WDPN32 PGTW 041500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 28W (TWENTYEIGHT) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 24.8N 143.3E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 109 NM EAST OF IWO TO
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 13 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
DEPRESSION 28W WITH DEEP CONVECTION CONFINED TO THE EASTERN
SEMICIRCLE OF A PARTIALLY EXPOSED LOW-LEVEL CIRCULATION CENTER
(LLCC). THE VORTEX APPEARS TO BE TILTED WITH HEIGHT. THE MID-LEVEL
CENTER IS ANALYZED TO BE OFFSET TO THE EAST, AND MODELS ARE
CURRENTLY REFLECTING THAT. ENVIRONMENTAL ANALYSIS INDICATES THAT
28W IS IN A FAVORABLE ENVIRONMENT CHARACTERIZED BY MODERATE
POLEWARD OUTFLOW ALOFT, LOW (10-15 KTS) VERTICAL WIND SHEAR, AND
WARM (29-30 C) SEA SURFACE TEMPERATURES. THE INITIAL POSITION IS
PLACED WITH MEDIUM CONFIDENCE BASED ON THE PARTIALLY EXPOSED LLCC
IN ANIMATED EIR IMAGERY. THE INITIAL INTENSITY OF 30 KTS IS
ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE AGENCY DVORAK FIXES OF
T1.5-2.0 AND AN EARLY CIMSS DPRINT ESTIMATE OF 29 KTS.

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

CURRENT STEERING MECHANISM: WEAK STEERING PATTERN WITH RIDGING TO
BOTH THE EAST AND WEST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T1.5 - 25 KTS
   CIMSS D-PRINT: 29 KTS AT 041200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: DRY AIR OVER THE WESTERN PERIPHERY

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: 28W IS FORECAST TO TRACK SLOWLY WESTWARD
THROUGH TAU 24 AS THE STEERING PATTERN REMAINS WEAK. AFTER TAU 24,
TRACK SPEEDS ARE EXPECTED TO INCREASE AS RIDGING TO THE EAST
EXTENDS NORTH OF THE SYSTEM AND BECOMES THE PRIMARY STEERING
MECHANISM. NEAR TAU 72, 28W IS FORECAST TO TURN NORTHWESTWARD AS IT
BEGINS TO ROUND THE SOUTHWESTERN EXTENT OF THE RIDGE. A TRACK
NORTHWARD, ALONG THE WESTERN EXTENT OF THE RIDGE, IS FORECAST TO
OCCUR NEAR THE END OF THE FORECAST PERIOD (TAU 120). HOWEVER, THERE
IS ALSO A STRONG RIDGE CENTERED OVER EASTERN CHINA THAT COULD
POTENTIALLY EXTEND TO THE NORTH OF 28W NEAR TAU 72, WHICH WOULD
PREVENT A RECURVE AND CAUSE A FURTHER WESTWARD TRACK. IF A MORE
SOUTHERN TRACK IS TAKEN DURING THE FIRST 72 HOURS, A FURTHER
WESTWARD TRACK WILL BE MORE LIKELY, AND POTENTIAL IMPACTS TO
OKINAWA WOULD BE MORE LIKELY. HOWEVER, A MORE NORTHWARD TRACK
DURING THE FIRST 72 HOURS WOULD FAVOR AN EARLY RECURVE. REGARDING
INTENSITY, 28W IS FORECAST TO STEADILY INTENSIFY THROUGH TAU 48 TO
AROUND 60 KTS. THE ENVIRONMENT WILL BE FAVORABLE, BUT IT WILL TAKE
SOME TIME FOR THE VORTEX TO BE FULLY ALIGNED. ONCE THAT HAPPENS,
RAPID INTENSIFICATION (RI) IS LIKELY TO OCCUR. A STRONG POLEWARD
OUTFLOW CHANNEL IS EXPECTED TO FORM NEAR TAU 60, WHICH WILL FURTHER
AID IN THE POTENTIAL RI. AN INTENSITY OF AROUND 105 KTS IS
CURRENTLY FORECAST FOR TAU 96, AS 28W BEGINS TO RECURVE. NEAR TAU
120, SOUTHWESTERLY SHEAR IS EXPECTED TO INCREASE, PUTTING A CAP ON
INTENSIFICATION. 

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE, WITH THE
EXCEPTION OF GALWEM, IS IN MODERATE AGREEMENT THROUGH TAU 72, WITH
A 195 NM CROSS-TRACK SPREAD AT THAT TIME. ECMWF IS THE SOUTHERNMOST
MEMBER WHILE GFS IS THE NORTHERNMOST MEMBER. AS A RESULT, ECMWF
TAKES THE SYSTEM WESTWARD AFTER TAU 96 RATHER THAN RECURVING WHILE
GFS HAS THE EARLIEST RECURVE. GUIDANCE AGREEMENT BECOMES VERY POOR
AFTER TAU 96 WITH A 490 NM CROSS-TRACK SPREAD AT TAU 120. THE JTWC
TRACK FORECAST IS HEDGED CLOSER TO THE ECMWF ENSEMBLE MEAN, GFS
ENSEMBLE MEAN, AND EC-AIFS WITH MEDIUM CONFIDENCE THROUGH TAU 72
AND LOW CONFIDENCE THEREAFTER. INTENSITY GUIDANCE IS IN RELATIVELY
GOOD AGREEMENT WITH ALL MODELS DEPICTING INTENSIFICATION THROUGH
TAU 96. PEAK INTENSITIES RANGE FROM 90 KTS (GFS) TO 115 KTS
(COAMPS-TC). THE JTWC INTENSITY FORECAST IS PLACED CLOSE TO THE
MULTI-MODEL CONSENSUS WITH OVERALL MEDIUM CONFIDENCE.

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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