WDPN31 PGTW 252100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 18W (CIMARON) 
WARNING NR 006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 27.9N 132.8E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 567 NM SOUTHWEST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
DEPRESSION 18W (CIMARON) WITH A WELL-DEFINED, EXPOSED LOW-LEVEL
CIRCULATION CENTER (LLCC). SPARSE FLARING CONVECTION CAN BE SEEN
ALONG THE EASTERN SEMICIRCLE OF THE CIRCULATION BLOWING OFF TO THE
EAST DUE TO THE STRONG WESTERLY VERTICAL WIND SHEAR. ENVIRONMENTAL
ANALYSIS INDICATES THAT 18W IS IN A MARGINAL ENVIRONMENT FOR
FURTHER DEVELOPMENT CHARACTERIZED BY WEAK OUTFLOW ALOFT, MODERATE
TO HIGH (20-25 KTS) VERTICAL WIND SHEAR OFFSET BY WARM (28-29C) SEA
SURFACE TEMPERATURES. THE INITIAL POSITION IS PLACED WITH HIGH
CONFIDENCE BASED ON THE COMPLETELY EXPOSED LLCC IN THE ANIMATED EIR
SATELLITE IMAGERY. THE INITIAL INTENSITY OF 30 KTS IS ASSESSED WITH
MEDIUM CONFIDENCE BASED ON THE AGENCY DVORAK ESTIMATES AND
OBJECTIVE INTENSITY ESTIMATES LISTED BELOW.

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

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: T1.5 - 25 KTS
   CIMSS ADT: 25 KTS AT 251800Z
   CIMSS AIDT: 34 KTS AT 251800Z
   CIMSS DPRINT: 25 KTS AT 251800Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: WEAK EASTWARD
   OTHER FACTORS: DRY AIR OVER THE WESTERN PORTION OF THE
CIRCULATION.

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: 18W IS FORECAST TO CONTINUE DRIFTING GENERALLY
WESTWARD OVER THE NEXT 12 HOURS AS IT REMAINS IN THE WEAK STEERING
ENVIRONMENT. BETWEEN TAU 12 AND 24 THE SUBTROPICAL RIDGE (STR) TO
THE WEST WILL GET BROKEN DOWN BY AN INCOMING TROUGH IN THE LONGWAVE
PATTERN WHICH WILL CAUSE THE EASTERN STR TO BECOME THE PRIMARY
STEERING MECHANISM. AFTER TAU 24, 18W IS FORECAST TO BEGIN QUICKLY
TRACKING NORTHEASTWARD, ALONG THE WESTERN PERIPHERY OF THE STR TO
ITS EAST AS IT APPROACHES THE BAROCLINIC ZONE. EXTRATROPICAL
TRANSITION (ETT) IS FORECAST TO BE COMPLETED AROUND TAU 72 AS THE
SYSTEM TRACKS ALONG THE COAST OF JAPAN. HIGH VERTICAL WIND SHEAR
VALUES AND LOWERING SST WILL INHIBIT ANY FURTHER DEVELOPMENT EXCEPT
A SLIGHT INCREASE BACK UP TO 30 KTS AS THE SYSTEM BEGINS
STRENGTHENING AS AN EXTRATROPICAL LOW. WITH ALL THIS IN MIND, THERE
IS ANOTHER, QUITE DIFFERENT POSSIBILITY FOR 18W. DUE TO THE DRY AIR
AND HIGH VERTICAL WIND SHEAR, 18W COULD POSSIBLY COMPLETELY
DISSIPATE AROUND TAU 36-48 SOUTH TO SOUTHEAST OF KYUSHU. AS MODEL
GUIDANCE CONTINUES TO COME IN, THERE IS A POSSIBILITY THAT A CHANGE
TO THE FORECAST PHILOSOPHY OF 18W OCCURS, DEPICTING A DISSIPATION
SCENARIO.

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN OVERALL POOR
AGREEMENT REGARDING THE TRACK OF 18W. ENSEMBLES ARE BEGINNING TO
HAVE SIGNIFICANT VARIANCE BETWEEN MEMBERS. THE ECMWF ENSEMBLE HAS
MANY MEMBERS DISSIPATE SOUTHEAST OF KYUSHU. THE GFS ENSEMBLE IS
BEGINNING TO HAVE MORE MEMBERS DO A SIMILAR DISSIPATION BUT REMAINS
WITH MANY MEMBERS CONTINUING THE EXTRATROPICAL TRANSITION. THE
MAJORITY OF DETERMINISTIC MODELS SUGGEST ETT AS WELL. THE JTWC
TRACK FORECAST IS PLACED WITH LOW CONFIDENCE BASED ON THE CHANCE
FOR A DISSIPATION SCENARIO VICE ETT. INTENSITY GUIDANCE BETWEEN
MODELS SUPPORTING ETT IS IN RELATIVELY GOOD AGREEMENT WITH A 25-30
KNOT SYSTEM AND SLIGHT INTENSIFICATION NEAR THE END OF THE FORECAST
AS IT BECOMES BAROCLINIC IN NATURE. AS A RESULT, THE JTWC INTENSITY
FORECAST IS PLACED WITH MEDIUM CONFIDENCE. 

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