WDPN31 PGTW 241500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 18W (EIGHTEEN) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 27.9N 135.1E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 502 NM SOUTH-SOUTHWEST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: EAST-NORTHEASTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY AND A 241154Z MHS 
89GHZ MICROWAVE IMAGE DEPICT IMPROVED TC STRUCTURE, INCLUDING A LOW 
LEVEL CIRCULATION CENTER (LLCC) THAT IS NO LONGER EXPOSED WITH DEEP 
CONVECTION BUILDING OVER THE CENTER, LOW LEVEL CLOUD LINES WRAPPING 
INTO THE CENTER ALONG THE NORTHERN AND SOUTHERN SEMICIRCLES, AND 
CONVECTIVE BANDING ORGANIZING ALONG THE EASTERN SEMICIRCLE. RECENT 
SCATTEROMETRY DATA SHOWS A BELT OF 25-30 KTS ACROSS THE SOUTHEASTERN 
QUADRANT UNDER THE AREA OF DEEPEST CONVECTION. THE INITIAL POSITION IS 
PLACED WITH MEDIUM CONFIDENCE BASED ON THE SCATTEROMETRY AND ANIMATED 
EIR IMAGERY. THE INITIAL INTENSITY OF 30 KTS IS ASSESSED WITH MEDIUM 
CONFIDENCE HEDGED ABOVE THE SUBJECTIVE AGENCY DVORAK ESTIMATES BASED 
ON SCATTEROMETRY DATA. IT SHOULD ALSO BE NOTED THAT PGTW MEASURED A 
DATA-T OF T2.0, WHICH WAS CONSTRAINED BY THE MET TO THE FINAL-T 
ESTIMATE OF T1.5.   

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: COMPETING STEERING FLOW IN A COL
BETWEEN SUBTROPICAL RIDGES TO THE WEST AND EAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   KNES: T1.5 - 25 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 27-29 CELSIUS
   OUTFLOW: WEAK EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: THE SYSTEM IS ENJOYING A MARGINALLY FAVORABLE
ENVIRONMENT FOR A SHORT DURATION ONLY AS IT DEVELOPS UNDERNEATH
WEAK RIDGING AND DIFFLUENT FLOW ALOFT. THE TRACK MOTION IS SLOW AND
ERRATIC DUE TO ITS CURRENT LOCATION IN A COL AREA BETWEEN TWO
COMPETING SUBTROPICAL RIDGES TO THE EAST AND WEST. AN UPSTREAM
TROUGH WILL DIG INTO THE LONG WAVE PATTERN, ERODING THE RIDGE TO
THE WEST, AT WHICH POINT THE RIDGE TO THE EAST WILL START TO BECOME
THE MORE DOMINANT STEERING MECHANISM BETWEEN TAU 24 AND TAU 36.
THIS WILL DRIVE THE SYSTEM GENERALLY POLEWARD AND EVENTUALLY
ACCELERATE NORTHEASTWARD, TOPPING OUT AT AROUND A 35-KNOT
INTENSITY. AS THE SYSTEM MIGRATES AWAY FROM LOWER LATITUDES, IT
WILL QUICKLY ENCOUNTER HARSH VERTICAL WIND SHEAR VALUES OF GREATER
THAN 30 KNOTS, BECOME FULLY ENTRENCHED IN DRY AIR, AND CEASE ANY
FURTHER TROPICAL DEVELOPMENT. THE INCREASINGLY HOSTILE ENVIRONMENT
WILL BRING ABOUT DISSIPATION OF 18W BY TAU 72.

MODEL DISCUSSION: GLOBAL ENSEMBLE MODEL GUIDANCE AGREES WITH
INITIAL ERRATIC MOTION FOLLOWED BY AN ACCELERATING TRACK TO THE
NORTHEAST. ADDITIONAL TRACK GUIDANCE INITIALLY STRUGGLED WITH THE
WEAK STEERING ENVIRONMENT BUT HAS RECENTLY COME INTO BETTER
AGREEMENT. THE INTENSITY MODEL GUIDANCE HAS A FAIR HANDLE ON THE 
AMOUNT OF DEVELOPMENT THAT CAN BE EXPECTED, AND THE JTWC FORECAST 
DIVERGES FROM THE GUIDANCE IN THE LATER TAUS DUE TO THE ANALYZED 
LIKELIHOOD OF A DISSIPATION SCENARIO. THERE REMAINS A POSSIBILITY THAT 
THE SYSTEM COULD HOLD TOGETHER AND REINTENSIFY AS AN EXTRA-TROPICAL 
LOW EAST OF TOKYO.

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM//
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