KTS
   DEMS: T2.0 - 30 KTS
   CIMSS SATCON: 35 KTS AT 251210Z
   CIMSS ADT: 33 KTS AT 251210Z
   CIMSS AIDT: 32 KTS AT 251210Z
   CIMSS D-MINT: 28 KTS AT 250857Z
   CIMSS D-PRINT: 28 KTS AT 251210Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY UNFAVORABLE
   VWS: 30+ KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE WESTWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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: THE SHALLOW VORTEX OF TROPICAL DEPRESSION (TD)
18W IS EMBEDDED WITHIN LOW- TO MID-LEVEL SOUTHWESTERLY FLOW, WHICH
WILL STEER THE CIRCULATION TOWARD THE NORTHEAST. A POLEWARD TURN
WILL COMMENCE AT TAU 12 AS THE SHALLOW SOUTHWESTERLY WINDS ORIENT
SOUTH-SOUTHWESTERLY ON THE SOUTHERN SIDE OF A MONSOON TROUGH OVER
SOUTHERN CHINA. WITH THE LACK OF CONVECTION, CONTINUED LAND
INTERACTION, AND STRONG VWS, TD 18W WILL CONTINUE TO WEAKEN. WHILE
THE FORECAST GOES OUT TO TAU 36, IT IS QUITE POSSIBLE THAT THE
CIRCULATION WILL DISSIPATE SOONER. THE HOSTILE SHEAR WILL PERSIST
FOR THE NEXT 24 HOURS, AFTER WHICH TIME THE SYSTEM WILL HAVE
PROGRESSED INLAND AND THEREFORE WILL BE UNABLE TO TAP INTO THE MORE
FAVORABLE ENVIRONMENTAL CONDITIONS.

MODEL DISCUSSION: THE TRACK GUIDANCE ALL DEPICT A NORTHEAST TO
NORTH-NORTHEAST TRAJECTORY THROUGH TAU 36. THE SPREAD INCREASES
QUICKLY, THOUGH THIS PHENOMENON IS LIKELY THE RESULT OF MODELS
HAVING DIFFICULTY TRACKING A POORLY DEFINED LLCC OR A TROUGH-LIKE
FEATURE. THE CIRCULATION HAS BEEN ON THE EQUATORWARD SIDE OF THE
GUIDANCE ENVELOPE. THEREFORE, THE JTWC TRACK FORECAST LEANS TOWARD
THE EQUATORWARD EDGE, ALIGNING MOST CLOSELY WITH THE GFS AND THE
EXPERIMENTAL GOOGLE DEEPMIND SOLUTIONS. THE INTENSITY AIDS AGREE ON
A STEADY TO RAPID WEAKENING TREND, WHICH TRACKS WITH THE FORECAST
PHILOSOPHY. THE JTWC INTENSITY FORECAST IS SET NEAR THE CENTER OF
THE GUIDANCE ENVELOPE THROUGH TAU 36.

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