WDPN33 PGTW 131500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (NANGKA) 
WARNING NR 008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 18W (NANGKA), LOCATED APPROXIMATELY 
286 NM EAST-SOUTHEAST OF HANOI, VIETNAM, HAS TRACKED WEST 
NORTHWESTWARD AT 08 KNOTS OVER THE PAST SIX HOURS. ANIMATED
ENHANCED INFRARED SATELLITE IMAGERY DEPICTS A STRENGTHENED 
SYSTEM, WITH CONVECTION HAVING CONSOLIDATED AND DEEPENED 
NEAR THE ASSESSED LLCC OVER THE PAST FEW HOURS. RADAR 
IMAGERY DEPICTS THAT THE SYSTEM MADE LANDFALL VERY NEAR THE 
131200Z HOUR JUST SOUTH OF QIONGHAI, CHINA. THE RADAR IMAGERY 
ALSO DEPICTS A VERY CLEAR, ALMOST EYE LIKE FEATURE AS THE 
SYSTEM MADE LANDFALL, WHICH WHEN COMBINED WITH A 131040Z 
SSMIS 36 GHZ MICROWAVE IMAGE, PROVIDED HIGH CONFIDENCE TO 
THE INITIAL POSITION. THE INITIAL INTENSITY HAS BEEN UPGRADED
TO 50 KNOTS WITH MODERATE CONFIDENCE, HEDGED UPWARD FROM 
DVORAK CURRENT INTENSITY ESTIMATES OF T2.5 FROM BOTH PGTW AND
RJTD, IN LIGHT OF AN ADVANCED DVORAK TECHNIQUE ESTIMATE OF 
T3.3 (51 KNOTS) AND A SATELLITE CONSENSUS ESTIMATE OF 51 
KNOTS AT 0600Z. COMBINING THE AUTOMATED ANALYZES WITH THE 
OVERALL STRUCTURE EVIDENT IN THE RADAR LEADS TO THE HIGHER 
INTENSITY ESTIMATE. ENVIRONMENTAL ANALYSIS INDICATES THE 
SYSTEM LIES IN A GENERALLY FAVORABLE ENVIRONMENT WITH LOW 
(10-15KT) VERTICAL WIND SHEAR, AND GOOD DIVERGENT WESTWARD 
OUTFLOW. THE CYCLONE IS TRACKING ALONG THE SOUTHWESTERN 
PERIPHERY OF A DEEP-LAYERED SUBTROPICAL RIDGE (STR) TO THE 
NORTH.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS 
PROGNOSTIC REASONING MESSAGE.
   B. TS 18W HAS MADE LANDFALL ALONG THE EAST COAST OF HAINAN 
ISLAND AS PREVIOUSLY DISCUSSED. THE TRACK OVER THE PAST SIX 
HOURS HAS BEEN SLIGHTLY MORE NORTH THAN PREVIOUSLY FORECAST IN 
RESPONSE TO DEVELOPMENT OF A SMALL WEAKNESS AND SHIFT IN 
ORIENTATION IN THE STR. THE FORECAST TRACK IS EXPECTED TO
FLATTEN TO MORE WESTWARD OVER THE NEXT 12 TO 24 HOURS AS THE 
STR BECOMES MORE EAST-WEST ORIENTED, AND THE LLCC IS FORECAST 
TO REEMERGE OVER THE SOUTHERN GULF OF TONKIN BY TAU 12. A CLOSE 
TO DUE WESTWARD TRACK IS FORECAST THEREAFTER THROUGH LANDFALL
ALONG THE VIETNAMESE COASTLINE AROUND TAU 30. NOW THAT THE 
SYSTEM IS OVER HAINAN, A SHORT-TERM WEAKENING IS EXPECTED DUE 
PRIMARILY TO TERRAIN INTERACTION. ONCE THE SYSTEM AGAIN MOVES 
OVER WATER, ANOTHER ROUND OF INTENSIFICATION WILL OCCUR AS THE 
SYSTEM TAPS INTO DIVERGENT WESTWARD OUTFLOW AND TRANSITS VERY 
WARM (NEAR 30 DEG CELSIUS) WATERS WITH LOW VWS, WITH A 
FORECASTED PEAK OF 55 KNOTS AT TAU 24. AFTER MAKING LANDFALL 
OVER NORTHERN VIETNAM, THE SYSTEM WILL RAPIDLY WEAKEN OVER THE 
MOUNTAINOUS TERRAIN OF VIETNAM AND WILL DISSIPATE AS A TROPICAL 
CYCLONE OVER NORTHERN LAOS BY TAU 72. NUMERICAL MODELS REMAIN 
IN GOOD AGREEMENT THOUGH THE ENTIRE FORECAST PERIOD WITH ONLY 
MINIMAL SPREAD AFTER LANDFALL IN VIETNAM. THE JTWC FORECAST 
TRACK LIES ON THE NORTHERN SIDE OF THE GUIDANCE ENVELOPE 
THROUGH THE FORECAST PERIOD WITH HIGH CONFIDENCE.//
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