Advisories

WDPN32 PGTW 051500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 13W (KUJIRA) WARNING NR 
004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.8N 119.8E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 204 NM NORTH-NORTHWEST OF MANILA,
PHILIPPINES
   MOVEMENT PAST 6 HOURS: EASTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED SHORTWAVE INFRARED (SWIR) SATELLITE IMAGERY DEPICTS A
COMPACT LOW-LEVEL CIRCULATION CENTER (LLCC) LOCATED JUST NORTHWEST
OF VIGAN, PHILIPPINES. OVER THE PAST 2-3 HOURS, DEEP CONVECTION HAS
RE-DEVELOPED ALONG THE SOUTHWESTERN PERIPHERY OF THE VORTEX AND
EXPANDED NORTHWARD, ENVELOPING THE LLCC. OUTER CONVECTIVE BANDS ALONG 
THE EASTERN FLANK OF TS 13W ARE RESOLVED ON THE PAGASA RADAR LOOP,
PROVIDING HIGHER CONFIDENCE IN THE CENTER POSITION (ASSESSED WITH 
MEDIUM CONFIDENCE). THE INITIAL INTENSITY IS ASSESSED AT 35 KNOTS WITH 
MEDIUM CONFIDENCE, CONSISTENT WITH RECENT SCATTEROMETRY AND CIMSS ADT 
AND D-PRINT ESTIMATES. WHILE CIMSS ANALYSIS INDICATES 15 KNOTS OF 
DEEP-LAYER VERTICAL WIND SHEAR (VWS), CIRA DERIVED MOTION VECTOR 
(DMV) WINDS AND WATER VAPOR IMAGERY INDICATE A SIGNIFICANT WEAKENING 
OF UPPER-LEVEL NORTHEASTERLY WINDS ALONG THE SYSTEM'S NORTHEASTERN 
FLANK, RESULTING IN A LOWER SHEAR ENVIRONMENT OVER THE CORE. THIS 
DIAGNOSIS IS SUBSTANTIATED BY RECENT CONVECTIVE RE-DEVELOPMENT AND 
RAPID VERTICAL VORTEX ALIGNMENT OBSERVED IN THE SWIR ANIMATIONS. 
OVERALL, THE ENVIRONMENT IS CHARACTERIZED AS MARGINAL; WHILE PROXIMITY 
TO LAND IMPEDES CONSOLIDATION, HIGH SSTS (29-30C), LOW VWS, AND STRONG 
EQUATORWARD OUTFLOW FAVOR DEVELOPMENT.

INITIAL WIND RADII BASIS: PERSISTENCE 

CURRENT STEERING MECHANISM: WEAK STEERING ENVIRONMENT, DOMINATED BY
THE LOWER-LEVEL SOUTHWESTERLY MONSOONAL SURGE FLOW. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T2.0 - 30 KTS
   KNES: T2.0 - 30 KTS
   CIMSS ADT: 35 KTS AT 051200Z
   CIMSS AIDT: 31 KTS AT 051200Z
   CIMSS D-PRINT: 33 KTS AT 051210Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG EQUATORWARD
   OTHER FACTORS: PROXIMITY TO LAND. 

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 13W HAS TURNED EASTWARD AND ACCELERATED
TOWARDS WESTERN LUZON SINCE THE 0600Z ANALYSIS. LOW-LEVEL
SOUTHWESTERLIES ASSOCIATED WITH A MONSOON SURGE ARE INTENSIFYING,
STEERING TS 13W EAST-SOUTHEASTWARD THROUGH TAU 24. LANDFALL ALONG THE 
COAST BETWEEN VIGAN AND LAOAG IS EXPECTED WITHIN THE NEXT FEW HOURS, 
FOLLOWED BY RAPID ACCELERATION ACROSS NORTHERN LUZON AND OFFSHORE 
EMERGENCE INTO THE PHILIPPINE SEA BETWEEN TAU 12 AND TAU 18. THROUGH 
THE REMAINDER OF THE FORECAST PERIOD, THE SYSTEM WILL CONTINUE ON AN 
EAST-NORTHEASTWARD TRAJECTORY ALONG THE NORTHERN PERIPHERY OF THE 
SURGING MONSOON SOUTHWESTERLIES. REGARDING INTENSITY, BRIEF 
INTENSIFICATION UP TO 40 KNOTS IS POSSIBLE IMMEDIATELY PRIOR TO 
LANDFALL; HOWEVER, TERRAIN INTERACTION OVER NORTHERN LUZON WILL 
INDUCE STEADY WEAKENING. DUE TO THE RAPID TRANSLATION SPEED, TS 13W 
WILL EMERGE OFFSHORE AS A 25-30 KNOT TROPICAL DEPRESSION BEFORE 
DEGENERATING INTO AN OPEN TROUGH BY TAU 24, FEATURING AN ASYMMETRIC 
WIND FIELD WITH 25-30 KNOT MONSOON SURGE WINDS CONFINED TO THE 
SOUTHERN FLANK. AS THE SYSTEM OPENS INTO A TROUGH, IT WILL LOSE 
TROPICAL CHARACTERISTICS WHILE MAINTAINING WINDS ABOVE WARNING 
THRESHOLDS. BEYOND THE END OF THE FORECAST PERIOD, THE REMNANT 
VORTICITY WILL TRACK NORTHEASTWARD AND BE ABSORBED INTO THE EXPANSIVE 
CIRCULATION OF TY 12W PASSING TO THE NORTH.  

MODEL DISCUSSION: TRACK GUIDANCE IS IN GOOD AGREEMENT ON THE
OVERALL TRACK SCENARIO, WITH RELIABLE NUMERICAL MODEL GUIDANCE AND 
THE JTWC SUPER ENSEMBLE CONSISTENTLY DEPICTING AN EASTWARD TRACK INTO 
THE PHILIPPINE SEA. THE JTWC FORECAST IS POSITIONED NEAR THE AI 
CONSENSUS MEAN WITH MEDIUM CONFIDENCE. INTENSITY GUIDANCE IS MIXED; 
HWRF INDICATES UNREALISTIC INTENSIFICATION TO 45 KNOTS OVER LAND, 
WHICH IS CONSIDERED LOW PROBABILITY AND DISCOUNTED. THE REMAINDER OF 
THE GUIDANCE DEPICTS WEAKENING ACROSS LUZON, FOLLOWED BY FLAT-LINING 
OR SLIGHT RE-INTENSIFICATION WITHIN THE MONSOON ENVELOPE. DECAY-SHIPS 
AND GOOGLE DEEPMIND ARE THE ONLY MEMBERS WEAKENING THE SYSTEM BELOW 
25 KNOTS. THE JTWC FORECAST IS PLACED NEAR THE DECAY-SHIPS GUIDANCE 
WITH MEDIUM CONFIDENCE. 

FORECAST CONFIDENCE:
   TRACK 00-72 HR: MEDIUM
   INTENSITY 00-72 HR: MEDIUM//
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