WDPN34 PGTW 122100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 27W (USAGI) WARNING NR 
008//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 14.7N 128.9E
   INITIAL INTENSITY: 60 KTS
   GEOGRAPHIC REFERENCE: 459 NM EAST OF MANILA, PHILIPPINES
   MOVEMENT PAST 6 HOURS: WESTWARD AT 15 KTS
   SIGNIFICANT WAVE HEIGHT: 28 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
STORM 27W (USAGI) WITH WELL-DEFINED CONVECTIVE BANDING TIGHTLY
WRAPPING INTO THE OBSCURED LOW-LEVEL CIRCULATION CENTER (LLCC).
ENVIRONMENTAL ANALYSIS INDIACTES THAT 27W IS IN A FAVORABLE
ENVIRONMENT FOR INTENSIFICATION CHARACTERIZED BY MODERATE
EQUATORWARD OUTFLOW ALOFT, LOW (5-10 KTS) VERTICAL WIND SHEAR, AND
WARM (29-30 C) SEA SURFACE TEMPERATURES. THE INITIAL POSITION IS
PLACED WITH MEDIUM CONFIDENCE BASED ON THE ANIMATED EIR. THE
INITIAL INTENSITY OF 60 KTS IS ASSESSED WITH MEDIUM CONFIDENCE
BASED ON THE AGENCY DVORAK AND OBJECTIVE INTENSITY ESTIMATES LISTED
BELOW.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF A SUBTROPICAL
RIDGE (STR) TO THE NORTH. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.0 - 65 KTS
   RJTD: T4.0 - 65 KTS
   RCTP: T4.0 - 65 KTS
   KNES: T3.5 - 55 KTS
   CIMSS SATCON: 61 KTS AT 121730Z
   CIMSS ADT: 45 KTS AT 121800Z
   CIMSS AIDT: 55 KTS AT 121800Z
   CIMSS DPRINT: 62 KTS AT 121800Z
   CIMSS DMINT: 63 KTS AT 121828Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD

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: 27W IS FORECAST TO TRACK WEST-NORTHWESTWARD
THROUGH TAU 36, ALONG THE SOUTHWESTERN PERIPHERY OF THE STR TO THE
NORTH. NEAR TAU 36, THE SYSTEM WILL BEGIN TO ROUND THE RIDGE AXIS
AS IT ENTERS THE LUZON STRAIT. AFTER TAU 72, A BIFURCATION IN
GUIDANCE STILL REMAINS WITH SOME MODELS SUGGESTING A SOUTHWESTWARD
TRACK INTO THE SOUTH CHINA SEA AFTER TAU WHILE OTHERS SUGGESTS A
RECURVE. THE MAIN REASONING FOR THE TURN TO THE SOUTHWEST IS AN
INCOMING COLD SURGE FLOWING SOUTHWESTWARD NEAR TAIWAN. THE VORTEX
IS FORECAST TO BE STRONG ENOUGH TO NOT GET SHOVED BY THE SURGE AND
CONTINUE WITH A RECURVE. REGARDING INTENSITY, 27W IS FORECAST TO
BEGIN RAPIDLY INTENSIFYING OVER THE NEXT 36 HOURS TO A PEAK OF 100
KTS IN RESPONSE TO THE HIGHLY CONDUCIVE ENVIRONMENT. AFTER TAU 48,
THE ENVIRONMENT WILL QUICKLY WORSEN DUE TO INCREASING VERTICAL WIND
SHEAR AND COOLING SEA SURFACE TEMPERATUERS. ADIITIONALLY, THE COLD
SURGE BRINING IN COLD AND DRY AIR WHICH WILL FURTHER AID IN
WEAKENING THE SYSTEM TO AROUND 35 KTS AT THE END OF THE FORECAST
PERIOD. 

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT
THROUGH TAU 48, BUT THEN VERY QUICKLY DIVERGES DUE TO THE MENTIONED
BIFURACTION. THE ECMWF ENSEMBLE CONTINUES TO HAVE NEAR 100 PERCENT
OF MEMBERS RECURVING WHILE THE GFS ENSEMBLE IS SPLIT BETWEEN THE
RECURVE AND THE SOUTHWESTWARD TRACK INTO THE SOUTH CHINA SEA. THE
JTWC TRACK FORECAST HAS A RECURVE, IN LINE WITH THE ECMWF ENSEMBLE
AS WELL AS THE GALWEM, UKMET, AND COAMPS-TC TRACKERS. INTENSITY
GUIDANCE IS IN AGREEMENT ON RAPID INTENSIFICATION (RI) WITH THE
EXCEPTION OF GFS. MULTIPLE RI AIDS ARE TRIGGERING (DTOP AND RICN).
THE COAMPS-TC ENSEMBLE ALSO SUGGESTS A HIGH PROBABILITY OF RI
THROUGH TAU 48. THE JTWC INTENSITY FORECAST IS PLACED WITH OVERALL
MEDIUM CONFIDENCE AS A RESULT.

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