WDPN32 PGTW 130300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 25W (VAMCO) WARNING NR 016//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 25W (VAMCO), LOCATED APPROXIMATELY 417 NM EAST
OF DA NANG, VIETNAM, HAS TRACKED WEST-NORTHWESTWARD AT 08 KNOTS
OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY 
(MSI) DEPICTS TIGHTLY CURVED BANDS OF CONVECTION WRAPPING INTO AN
OBSCURED LOW LEVEL CIRCULATION CENTER WITH THE EYE FEATURE FILLING
OVER THE PAST 6 HOURS. THE INITIAL POSITION IS PLACED WITH HIGH 
CONFIDENCE BASED ON THE MICROWAVE EYE FEATURE PRESENT IN A 122317Z
SSMIS 37 GHZ IMAGE. THE INITIAL INTENSITY OF 75 KTS IS PLACED WITH 
FAIR CONFIDENCE BASED ON A PGTW DVORAK CURRENT INTENSITY ESTIMATE
OF T4.5 (77 KTS). TY VAMCO IS TRACKING ALONG THE SOUTHERN PERIPHERY 
OF A SUBTROPICAL RIDGE (STR) POSITIONED TO THE NORTHEAST AND THROUGH 
A FAVORABLE ENVIRONMENT WITH LOW (5-10 KTS) VERTICAL WIND SHEAR, 
WARM (27-28 CELSIUS) SEA SURFACE TEMPERATURES (SST) AND WELL 
ESTABLISHED POLEWARD AND EQUATORWARD OUTFLOW ALOFT. 
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS 
PROGNOSTIC REASONING MESSAGE.
   B. TY 25W WILL CONTINUE TO TRACK GENERALLY WESTWARD TO WEST-
NORTHWESTWARD THROUGH TAU 24 UNDER THE STEERING INFLUENCE OF THE 
AFOREMENTIONED STR. DURING THIS TIME, THE ENVIRONMENT WILL SLOWLY 
DEGRADE AS VWS BECOMES MODERATE (10-15 KTS) AND SST BEGIN TO DROP 
(26-27 CELSIUS). AS A RESULT, THE ENVIRONMENT WILL NOT BE CONDUCIVE 
FOR FURTHER INTENSIFICATION BUT WILL SUSTAIN THE 75 KTS INTENSITY 
THROUGH TAU 24. AFTER TAU 24, TY VAMCO WILL ASSUME A MORE 
NORTHWESTWARD TRACK AS IT APPROACHES A WEAKNESS IN THE STEERING 
RIDGE. THE ENVIRONMENT WILL CONTINUE TO BECOME MORE UNFAVORABLE AS 
SST COOL (< 26 CELSIUS) AND VWS CONTINUES TO BE MODERATE. THESE 
CONDITIONS WILL WEAKEN THE SYSTEM TO 60 KTS BY TAU 48 AT WHICH TIME 
IT WILL BEGIN TO DISSIPATE OVER WATER. AFTER TAU 48, THE SYSTEM WILL 
MAKE LANDFALL NORTH OF HUE, VIETNAM. AS TY 25W TRACKS OVER THE 
RUGGED TERRAIN OF NORTHERN VIETNAM AND INTO LAOS IT WILL BEGIN TO 
RAPIDLY DECAY DUE TO INCREASING VWS AND TERRAIN INTERACTION. BY TAU 
72, THIS WEAKENING WILL LEAD TO FULL DISSIPATION OVER LAND. 
NUMERICAL MODEL TRACK GUIDANCE IS IN FAIR AGREEMENT THROUGH TAU 36 
BUT YIELDS POOR GUIDANCE THEREAFTER AS MODELS DIVERGE. FOR EXAMPLE, 
THERE IS A 300 NM SPREAD IN TRACK SOLUTIONS AT LANDFALL. THIS LARGE 
SPREAD IS DUE LARGELY TO THE UKMET ENSEMBLE TRACK WHICH BRINGS THE 
VORTEX FARTHER NORTH TOWARDS THE BORDER OF VIETNAM AND CHINA. WHEN 
THIS TRACK IS REMOVED, THE SPREAD IN MODEL SOLUTIONS AT LANDFALL 
DECREASES TO 200 NM. THIS LARGE SPREAD IN NUMERICAL MODEL TRACK 
GUIDANCE LENDS OVERALL FAIR CONFIDENCE IN THE JTWC FORECAST TRACK 
WHICH IS PLACED NEAR THE MULTI-MODEL CONSENSUS.//
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