WDPN31 PGTW 250900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (CIMARON) WARNING NR 
004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
 
SUMMARY:
   INITIAL POSITION: 28.2N 133.9E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 518 NM SOUTHWEST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: WESTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET
 
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TS 18W (CIMARON) REACHED ITS PEAK FORECASTED INTENSITY OF 35 KTS
AND QUICKLY BECAME DECOUPLED, AS DEPICTED BY ANIMATED MULTISPECTRAL
SATELLITE IMAGERY (MSI). A WELL-DEFINED, WELL-WRAPPED LOW LEVEL
CIRCULATION CENTER (LLCC) BECAME FULLY EXPOSED, TRACKING WESTWARD,
WHILE THE ASSOCIATED DEEP CONVECTION AND DIFFUSE RADIAL OUTFLOW
SHEARED TO THE EAST. MODEST CONVECTION IS OBSERVED ON ANIMATED MSI
IN THE INNERMOST EASTERN SEMICIRCLE BUT IS ALSO SHEARING EAST. DUE
TO THE FULLY EXPOSED LLCC, THE INITIAL POSITION IS PLACED WITH HIGH
CONFIDENCE. THE INITIAL INTENSITY OF 35 KTS IS ALSO ASSESSED WITH
HIGH CONFIDENCE BASED ON AN EARLIER 250114Z ASCAT IMAGE THAT
REVEALED A BROAD PATCH OF 35 KTS LOCATED IN THE NORTHWEST QUADRANT
WITH A FEW ISOLATED MEASUREMENTS OF 40 KTS UNDER CONVECTION AND
CONTINUES TO BE SUPPORTED BY OBJECTIVE INTENSITY ESTIMATES RANGING
FROM 31 TO 39 KTS.
 
INITIAL WIND RADII BASIS: SCATTEROMETER DATA
 
CURRENT STEERING MECHANISM: WEAK STEERING DUE TO A COL BETWEEN
SUBTROPICAL RIDGES TO THE WEST AND EAST.
 
 
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T2.0 - 30 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.0 - 30 KTS
   CIMSS SATCON: 39 KTS AT 250127Z
   CIMSS ADT: 37 KTS AT 250530Z
 
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: WEAK RADIAL
 
ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: HIGH
 
3. FORECAST REASONING.
 
SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.
 
FORECAST DISCUSSION: THE SYSTEM IS DRIFTING SLOWLY WESTWARD DUE TO
A WEAK STEERING ENVIRONMENT AS IT TRACKS IN A COL REGION BETWEEN
TWO SUBTROPICAL RIDGES TO THE WEST AND EAST. THE ENVIRONMENT IS
ASSESSED AS MARGINALLY FAVORABLE BECAUSE THE VERTICAL WIND SHEAR
(VWS) HAS TEMPORARILY DECREASED AND SEA SURFACE TEMPERATURES REMAIN
CONDUCIVE FOR TROPICAL DEVELOPMENT. DIFFLUENCE ALOFT HAS ALLOWED
THE SYSTEM TO INTENSIFY, BUT THE UPPER LEVEL ENVIRONMENT IS
EXPECTED TO BECOME INCREASINGLY UNFAVORABLE AS STRONG SOUTHWESTERLY
FLOW APPROACHES DUE TO A TROUGH DIGGING INTO THE LONG WAVE PATTERN
NORTHWEST OF THE SYSTEM. THIS TROUGH WILL CAUSE THE RIDGE TO THE
WEST TO ERODE, AT WHICH POINT THE RIDGE TO THE EAST WILL BECOME THE
DOMINANT STEERING MECHANISM AROUND TAU 36. A NORTHEASTWARD
ACCELERATION WILL OCCUR NEAR TAU 48, BUT THIS WILL ALSO CAUSE THE
SYSTEM TO ELONGATE AND ENTRAIN MORE DRY AIR FROM THE WEST. HENCE,
THE SYSTEM WILL BEGIN TO SHOW SIGNS OF EXTRATROPICAL TRANSITION
AFTER TAU 48 AND BECOME FULLY EXTRATROPICAL BY TAU 72. DUE TO THE
WEAK NATURE OF THIS STORM, THERE IS A STRONG POSSIBILITY IT WILL
DISSIPATE PRIOR TO UNDERGOING EXTRATROPICAL TRANSITION.
 
MODEL DISCUSSION: TRACK GUIDANCE HAS COME INTO BETTER AGREEMENT,
WITH CONSENSUS AROUND THE INITIAL WESTWARD DRIFT AND AN EVENTUAL
TURN AND ACCELERATION TO THE NORTHEAST. TRACK SPEEDS VARY, WITH THE
MORE RELIABLE MODELS FAVORING A SLIGHTLY SLOWER TRACK AND HOLDING
THE SYSTEM OFFSHORE AS IT NEARS TOKYO. THE ECMWF ENSEMBLE (ECENS)
HAS A WIDER TRACK SPREAD EARLY IN THE FORECAST PERIOD DUE TO THE
WEAK STEERING ENVIRONMENT AND UNCERTAINTY ABOUT STORM DEPTH BUT
SHOWS A TIGHTENING CONSENSUS AFTER TAU 48. CONVERSELY, THE GFS
ENSEMBLE (GEFS) HAS MUCH LESS CERTAINTY IN THE TRACK AFTER TAU 48,
LIKELY DUE TO A WEAKENING SIGNAL AND POINTING TOWARDS A DISSIPATION
SCENARIO. THE INTENSITY GUIDANCE HAS CONSIDERABLE UNCERTAINTIES AND
OVERALL POOR AGREEMENT, WITH THE JTWC FORECAST UNDERCUTTING THE
CONSENSUS MEAN AT TAU 24 THROUGH TAU 48 DUE TO THE HIGH LIKELIHOOD
OF DISSIPATION. IF THE SYSTEM DOES MANAGE TO HOLD TOGETHER AS
FORECASTED, THE INTENSITY IS EXPECTED TO INCREASE CLOSER TO THE
CONSENSUS MEAN ONCE THE SYSTEM BECOMES FULLY EXTRATROPICAL.
 
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: HIGH
   INTENSITY 72-120 HR: MEDIUM//
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