WDPN32 PGTW 122100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 25W (VAMCO) WARNING NR 015//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 25W (VAMCO), LOCATED APPROXIMATELY 465 NM EAST
OF DA NANG, VIETNAM, HAS TRACKED WESTWARD AT 11 KNOTS OVER THE
PAST SIX HOURS. ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY 
DEPICTS A TRANSIENT EYE FEATURE SURROUNDED BY TIGHTLY CURVED BANDS
OF CONVECTION. THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE
BASED ON THE PRESENCE OF THE EYE FEATURE IN THE EIR LOOP. THE 
INITIAL INTENSITY OF 75 KTS IS PLACED WITH FAIR CONFIDENCE AND IS
HEDGED HIGHER THAN MULTI-AGENCY DVORAK CURRENT INTENSITY ESTIMATES
OF T4.0 (65 KTS, PGTW) AND T3.5 (55 KTS, RJTD) DUE TO CONSISTENTLY
HIGHER INTENSITY TRENDS NOTED OVER THE PAST 6HOURS FROM OBJECTIVE
DVORAK METHODS, AND A 86 KTS INTENSITY CALCULATED IN A 121347Z 
CIMSS SATELLITE CONSENSUS INTENSITY ESTIMATE. 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 (28-20 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 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 322 
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 233 
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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