WDPN31 PGTW 250900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 13W (THIRTEEN) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 15.9N 109.5E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 69 NM EAST OF DA NANG, VIETNAM
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 15 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
DEPRESSION 13W (THIRTEEN) AS HAVING A TIGHT MESOVORTEX AMONG A BROAD 
AREA OF CYCLONIC TURNING SPANNING THE ENTIRETY OF THE SOUTH CHINA SEA, 
WITH UPPER-LEVEL FORMATIVE BANDS OF CONVECTION ALONG THE NORTHERNMOST 
AND SOUTHERNMOST PERIPHERIES. PERSISTENT FLARING CONVECTION NEAR THE 
LOW-LEVEL CIRCULATION CENTER (LLCC) QUICKLY DRIFTS SOUTHWESTWARD ALOFT 
WITH MUCH OF THE HIGH CLOUD ETAGE EXPERIENCING SIGNIFICANT 
NORTHEASTERLY SHEAR. A 250232Z ASCAT-C SCATTEROMETRY IMAGE SHOWS 25 
KNOT WINDS SURROUNDING THE ENTIRE EASTERN PERIPHERY, EXTENDING UP TO 
40NM OUTWARDS OF THE LLCC BUT FAILED TO CAPTURE DATA WEST OF THE LLCC. 
LOW-LEVEL FORMATIVE SPIRAL BANDING IS OBSERVED WRAPPING INTO THE LLCC 
IN A 250554Z AMSR2 36GHZ MICROWAVE IMAGE. THE INITIAL POSITION IS 
PLACED WITH MEDIUM CONFIDENCE BASED ON A 250600Z HIMIWARI-9 AHI 1KM 
VISIBLE IMAGE. THE INITIAL INTENSITY OF 25 KTS IS ASSESSED WITH HIGH
CONFIDENCE BASED ON THE PREVIOUSLY MENTIONED SCATTEROMETRY IMAGE.

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

CURRENT STEERING MECHANISM: RIDGING OVER SOUTHERN CHINA.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.0 - 25 KTS
   RJTD: T1.0 - 25 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: MODERATE POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: TROPICAL DEPRESSION 13W (THIRTEEN) IS FORECAST
TO BE A SHORT-LIVED (ABOUT 24 HOURS) SYSTEM OF LOW TROPICAL
DEPRESSION INTENSITY.  PRIOR TO MAKING LANDFALL NEAR HUE, VIETNAM,
THE SYSTEM IS ANTICIPATED TO INTENSIFY ON A NORTHWESTWARD TRACK IN
A FAVORABLE ENVIRONMENT WITH WARM SEA SURFACE TEMPERATURES (30-31
DEGREES CELSIUS) AND LOW VERTICAL WIND SHEAR (10-15 KNOTS). JUST
AFTER LANDFALL, HOWEVER, THE SYSTEM WILL SLOWLY DECAY, MOVING
WESTWARD OVER LAND UNTIL DISSIPATION OCCURS BETWEEN TAU 24 AND TAU
36. THE FORECAST CONFIDENCE FOR BOTH TRACK AND INTENSITY IS MEDIUM.

MODEL DISCUSSION: GLOBAL NUMERICAL MODELS ARE IN EXCEPTIONAL
AGREEMENT THAT TROPICAL DEPRESSION 13W (THIRTEEN) WILL TRACK
NORTHWEST THROUGH TAU 12, AND THEN SHIFT TO A MORE WESTWARD TRACK
POST TAU 12 THROUGH DISSIPATION.  INTENSITY GUIDANCE IS IN GOOD
AGREEMENT THAT THIS SYSTEM WILL PEAK AROUND 30 KTS BEFORE
IT MAKES LANDFALL AND WILL BEGIN DISSIPATION THEREAFTER.  CROSS-TRACK
AND ALONG-TRACK SPREADS ARE NEGLIGIBLE, BUT A HANDFUL OF GFS AND ECMWF 
ENSEMBLE MEMBERS (AROUND 10) RESOLVE TD 13W TRACK TO STAY OVER WATER
LONGER, ALLOWING SLIGHTLY MORE TIME FOR INTENSIFICATION TO OCCUR
BEFORE LANDFALL. BOTH CONTRIBUTE TO THE FORECAST TRACK AND
INTENSITY CONFIDENCE OF MEDIUM.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM//
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