WDPN34 PGTW 120900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 27W (USAGI) WARNING NR 
006//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 14.0N 131.7E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 396 NM NORTH-NORTHWEST OF KAYANGEL
   MOVEMENT PAST 6 HOURS: WESTWARD AT 20 KTS
   SIGNIFICANT WAVE HEIGHT: 27 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
STORM (TS) 27W (USAGI) WITH IMPROVED AND FORMATIVE CONVECTIVE
BANDING WRAPPING AROUND THE WESTERN PERIPHERY OF THE SYSTEM AND
ALONG THE EASTERN FLANK OF THE LOW-LEVEL CIRCULATION CENTER (LLCC).
INCREASED CONVECTION IN CLOSE PROXIMITY TO THE LLCC OVER THE LAST
FEW HOURS HAS ALSO COMPLETELY OBSCURED THE CYCLONICALLY TURNING
LOW-LEVEL CLOUD LINES, HOWEVER, A 120425Z 89 GHZ AMSR2 MICROWAVE
IMAGE HAS REVEALED AN IMPROVED AND FURTHER CONSOLIDATING CENTER.
ANIMATED WATER VAPOR IMAGERY SHOWS MODERATE EQUATORWARD OUTFLOW
ALOFT, WITH FRAGMENTED STRIATIONS EXTENDING SOUTHWARD. THE INITIAL
POSITION IS PLACED WITH HIGH CONFIDENCE BASED ON THE AFOREMENTIONED
120425Z 89 GHZ AMSR2 IMAGE. THE INITIAL INTENSITY IS ASSESSED WITH
MEDIUM CONFIDENCE BASED ON A 120425Z GCOMW1 WINDSPEED IMAGE,
SHOWING AN ISOLATED SWATH OF 50-55 KTS WITHIN THE EASTERN QUADRANT
OF TS 27W.

INITIAL WIND RADII BASIS: PERSISTENCE

CURRENT STEERING MECHANISM: THE SUBTROPICAL RIDGE (STR) LOCATED
DIRECTLY TO THE NORTH, EXTENDING WESTWARD OVER TAIWAN.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   RJTD: T3.0 - 45 KTS
   KNES: T3.5 - 55 KTS
   CIMSS ADT: 51 KTS AT 120530Z
   CIMSS AIDT: 41 KTS AT 120530Z
   CIMSS D-MINT: 42 KTS AT 120630Z
   CIMSS D-PRINT: 40 KTS AT 120630Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TROPICAL STORM 27W IS FORECASTED TO CONTINUE
ITS NORTHWESTWARD TRACK ALONG THE SOUTHWESTERN PERIPHERY OF THE
SUBTROPICAL RIDGE LOCATED DIRECTLY TO THE NORTHEAST OVER THE 96
HOURS. AFTER TAU 96 AND WHEN TS 27W ENTERS THE LUZON STRAIT, A
DISTINCT BIFURCATION IN MODEL SOLUTIONS ILLUSTRATES TWO POTENTIAL
OUTCOMES FOR THE SYSTEM, WITH THE JTWC FORECAST CHARACTERIZING THE
NORTHEASTWARD APPROACH JUST EAST OF TAIWAN. THE SECOND SOLUTION FOR
TS 27W IS A CONTINUED WESTWARD TRACK JUST NORTH OF LUZON AFTER TAU
96 DUE TO A STRONG NORTHEASTERLY COLD SURGE EVENT FORCING THE
LOW-LEVEL CIRCULATION TOWARD THE SOUTH CHINA SEA. THE JTWC TRACK IS
ALIGNED AHEAD OF THE EXPECTED COLD SURGE EVENT AFTER TAU 96,
RECURVING JUST EAST OF TAIWAN INTO THE END OF THE FORECAST PERIOD. 
WITH REGARD TO THE INTENSITY FORECAST OF 27W, STEADY INTENSIFICATION 
IS EXPECTED INTO TAU 48 WITH A LARGELY CONDUCIVE ENVIRONMENT AS SEA
SURFACE TEMPERATURES REMAIN BETWEEN 29 C AND 30 C, VERTICAL WIND
SHEAR (VWS) REMAINS LOW OF LESS THAN 15 KTS, AND THE SYSTEMS CORE
REMAINS MOIST. FOLLOWING TAU 48, ENCROACHING DRY AIR FROM THE WEST
AND INCREASED VWS OF GREATER THAN 20 KTS BEGINS THE INITIAL
WEAKENING PHASE UNTIL TAU 120. ADDITIONALLY, SEA SURFACE
TEMPERATURES DIP BELOW 26 C, FURTHER WEAKENING THE SYSTEM AND
CUTTING OFF A SIGNIFICANT ENERGY SOURCE FOR SUSTAINMENT. BY TAU
120, INTENSITIES ARE FORECASTED TO DECREASE TO 40 KTS AS THE TRACK
PROGRESSES FURTHER NORTHWARD TOWARD COOL, DRIER AIR, AND COOLER SEA
SURFACE TEMPERATURES. 

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE REMAINS IN GOOD
AGREEMENT THROUGH TAU 48, WITH A CROSS-TRACK SPREAD OF JUST 80 NM
AS TS 27W PROGRESSES WESTWARD THROUGH THE PHILIPPINE SEA. WITH
TIMING DISCREPANCIES BECAUSE OF THE AFOREMENTIONED COLD SURGE
EVENT, A SIGNIFICANT BIFURCATION OCCURS AFTER TAU 48 AND INTO THE
REMAINDER OF THE FORECAST PERIOD. THE JTWC TRACK FORECAST IS
ALIGNED CLOSELY WITH THE MULTI-MODEL CONSENSUS, ILLUSTRATING A
RECURVE SCENARIO JUST EAST OF TAIWAN ALONG THE SUBTROPICAL RIDGE
AXIS BETWEEN TAU 96 AND TAU 120. INTENSITY GUIDANCE IS IN MODERATE
AGREEMENT WITH A STEADY INTENSIFICATION PHASE EXPECTED UNTIL TAU 72
TO 100 KTS, THEN BEGINNING ITS INITIAL WEAKENING PHASE THEREAFTER
TO 40 KTS BY TAU 120.

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