WDPN34 PGTW 112100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 27W (TWENTYSEVEN) 
WARNING NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 12.8N 135.0E
   INITIAL INTENSITY: 45 KTS
   GEOGRAPHIC REFERENCE: 823 NM EAST OF MANILA, PHILIPPINES
   MOVEMENT PAST 6 HOURS: WESTWARD AT 15 KTS
   SIGNIFICANT WAVE HEIGHT: 20 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TROPICAL
STORM 27W WITH AN IMPROVING CONVECTIVE STRUCTURE OVER THE PAST 6
HOURS. A STRONG CONVECTIVE BAND ALONG THE WESTERN PERIPHERY OF THE
SYSTEM IS BECOMING MORE DEFINED AND IS WRAPPING INTO THE CIRCULATION 
CENTER. ENVIRONMENTAL ANALYSIS INDICATES 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 LOW-LEVEL CLOUD LINES IN THE ANIMATED EIR 
IMAGERY. THE INITIAL INTENSITY OF 45 KTS IS ASSESSED WITH MEDIUM 
CONFIDENCE BASED ON THE AGENCY DVORAK AND OBJECTIVE INTENSITY 
ESTIMATES LISTED BELOW.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK 

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

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS
   RJTD: T2.0 - 30 KTS
   KNES: T3.5 - 55 KTS
   CIMSS ADT: 49 KTS AT 111800Z
   CIMSS AIDT: 40 KTS AT 111800Z
   CIMSS DPRINT: 43 KTS AT 111800Z

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 CONTINUE TRACKING
WEST-NORTHWESTWARD ALONG THE SOUTHERN PERIPHERY OF A STR TO THE
NORTH THROUGH TAU 48. AFTER TAU 48, THE SYSTEM WILL BEGIN TRACKING
NORTHWESTWARD ALONG THE SOUTHWESTERN PERIPHERY OF THE STR. NEAR TAU
72, A BIFURCATION SCENARIO BEGINS WITH TWO DISTINCT POSSIBILITIES.
THE FIRST BEING A RECURVE EAST OF TAIWAN (ECMWF DEPICTION) AND THE
OTHER A WESTWARD TRACK INTO THE SOUTH CHINA SEA (GFS DEPICTION).
NEAR TAU 96, A COLD SURGE FLOWING TOWARDS TO SOUTHWEST IS THE CAUSE
FOR THE WESTWARD TRACK INTO THE SOUTH CHINA SEA. ON THE OTHER HAND,
IF THE SYSTEM TRACKS JUST QUICK ENOUGH TO OUTPACE THE ONSET OF THE
COLD SURGE, A RECURVE WILL ENSUE. THE JTWC TRACK IS PLACED CLOSER
TO THE RECURVE SCENARIO DUE TO THE EXPECTATION THAT THE VORTEX WILL
REMAIN STRONG ENOUGH TO NOT GET SHOVED SOUTHWESTWARD ALONG THE LOW-
LEVEL FLOW OF THE COLD SURGE. HOWEVER, THERE REMAINS LARGE UNCERTAINTY 
IN THE EXTENDED FORECAST AND THE TRACK COULD SHIFT. REGARDING THE 
INTENSITY OF 27W, THE ENVIRONMENT IS EXPECTED TO REMAIN CONDUCIVE 
OVERALL THROUGH TAU 72 WITH A PEAK INTENSITY OF 100 KTS. AFTER TAU 72, 
VERTICAL WIND SHEAR WILL INCREASE AND SEA SURFACE TEMPERATURES WILL 
DROP. ADDITIONALLY, THE COLD SURGE WILL BRING IN COLD AND DRY AIR 
TOWARD THE SYSTEM. AS A RESULT, 27W IS FORECAST TO QUICKLY WEAKEN TO 
AROUND 50 KTS AT TAU 120.

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT
THROUGH TAU 72, HOWEVER, AS PREVIOUSLY MENTIONED, A BIFURCATION
OCCURS AFTERWARDS. THE GFS SUGGESTS A WESTWARD AND THEN SOUTHWESTWARD 
TRACK INTO THE SOUTH CHINA SEA WHILE THE ECMWF SUGGESTS A RECURVE. DUE 
TO THIS DISCREPANCY, THE JTWC FORECAST IS PLACED WITH HIGH CONFIDENCE 
THROUGH TAU 72 AND LOW CONFIDENCE THEREAFTER. INTENSITY GUIDANCE IS IN 
OVERALL MODERATE AGREEMENT WITH INTENSIFICATION THROUGH TAU 72 AND 
WEAKENING AFTER. THE JTWC INTENSITY FORECAST IS PLACED CLOSE TO 
CONSENSUS WITH OVERALL MEDIUM CONFIDENCE.

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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