WDPN32 PGTW 210900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 20W (TWENTY) 
WARNING NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 20W (TWENTY), LOCATED APPROXIMATELY 93 
NM WEST OF IWO TO, JAPAN, HAS TRACKED SOUTHWESTWARD AT 04 KNOTS 
(KTS) OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE 
IMAGERY DEPICTS A FULLY EXPOSED, WELL DEFINED LOW LEVEL CIRCULATION 
CENTER (LLCC), LENDING GOOD CONFIDENCE TO THE INITIAL POSITION. 
ADDITIONALLY, A 210540Z GMI 89 GHZ MICROWAVE IMAGE REVEALS A CLEARLY-
DEFINED, SYMMETRIC LLCC WITH WEAK BANDING TO THE NORTH. THE INITIAL 
INTENSITY REMAINS AT 30 KTS BASED ON RECENT ASCAT DATA AND A DVORAK 
INTENSITY ESTIMATE OF T2.0/2.5 (30-35 KTS) BY KNES. ENVIRONMENTAL 
ANALYSIS INDICATES A MARGINAL ENVIRONMENT WITH STRONG (35-40 KTS) 
VERTICAL WIND SHEAR (VWS) BEING OFFSET BY WARM (28-29 CELSIUS) SEA 
SURFACE TEMPERATURES AND ROBUST EQUATORWARD OUTFLOW. TD 20W IS 
CURRENTLY TRACKING ALONG THE SOUTHEASTERN PERIPHERY OF A WEAKENED 
LOW LEVEL SUBTROPICAL RIDGE (STR) POSITIONED TO THE WEST.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS 
PROGNOSTIC REASONING MESSAGE.
   B. IN THE NEAR TERM, TD 20W SHOULD TRACK SLOWLY WESTWARD ALONG 
THE PERIPHERY OF THE WEAKENED STR THROUGH TAU 12. AFTERWARDS, AS A 
SHORTWAVE TROUGH POSITIONED TO THE NORTH PROPAGATES EASTWARD, THE 
STR SHOULD BEGIN TO REBUILD AND REORIENT POLEWARD BY TAU 12. THIS 
WILL ALLOW TD 20W TO TURN AND ACCELERATE NORTHWARD ALONG THE WESTERN 
PERIPHERY OF THE STR THROUGH TAU 36. THROUGH THE FORECAST PERIOD, TD 
20W IS EXPECTED TO GRADUALLY WEAKEN DUE TO PERSISTENT HIGH (40-45 
KTS) VWS UNTIL DISSIPATION BY TAU 36. A DEVELOPING FRONT SOUTHEAST 
OF THE KANTO PLAIN WILL ABSORB THE REMNANTS OF THE SYSTEM AFTER 
DISSIPATION. NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT. HOWEVER, 
DUE TO THE SLIGHTLY ERRATIC TRACK NATURE THROUGH TAU 12, THERE IS 
ONLY OVERALL FAIR CONFIDENCE IN THE JTWC FORECAST TRACK WHICH IS 
PLACED NEAR THE MULTI-MODEL CONSENSUS.//
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