WDPN32 PGTW 232100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 11W (SHANSHAN) WARNING NR 
010//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 18.8N 141.3E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 360 NM SOUTH OF IWO TO
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 18 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
PARTIALLY EXPOSED LOW-LEVEL CIRCULATION (LLC) SYSTEM OBSCURED BY
PLUMES OF PERSISTENT CONVECTION SHEARED NORTHEASTWARD BY A TROPICAL
UPPER TROPOSPHERIC TROUGH IN CLOSE PROXIMITY TO THE NORTHWEST. A
SLOT OF DRY AIR FROM THE WEST HAS CONTRIBUTED TO THE STAGNANT
INTENSIFICATION OVER THE LAST SIX HOURS. THE INITIAL POSITION IS
PLACED WITH HIGH CONFIDENCE BASED ON ANIMATED EIR AND IS SUPPORTED
BY A 231815Z F-18 SSMIS 91GHZ IMAGE DEPICTING DEEP CONVECTION
WRAPPING INTO A VORTEX CENTER FROM THE NORTHEAST AND AN EXPOSED
SOUTHWESTERN QUADRANT DUE TO DRY AIR ENTRAINMENT. THE INITIAL
INTENSITY OF 60 KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON AN
EARLIER GCOM W1 AMSR2 WIND SPEED IMAGE DEPICTING MOSTLY 55-60KT
WIND SPEEDS IN THE NORTHEASTERN PERIPHERY OF THE CIRCULATION
ALONGSIDE THE AGENCY AND OBJECTIVE FIXES LISTED BELOW.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: ON THE WESTERN PERIPHERY OF A RIDGE TO
THE EAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.0 - 65 KTS
   RJTD: T3.5 - 55 KTS
   RCTP: T4.0 - 65 KTS
   KNES: T4.0 - 65 KTS
   CIMSS SATCON: 59 KTS AT 231627Z
   CIMSS ADT: 59 KTS AT 231730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD AND EQUATORWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: MEDIUM

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL STORM (TS) 11W IS FORECAST TO
CONTINUE TRACKING NORTHWESTWARD BY THE RIDGE TO THE EAST THROUGH
TAU 72. BETWEEN TAU 72 AND 120, THE TS WILL TRACK AROUND THE RIDGE
ULTIMATELY NORTHEASTWARD BY THE END OF THE FORECAST.
INTENSIFICATION WILL INITIALLY BE HINDERED BY HIGH VERTICAL WIND
SHEAR THAT IS INTRODUCED BY A NEARBY TUTT CELL POSITIONED TO THE
NORTHWEST, POSITIONED SO CLOSELY THAT THE HIGH SHEAR HINDERS
INTENSIFICATION DESPITE THE FAVORABLE OUTFLOW ALSO INDUCED BY THE
TUTT. THE CAREFUL BALANCE BETWEEN HIGH SHEAR, WARM SEA SURFACE
TEMPERATURES, AND STRONG UPPER-LEVEL DIVERGENCE IS EXPECTED TO
CAUSE THE SYSTEM TO STRUGGLE TO INTENSIFY FOR THE FIRST 36 HOURS.
BY TAU 48, THE TUTT CELL IS EXPECTED TO PASS TO THE WEST AND THE
ENVIRONMENT WILL REMAIN FAVORABLE FOR INTENSIFICATION TO A PEAK OF
120KTS BY TAU 72. AFTER MAKING LANDFALL AT TAU 96, THE SYSTEM WILL
GRADUALLY WEAKEN AND BECOME SHALLOW DUE TO TOPOGRAPHIC INTERACTION.
AFTER COMING BACK OUT OVER WATER, EXTRATROPICAL TRANSITION WILL
START PRIOR TO TAU 120 WHEN THE CIRCULATION PASSES OVER COOL SEA
SURFACE TEMPERATURES AND ENTERS A BAROCLINIC ZONE. 

MODEL DISCUSSION: DETERMINISTIC MODEL GUIDANCE IS IN GOOD AGREEMENT
FOR THE FIRST 72 HOURS OF THE FORECAST WHILE UNDER THE STEERING
INFLUENCE OF THE FIRST RIDGE, BUT THEN VARIES AFTER TAU 72 TO OPEN
TO A 330NM CROSS-TRACK DISTANCE BY THE END OF THE FORECAST. THE
OPENING LIKELY IS DUE TO DIFFERENT SOLUTIONS INTERPRETING HOW
STRONGLY THE SUBTROPICAL RIDGE PICKS UP THE SYSTEM AND HOW QUICKLY.
DUE TO THE VARYING INTERPRETATIONS OF STEERING, THE INTENSITY
GUIDANCE ALSO IS NOT IN STRONG AGREEMENT. IF THE TRACK IS FARTHER
WEST, THERE WOULD BE A LONGER PERIOD THAT THE CIRCULATION IS IN A
FAVORABLE ENVIRONMENT, LEADING TO A LONGER PERIOD OF
INTENSIFICATION. IF THE TRACK IS FARTHER EAST, THERE COULD BE A
POTENTIAL THAT THE EXTRATROPICAL TRANSITION COULD COMMENCE EARLIER.
THE JTWC INTENSITY GUIDANCE REFLECTS THE VARIATIONS WITH A 70KT
SPREAD IN THE PEAK INTENSITY. 

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: HIGH
   TRACK 72-120 HR: MEDIUM
   INTENSITY 0 - 72 HR: MEDIUM
   INTENSITY 72-120 HR: LOW//
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