WDPN31 PGTW 070300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (TWENTYONE) WARNING 
NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 20.9N 146.0E
   INITIAL INTENSITY: 40 KTS
   GEOGRAPHIC REFERENCE: 127 NM NORTH OF AGRIHAN
   MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 17 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS DEEP 
CONVECTION PERSISTING OVER THE EASTERN PORTION OF A PARTIALLY EXPOSED 
LOW-LEVEL CIRCULATION. THE INITIAL POSITION IS PLACED WITH HIGH 
CONFIDENCE BASED ON VISIBLE IMAGERY, A 062330Z METOP-B 89 GHZ IMAGE, 
AND RECENT POSITION FIXES FROM PGTW, RJTD, AND KNES. THE INITIAL 
INTENSITY OF 40 KTS IS ASSESSED WITH MEDIUM CONFIDENCE, CONSISTENT 
WITH OBJECTIVE AUTOMATED DVORAK AND SATELLITE CONSENSUS INTENSITY 
ESTIMATES FROM UW-CIMSS AND ALSO SUPPORTED BY A 062028Z SMAP PASS. TS 
21W HAS INTENSIFIED OVER THE PAST SIX HOURS, DESPITE PERSISTENT 
EASTERLY FLOW ALOFT AND RESULTANT VERTICAL WIND SHEAR, DUE TO THE 
FAVORABLE INFLUENCES OF STRONG EASTWARD DIFFLUENCE ALOFT AND PASSAGE 
OVER VERY WARM WATER.

INITIAL WIND RADII BASIS: SMAP DATA FROM 062028Z

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE (STR) TO THE EAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.0 - 30 KTS
   CIMSS SATCON: 39 KTS AT 070000Z
   CIMSS ADT: 39 KTS AT 062330Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG EASTWARD
   OTHER FACTORS: PERSISTENT WESTERLY FLOW ALOFT OVER THE SYSTEM

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: NEAR-TERM FORECAST INTENSITIES HAVE
INCREASED FROM THE PREVIOUS WARNING. OTHERWISE, THERE ARE NO
SIGNIFICANT CHANGES TO THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TROPICAL STORM (TS) 21W WILL TRACK NORTHEASTWARD 
UNDER THE CONTINUED INFLUENCE OF SUBTROPICAL RIDGING TO THE EAST AND 
SOUTH DURING THE FORECAST PERIOD. TS 21W IS MOVING INTO AN AREA OF 
PERSISTENT MODERATE TO HIGH SOUTHWESTERLY VERTICAL WIND SHEAR (VWS), 
WHICH WILL IMPACT THE SYSTEM THROUGHOUT THE FORECAST PERIOD. DESPITE 
THE INCREASING VWS, TS 21W IS EXPECTED TO SLOWLY INTENSIFY THROUGH 
TAU 72, AS PASSAGE OVER WARM WATER AND FAVORABLE UPPER-LEVEL 
DIFFLUENCE FUEL THE SYSTEM. AFTER TAU 72, TS 21W IS EXPECTED TO 
ACCELERATE POLEWARD AND MERGE WITH A DEVELOPING MID-LATITUDE TROUGH 
APPROACHING FROM THE NORTHWEST. EXTRATROPICAL TRANSITION WILL BEGIN 
BEFORE TAU 96 AND COMPLETE BY TAU 120. FAVORABLE BAROCLINIC 
INTERACTION WITH THE MID-LATITUDE FLOW PATTERN DURING THE TRANSITION 
WILL INDUCE INTENSIFICATION AND BROADENING OF THE WIND FIELD AS THE 
SYSTEM TRANSITIONS TO A VERY STRONG EXTRATROPICAL LOW. 

MODEL DISCUSSION: NUMERICAL MODEL TRACK FORECAST GUIDANCE IS IN
GOOD AGREEMENT WITH SPREAD INCREASING MARGINALLY AFTER TAU 72 DUE
TO DIFFERING REPRESENTATIONS OF THE STEERING FLOW PATTERN AND 
INTERACTION WITH THE DEVELOPING MID-LATITUDE TROUGH AFTER TAU 72. 
HOWEVER, SPREAD REMAINS MINIMAL THROUGHOUT THE FORECAST PERIOD AND 
CONFIDENCE IN THE FORECAST TRACK IS HIGH. INTENSITY GUIDANCE IS ALSO 
IN GOOD AGREEMENT, WITH BOTH HAFS AND COAMPS-TC DEPICTING SLOW 
INTENSIFICATION IN THE NEAR TO MEDIUM-TERM, LIMITED BY PERSISTENT 
VWS, AND INTENSIFICATION DURING EXTRATROPICAL TRANSITION IN THE 
EXTENDED FORECAST PERIOD. DUE TO UNCERTAINTIES INTRODUCED BY RECENT 
INTENSIFICATION AND EXPECTED INTENSITY CHANGES ASSOCIATED WITH 
EXTRATROPICAL TRANSITION, THERE IS MEDIUM CONFIDENCE IN THE INTENSITY 
FORECAST DESPITE NOTED NUMERICAL MODEL AGREEMENT.

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