WDPN32 PGTW 201500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 20W (TWENTY) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 20W (TWENTY), LOCATED APPROXIMATELY 122 
NM WEST-NORTHWEST OF IWO TO, JAPAN, HAS TRACKED NORTHEASTWARD AT 05 
KNOTS(KTS) OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED 
SATELLITE IMAGERY REVEALS A PARTIALLY EXPOSED LOW LEVEL CIRCULATION 
CENTER (LLCC) WITH FLARING DEEP CONVECTION LOCATED OVER THE EASTERN 
SEMI-CIRCLE. A 201034Z ASCAT-A IMAGE REVEALS A DEFINED LLCC WITH A 
SMALL SWATH OF 30KT WINDS TO THE SOUTHEAST AND 25-29KT WINDS 
WRAPPING AROUND THE EASTERN QUADRANT, LENDING GOOD CONFIDENCE TO THE 
INITIAL INTENSITY. THE INTENSITY IS FURTHER SUPPORTED BY T1.5 (25 
KTS) BY KNES. ENVIRONMENTAL ANALYSIS REVEALS TD 20W IS CURRENTLY IN 
A MARGINALLY FAVORABLE ENVIRONMENT FOR DEVELOPMENT WITH WARM (28-29 
CELSIUS) SEA SURFACE TEMPERATURES AND ESTABLISHED EQUATORWARD 
OUTFLOW ALOFT BEING OFFSET BY HIGH (30-35 KT) VERTICAL WIND SHEAR 
(VWS). ADDITIONALLY, THE PROXIMITY TO TS 19W IS APPLYING PRESSURE TO 
THE WESTERN SEMICIRCLE, LIMITING THE FORMATION OF DEEP CONVECTION. 
DUE TO A EASTWARD-PROPAGATING TROUGH TO THE NORTH, TD 20W IS LOCATED 
WITHIN A WEAKENED AREA OF A SUBTROPICAL RIDGE (STR) POSITIONED TO 
THE EAST, CONTRIBUTING TO AN OVERALL WEAK STEERING ENVIRONMENT.
3. FORECAST REASONING.
   A. THIS IS THE FIRST PROGNOSTIC REASONING MESSAGE AND SETS THE 
FORECAST PHILOSOPHY.
   B. WITHIN THIS WEAK STEERING ENVIRONMENT, NUMERICAL MODEL 
GUIDANCE DEPICTS TD 20W REMAINING QUASI-STATIONARY THROUGH TAU 24 AS 
THE SYSTEM CONDUCTS A SLOW LOOP PRIOR TO TURNING GENERALLY 
NORTHWARD. AFTERWARDS, WITH THE PASSAGE OF THE TROUGH TO THE EAST, 
THE STR IS EXPECTED TO BUILD BACK IN, CAUSING THE SYSTEM TO 
ACCELERATE NORTHWARD WHERE TD 20W IS FORECAST TO DISSIPATE AS IT 
INTERACTS WITH AND BECOMES EMBEDDED WITHIN THE WESTERLY FLOW. THERE 
IS SOME SPREAD BETWEEN THE NUMERICAL MODEL GUIDANCE, PLACING OVERALL 
FAIR CONFIDENCE IN THE JTWC FORECAST TRACK WHICH LIES NEAR THE MULTI-
MODEL CONSENSUS AND THE GFS AND ECMWF TRACK SOLUTIONS.//
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