WDPN31 PGTW 092100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 04W (SINLAKU) WARNING NR 
004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 8.4N 151.6E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 57 NM NORTH-NORTHWEST OF CHUUK
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 03 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
STORM (TS) 04W (SINLAKU) WITH IMPROVING CONVECTIVE BANDING ALONG
THE WESTERN AND NORTHERN PERIPHERIES. THE LOW-LEVEL CIRCULATION
CENTER (LLCC) HAS ALSO BECOME MORE DEFINED OVER THE PAST 6 HOURS,
AND IS EVIDENT IN THE 091545Z AMSR2 89 GHZ MICROWAVE IMAGE. THE
INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE AMSR2
IMAGE AND ANIMATED EIR IMAGERY. THE INITIAL INTENSITY OF 35 KTS IS
ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY DVORAK FIXES
AND OBJECTIVE INTENSITY ESTIMATES LISTED BELOW. ENVIRONMENTAL
ANALYSIS INDICATES THAT 04W IS IN A FAVORABLE ENVIRONMENT
CHARACTERIZED BY MODERATE POLEWARD OUTFLOW ALOFT, LOW (10-15 KTS)
VERTICAL WIND SHEAR, AND WARM (28-29 C) SEA SURFACE TEMPERATURES.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT WITH A
SUBTROPICAL RIDGE (STR) POSITIONED EAST OF LUZON AND A
NEAR-EQUATORIAL RIDGE (NER) TO THE EAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   KNES: T3.0 - 45 KTS
   CIMSS SATCON: 36 KTS AT 091810Z
   CIMSS ADT: 39 KTS AT 091810Z
   CIMSS AIDT: 34 KTS AT 091810Z
   CIMSS D-MINT: 33 KTS AT 091556Z
   CIMSS D-PRINT: 35 KTS AT 091810Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TS 04W IS FORECAST TO MEANDER SLOWLY WESTWARD
OVER THE NEXT 36 HOURS AS IT STAYS PLACED WITHIN A WEAK STEERING
ENVIRONMENT. AFTER TAU 36, THE NER TO THE EAST IS EXPECTED TO BUILD
AND CAUSE 04W TO PICK UP SPEED IN THE NORTHWESTWARD DIRECTION. A
NORTHWESTWARD TRACK IS THEN FORECAST TO PERSIST THROUGH THE
REMAINDER OF THE FORECAST PERIOD. THE DIRECTION OF THE
NORTHWESTWARD TRACK IS STILL UNCERTAIN, WITH TWO DISTINCT SCENARIOS
BEGINNING TO FORM. THE FIRST SCENARIO IS A FLATTER TRACK THAT ENDS
UP TRACKING THE VORTEX SOUTH OF GUAM. THE SECOND SCENARIO IS A MORE
NORTHWARD TRAJECTORY, THAT ENDS UP TRACKING THE VORTEX TO THE NORTH
OF GUAM. THE JTWC TRACK FORECAST HAS STARTED TO SLIGHTLY DRIFT
FURTHER NORTH, CAUSING A CLOSER CPA TO GUAM AT AROUND 140000Z. A
LARGE FACTOR IN THE DIRECTION THAT 04W WILL TAKE WILL BE THE NEAR
TERM MOVEMENT. A SLOWER TRACK OVER THE NEXT 24-36 HOURS WILL LIKELY
ALLOW FOR A MORE NORTHWARD TRACK, UNDER THE INFLUENCE OF THE NER TO
THE EAST RATHER THAN GETTING CAUGHT BY THE STR NEAR LUZON.
REGARDING INTENSITY, 04W IS FORECAST TO STEADILY INTENSIFY OVER THE
96 HOURS WITH A PEAK INTENSITY OF 105 KTS. THE ENVIRONMENT IS
EXPECTED TO REMAIN FAVORABLE WITH LOW SHEAR, WARM SST, AND GOOD
DIVERGENCE ALOFT. ONE THING TO NOTE IS THE POSSIBILITY FOR AN
ENHANCED MID-LEVEL EASTERLY SHEAR COMPONENT, THAT COULD SLIGHTLY
HINDER DEVELOPMENT. ADDITIONALLY, THE LARGE SIZE OF THE SYSTEM MAY
BE A LIMITING FACTOR IN THE INTENSITY OF THE WIND FIELD. 

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE GENERALLY AGREES ON
VERY SLOW MOVEMENT THROUGH TAU 36. GFS IS THE FASTEST MODEL THROUGH
TAU 36, WHICH ALSO CORRESPONDS TO IT BEING THE SOUTHERNMOST TRACK.
AFTER TAU 36, THERE ENDS UP BEING TWO DISTINCT GROUPINGS. THE
FIRST GROUP INCLUDES THE GOOGLE DEEPMIND ENSEMBLE MEAN, THE
EC-AIFS, GALWEM, AND UKMET, WHICH SHOW A TRACK THAT PASSES CLOSER
TO TINIAN AND SAIPAN. THE SECOND GROUPING IS COMPRISED OF THE GFS
AND ECMWF (AND THEIR ENSEMBLE MEANS), WHICH TRACK THE VORTEX TO THE
SOUTH OF GUAM. AS A RESULT, CROSS-TRACK SPREAD AT TAU 72 IS 260 NM
WHICH OPEN UP TO 430 NM AT TAU 120. THE JTWC TRACK FORECAST IS
PLACED ON THE NORTHERN EDGE OF THE SECOND GROUPING, CLOSE TO THE
ECMWF ENSEMBLE MEAN WITH LOW CONFIDENCE. INTENSITY GUIDANCE AGREES
ON INTENSIFICATION THROUGH TAU 96 AND THEN LEVELING OFF THROUGH TAU
120. MULTIPLE RAPID INTENSIFICATION (RI) AIDS HAVE TRIGGERED THIS
MODEL RUN, ADDING UNCERTAINTY TO THE RATE OF INTENSIFICATION
THROUGH THE APPROACH TO THE MARIANAS. HAFS-A REMAINS THE MOST
AGGRESSIVE MODEL, SUGGESTING A PEAK OF AROUND 125 KTS. THE JTWC
INTENSITY FORECAST IS PLACED NEAR THE MULTI-MODEL CONSENSUS
THROUGHOUT THE FORECAST PERIOD WITH MEDIUM CONFIDENCE.


FORECAST CONFIDENCE:
   TRACK 00-72 HR: LOW
   TRACK 72-120 HR: LOW
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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