WDPN31 PGTW 140900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 21W (TWENTYONE) WARNING 
NR 008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.2N 156.6E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 230 NM SOUTHEAST OF MINAMI TORI SHIMA
   MOVEMENT PAST 6 HOURS: NORTH-NORTHEASTWARD AT 24 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TROPICAL STORM (TS) 21W IS EMBEDDED UNDER STRONG SOUTHERLY FLOW
WITHIN THE EASTERN PERIPHERY OF A BROAD, SUBTROPICAL UPPER-LEVEL
LOW POSITIONED NEAR 23N 152E. CONSEQUENTLY, ENVIRONMENTAL
CONDITIONS ARE MARGINAL WITH LOW TO MODERATE VERTICAL WIND SHEAR
OFFSET BY ROBUST POLEWARD OUTFLOW. ANIMATED MULTISPECTRAL SATELLITE
IMAGERY (MSI) DEPICTS A SMALL, WELL-DEFINED LOW-LEVEL CIRCULATION
(LLC) ACCELERATING POLEWARD WITH FLARING DEEP CONVECTION OVER THE
LLC. A 140641Z SSMIS 91GHZ COLOR COMPOSITE MICROWAVE IMAGE SHOWS
FRAGMENTED CONVECTIVE BANDING OVER THE EASTERN AND NORTHERN
QUADRANTS OF THE SMALL SYSTEM. THE SYSTEM IS LOCATED WITHIN THE
NORTHEASTERN PERIPHERY OF A LARGE LOW PRESSURE AREA, WHICH IS
DEVELOPING AT THE SURFACE IN ASSOCIATION WITH THE UPPER-LEVEL
SUBTROPICAL LOW. THE INITIAL POSITION IS PLACED WITH MEDIUM
CONFIDENCE BASED ON THE MSI. THE INITIAL INTENSITY OF 35 KTS IS
ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE PGTW DVORAK FIX.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

CURRENT STEERING MECHANISM: TS 21W IS TRACKING QUICKLY POLEWARD
WITHIN THE ENHANCED FLOW BETWEEN THE AFOREMENTIONED SUBTROPICAL LOW
AND A SUBTROPICAL RIDGE (STR) POSITIONED TO THE EAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T1.5 - 25 KTS
   CIMSS SATCON: 42 KTS AT 140312Z
   CIMSS ADT: 35 KTS AT 140540Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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 21W IS FORECAST TO TRACK NORTHWARD TO
NORTH-NORTHEASTWARD THROUGH TAU 24 ALONG THE WESTERN PERIPHERY OF
THE STR. ENVIRONMENTAL CONDITIONS WILL REMAIN MARGINAL AS THE
SYSTEM TRACKS WITHIN THE EASTERN PERIPHERY OF THE UPPER-LEVEL
SUBTROPICAL LOW WITH ROBUST POLEWARD OUTFLOW OFFSET BY DRY AIR
ENTRAINMENT AND MODERATE VERTICAL WIND SHEAR. GRADUAL WEAKENING IS
ANTICIPATED WITH DISSIPATION EXPECTED BY TAU 24. 

MODEL DISCUSSION: WITH THE EXCEPTION OF JGSM, NUMERICAL MODEL
GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU 24 WITH A 70NM SPREAD IN
SOLUTIONS AT TAU 24. THE 140000Z ECMWF (EPS) AND GFS (GEFS)
ENSEMBLES SUPPORT THE JTWC FORECAST TRACK WITH MEDIUM OVERALL
CONFIDENCE. NUMERICAL MODELS CONTINUE TO INDICATE THAT TS 21W WILL
DISSIPATE AND THE REMNANTS WILL GET ABSORBED INTO A LARGER,
DEVELOPING SUBTROPICAL LOW (INVEST 91W). THIS NEWLY-DEVELOPED
SUBTROPICAL LOW COULD THEN DEEPEN AND TRACK POLEWARD WITH EXPANDING
GALE-FORCE WINDS. THIS COMPLEX SYNOPTIC PATTERN AND MERGER WILL BE
CLOSELY MONITORED.     

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