WDPN31 PGTW 250900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 22W (TRAMI) WARNING NR 
020//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.7N 117.1E
   INITIAL INTENSITY: 55 KTS
   GEOGRAPHIC REFERENCE: 321 NM SOUTH-SOUTHEAST OF HONG KONG
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 16 KTS
   SIGNIFICANT WAVE HEIGHT: 22 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A LARGE AND
SYMMETRICAL TROPICAL STORM (TS) 22W (TRAMI), MOVING INTO THE
CENTRAL PORTION OF THE NORTHERN SOUTH CHINA SEA (SCS). WELL-DEFINED
CONVECTIVE BANDS ARE EVIDENT, PARTICULARLY FOCUSED IN THE WESTERN
AND SOUTHERN PORTIONS OF THE ROTATION. THE INITIAL POSITION WAS
ASSESSED WITH MEDIUM CONFIDENCE BASED SOLELY ON THE ANIMATED MSI
AND AGENCY FIX POSITIONS DUE TO A LACK OF RECENT MICROWAVE IMAGERY.
THE INITIAL INTENSITY WAS ASSESSED WITH MEDIUM CONFIDENCE, TOWARDS
THE HIGHER END OF THE SUBJECTIVE AGENCY DVORAK ESTIMATES,
MAINTAINING PERSISTENCE WITH AN EARLIER 242203Z RCM-3 SAR PASS.
HIGHER WINDS WERE INDICATED IN THE SAR PASS, UNDER CONVECTION IN
THE MAIN INFLOW BAND TO THE SOUTH. IN THE UPPER-LEVELS THE SYSTEM
IS FACING MODERATE TO HIGH EASTERLY SHEAR (25-30 KNOTS) PER THE
LATEST CIMSS ANALYSIS BUT THE PRESENCE OF TRANSVERSE CIRRUS BANDS
TO THE EAST OF THE CENTER, AND A BOW-WAVE FEATURE IN THE WATER
VAPOR IMAGERY SUGGESTS THAT OUTFLOW ASSOCIATED WITH THE CONVECTION
IS PUSHING BACK AGAINST THE EASTERLY SHEAR. THIS IS GENERALLY
CONFIRMED BY THE CIMSS TC-SCALE ATMOSPHERIC MOTION VECTORS.
OTHERWISE, THE ENVIRONMENT IS FAVORABLE, WITH WARM SSTS AND GOOD
EQUATORWARD OUTFLOW ALOFT.

INITIAL WIND RADII BASIS: PERSISTENCE WITH EARLIER SAR DATA AND
SHIP OBSERVATIONS ACROSS THE REGION. 

CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) COMPLEX EXTENDING FROM EASTERN THAILAND TO
SOUTHEAST OF HONSHU.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.5 - 55 KTS
   RJTD: T3.0 - 45 KTS
   KNES: T3.0 - 45 KTS
   DEMS: T3.5 - 55 KTS
   CIMSS SATCON: 50 KTS AT 250555Z
   CIMSS ADT: 49 KTS AT 250530Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 25-30 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: STRONG EQUATORWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TS 22W WILL CONTINUE TRACKING WESTWARD ALONG
THE SOUTHERN SIDE OF THE EXTENSIVE STR COMPLEX TO THE NORTH FOR
36 HOURS, PASSING SOUTH OF HAINAN ISLAND AROUND TAU 36. A SLOW
CURVE TO THE SOUTH IS FORECAST TO BEGIN SHORTLY AFTER THE CLOSEST
POINT OF APPROACH TO HAINAN ISLAND AS THE STEERING RIDGE BEGINS TO
BREAK DOWN. A BRIEF PERIOD OF ADDITIONAL INTENSIFICATION IS
EXPECTED OVER THE NEXT 12 TO 24 HOURS AS THE CYCLING CONVECTION
CONTINUES TO PUSH BACK AGAINST THE MODERATE TO HIGH EASTERLY SHEAR.
HOWEVER, BY TAU 48, AN INFLUX OF DRY AIR IS EXPECTED TO PUNCH INTO
THE CORE OF THE SYSTEM, INHIBITING CONVECTION PARTICULARLY ALONG
THE EASTERN SIDE, AND ALLOWING FOR THE SHEAR TO PENETRATE TO THE
INNER CORE, MARKING THE BEGINNING OF A GENERAL WEAKENING TREND. BY
TAU 48, THE SYSTEM WILL BE TRACKING MORE SOUTHWARD ALONG THE COAST
OF VIETNAM AND BEGINNING A SHARP TURN BACK TO THE EAST, WHILE
SIMULTANEOUSLY BEGINNING A RAPID WEAKENING PHASE DUE TO CONTINUED
DRY AIR ENTRAINMENT AND INCREASING SHEAR. A LANDFALL ALONG THE
COAST OF VIETNAM NEAR HUE CITY OR DA NANG CANNOT BE RULED OUT,
THOUGH THE CURRENT FORECAST DOES NOT EXPRESSLY ANTICIPATE THIS
EVENTUALITY. REGARDLESS, THE PROXIMITY TO LAND WILL ACCELERATE THE
WEAKENING TREND AND AS THE VORTEX SHALLOWS, THE SYSTEM COMES UNDER
THE STEERING INFLUENCE OF THE LOW-LEVEL WEST-SOUTHWESTERLY FLOW
ALONG THE NORTH SIDE OF A NEAR EQUATORIAL RIDGE (NER) IN THE NATUNA
SEA. A SHARP TURN EASTWARD IN EXPECTED AFTER TAU 72, WITH THE
SYSTEM STEADILY WEAKENING THROUGH THE REMAINDER OF THE FORECAST
PERIOD.  

MODEL DISCUSSION: TRACK GUIDANCE IS IN REMARKABLY GOOD AGREEMENT,
CONSIDERING THE HIGHLY COMPLEX NATURE OF THE STEERING PATTERN. ALL
MEMBERS OF THE CONSENSUS AGREE ON A SHARP TURN SOUTH THEN EAST
BEGINNING AT TAU 48, THOUGH THERE ARE DIFFERENCES HOW FAR WEST THE
SYSTEM GETS BEFORE STARTING THE TURN. THE GFS TAKES THE SYSTEM
FURTHEST WEST AND INTO VIETNAM AFTER TAU 60, WHILE UKMET ENSEMBLE
MEAN KEEPS THE SYSTEM FURTHEST OFFSHORE. THE JTWC FORECAST CLOSELY
TRACES THE ECMWF, WHICH MAKES A HEAD FAKE TOWARDS DA NANG, VIETNAM
BEFORE TURNING EAST AT TAU 60. THE MAIN ISSUE AFTER TAU 72 IS THE
SPEED OF THE EASTWARD TRACK, WITH THE GFS VERY SLOWLY MOVING THE
SYSTEM SOUTHEASTWARD WHILE THE UKMET, NAVGEM AND ECMWF MOVE THE
SYSTEM MUCH FASTER AND FURTHER TO THE EAST. THE JTWC FORECAST AGAIN 
TRACES
THE ECMWF THROUGH THE EXTENDED TAUS, WITH MEDIUM CONFIDENCE.
INTENSITY GUIDANCE IS IN GOOD AGREEMENT WITH A SHORT
INTENSIFICATION PERIOD AT THE BEGINNING OF THE FORECAST FOLLOWED BY
RAPID WEAKENING, ESPECIALLY AFTER TAU 48. 

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