WDPN32 PGTW 042100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 13W (THIRTEEN) 
WARNING NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 31.2N 146.5E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 431 NM SOUTHEAST OF YOKOSUKA, JAPAN
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 15 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY SHOWS A MIDGET
SYSTEM THAT CONTINUES TO STRUGGLE WITH FLARING CONVECTION PARTLY
OBSCURING THE LOW-LEVEL CIRCULATION CENTER (LLCC). WATER VAPOR
IMAGERY REVEALS WEAK RADIAL OUTFLOW, EXACERBATED BY SUBSIDENCE
ALONG THE SOUTHERN FLANK CAUSED BY A TUTT CELL TO THE SOUTHWEST.
THE INITIAL POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON A
RAGGED BUT DEFINED LLCC FEATURE IN THE 041754 SSMIS 91GHZ IMAGE.
THE INITIAL INTENSITY OF 25 KTS IS ALSO ASSESSED WITH HIGH CONFIDENCE 
BASED ON THE PGTW DVORAK INTENSITY ESTIMATE OF T1.5 AND THE 041615Z 
AMSR2 WINDSPEED PRODUCT THAT SHOWED MAX WINDS OF 23KTS ALONG THE 
NORTHEAST QUADRANT AND REFLECTS THE 6-HR CONVECTIVE DEGRADATION 
EVIDENT IN THE EIR LOOP. ANALYSIS INDICATES A MARGINALLY FAVORABLE 
ENVIRONMENT WITH WARM SST, LIGHT VWS, AND WEAK RADIAL OUTFLOW OFFSET 
BY SUBSIDENCE AND COOL DRY AIR INTRUSION AT THE LOW LEVELS.

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

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   KNES: T2.0 - 30 KTS
   CIMSS SATCON: 39 KTS AT 041534Z
   CIMSS ADT: 33 KTS AT 041730Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 10-15 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: WEAK RADIAL
   OTHER FACTORS: COOL DRY AIR INTRUSION AT THE LOW LEVELS.

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   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 13W HAS CRESTED THE RIDGE AXIS AND WILL
TRACK MORE NORTH-NORTHEASTWARD THEN ACCELERATE NORTHEASTWARD AS IT
ROUNDS THE POLEWARD SIDE OF THE STEERING STR. THE MARGINALLY
FAVORABLE ENVIRONMENT WILL FUEL A SLIGHT INTENSIFICATION TO 30KTS
BY TAU 12. AFTERWARD, INCREASING VWS, SUSTAINED SUBSIDENCE ALOFT,
AND COOL DRY AIR INTRUSION NEAR THE SURFACE WILL INHIBIT ANY
FURTHER INTENSIFICATION. CONCURRENTLY BY TAU 36, THE SYSTEM WILL
BEGIN EXTRA-TROPICAL TRANSITION (ETT) AND BY TAU 48, WILL BECOME A
COLD CORE LOW WITH NEAR GALE-FORCE WINDS. THERE IS A POSSIBILITY
THAT TD 13W WILL DISSIPATE PRIOR TO COMPLETION OF ETT.

MODEL DISCUSSION: NUMERICAL MODELS ARE IN TIGHT AGREEMENT WITH AN
EVEN SPREAD TO A MERE 76-NM BY TAU 48, LENDING HIGH CONFIDENCE IN
THE JTWC TRACK FORECAST. GIVEN THE VARIABILITY ASSOCIATED WITH ETT
AND THE POSSIBILITY OF AN EARLY DISSIPATION, THERE IS ONLY MEDIUM
CONFIDENCE IN THE JTWC INTENSITY FORECAST. 

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