Tropical Storm VAMCO Advisory 11月13日

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 6 HOURS 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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