WDPN31 PGTW 091500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 04W (FOUR) WARNING 
NR 003//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 8.1N 151.8E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 527 NM SOUTHEAST OF ANDERSEN AFB
   MOVEMENT PAST 6 HOURS: WESTWARD AT 03 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
CONSOLIDATING LOW-LEVEL CIRCULATION CENTER (LLCC) AND SPIRAL BANDS
DEVELOPING WITHIN THE NORTHWEST AND NORTHEAST QUADRANTS. THESE
BANDS ARE BEGINNING TO WRAP AROUND AND CLOSE OFF THE SOUTHERN
PERIPHERY OF THE LLCC. LOCATED SOUTH OF A WEAKNESS IN THE
SUBTROPICAL RIDGE, THE SYSTEM IS MOVING DUE WEST AT 03 KTS DUE TO A
VERY WEAK STEERING FLOW. ENVIRONMENTAL ANALYSES INDICATE LOW
VERTICAL WIND SHEAR (VWS), HIGH MID-LEVEL MOISTURE, WARM SEA
SURFACE TEMPERATURES (SST), AND ROBUST POLEWARD AND WESTWARD
EXHAUST, ALL OF WHICH ARE FAVORABLE FOR INTENSIFICATION. RECENT
SCATTEROMETER PASSES DEPICT AN ASYMMETRIC WIND FIELD, WITH 30 KT
EASTERLIES OVER THE NORTHERN SEMICIRCLE AND 10-20 KT WESTERLIES TO
THE SOUTH. GIVEN THIS DATA, THE INITIAL INTENSITY IS SET AT 30 KTS
WITH HIGH CONFIDENCE AS TD 04W IS ON THE VERGE OF BECOMING A
TROPICAL STORM. THE INITIAL POSITION IS SET WITH MEDIUM CONFIDENCE
BECAUSE THE LOWER-LEVEL CLOUD LINES HAVE BEEN PARTLY OBSCURED BY
MID- AND HIGH-LEVEL CLOUDS.

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

CURRENT STEERING MECHANISM: COMBINATION OF THE BETA EFFECT AND AN
EXTENSION OF A DEEP LAYER, NEAR-EQUATORIAL RIDGE (NER) TO THE EAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.5 - 35 KTS
   CIMSS ADT: 37 KTS AT 091140Z
   CIMSS AIDT: 35 KTS AT 091140Z
   CIMSS D-PRINT: 30 KTS AT 091210Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: HIGH
   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: TD 04W WILL CONTINUE TO DRIFT WESTWARD WITHIN
A WEAK STEERING ENVIRONMENT. AFTER TAU 24, THE SYSTEM SHOULD BEGIN
TO GAIN LATITUDE AS THE NER TO THE EAST STRENGTHENS AND PUSHES IT
TOWARDS THE NORTHWEST. THIS MOTION WILL THEN PERSIST THROUGH THE
FORECAST PERIOD UNDER THE NEW REGIME. THE SIGNIFICANT CHALLENGE IS
IN DETERMINING THE POLEWARD EXTENT OF THE TRACK, WHICH WILL DEPEND
ON HOW QUICKLY THE NER BUILDS. A STRONGER NER WILL STEER THE SYSTEM
CLOSER TO GUAM AND THE NORTHERN MARIANA ISLANDS, WHILE A WEAKER
RIDGE WILL KEEP THE TD 04W ON A MORE ZONAL TRACK SOUTH OF GUAM. THE
FORECAST TRACK IS LAID ON A MORE LIKELY SOLUTION IN BETWEEN THE TWO
EXTREMES. TD 04W WILL REMAIN IN A FAVORABLE ENVIRONMENT FOR
DEVELOPMENT THROUGH THE ENTIRE FORECAST PERIOD, CHARACTERIZED BY
LOW VWS OF 5-10 KTS, WARM SST OF 28-29 DEGREES CELSIUS, HIGH
MID-LEVEL MOISTURE, AND UPPER-LEVEL DIVERGENCE. THE INITIAL
LIMITING FACTORS AGAINST RAPID INTENSIFICATION ARE THE SIZE AND
ASYMMETRIC NATURE OF THE SYSTEM. ONCE THE SYSTEM DEVELOPS AN INNER
CORE AND TIGHTENS UP, IT SHOULD INTENSIFY MORE QUICKLY, REACHING
115-120 KTS AS IT MAKES A VERY CLOSE APPROACH TO GUAM. WHILE IT IS
TOO EARLY TO DETERMINE PROXIMITY TO THE ISLANDS WITH HIGH FIDELITY,
THE IMPACTS WILL BE FELT FAR FROM THE CENTER AS THE LARGE WIND
FIELD ENVELOPS MUCH OF THE MARIANA ISLANDS.

MODEL DISCUSSION: THE CROSS-TRACK SPREAD IN MODEL GUIDANCE IS LESS
THAN IDEAL, INCREASING FROM 100 NM AT TAU 48 TO A STAGGERING 480 NM
AT TAU 120. THIS PUTS THE AREA SPANNING FROM SAIPAN TO GUAM WELL
WITHIN THE AREA OF UNCERTAINTY. THE GFS AND ENSEMBLE MEAN REMAIN ON
THE SOUTHERN SIDE OF THE ENVELOPE, AND THEY BRING THE SYSTEM TO 170
NM SOUTHWEST OF GUAM. THE EXPERIMENTAL AI AND UKMET MODELS LEAN
TOWARDS THE OTHER END, TAKING THE SYSTEM 190 NM NORTHEAST OF GUAM.
THE FORECAST TRACK IS PLACED WITH MEDIUM CONFIDENCE NEAR THE
MULTI-MODEL CONSENSUS, WHICH IS NEAR THE HAFS-A AND HWRF MESOSCALE
MODELS AND ECMWF, THROUGH TAU 72 AND LOW CONFIDENCE THEREAFTER. THE
FORECAST INTENSITY FOLLOWS THE CONSENSUS DEPICTING STEADY
DEVELOPMENT FOR THE FIRST 48 HOURS. AFTER TAU 48 AND ONCE THE INNER
CORE FORMS, THE FORECAST CALLS FOR A STEEPER RATE OF
INTENSIFICATION, HEDGING CLOSER TO THE MESOSCALE AND EXPERIMENTAL
AI MODELS. THE FORECAST TAPERS OFF THE INTENSITY TREND TOWARDS THE
END OF THE FORECAST PERIOD AT THE UPPER END OF THE MODEL GUIDANCE.
WITH SUCH A PERSISTENTLY FAVORABLE ENVIRONMENT, HIGHER INTENSITIES
TOWARDS THE END OF THE FORECAST PERIOD ARE NOT OUT OF THE QUESTION.

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