WDPN31 PGTW 110900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (CONSON) WARNING NR 
022//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 15.6N 109.5E
   INITIAL INTENSITY: 50 KTS
   GEOGRAPHIC REFERENCE: 73 NM EAST-SOUTHEAST OF DA NANG, VIETNAM
   MOVEMENT PAST 6 HOURS: WESTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 18 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS FLARING DEEP
CONVECTION SHEARED TO THE WEST OF A PARTIALLY EXPOSED LOW LEVEL
CIRCULATION CENTER (LLCC). A SYMMETRICAL AND SOMEWHAT IMPRESSIVE
CONVECTIVE BURST FLARED UP AROUND THE 0600Z HOUR BUT HAS SINCE
DISSIPATED. ANIMATED RADAR IMAGERY FROM VIETNAM SUPPORTS THE
INITIAL POSITION OF THE LLCC ON THE EASTERN EDGE OF THIS AREA OF
INTENSE CONVECTION WITH MEDIUM CONFIDENCE. THE INITIAL INTENSITY IS
HELD AT A GENEROUS 50 KNOTS, HEDGED SLIGHTLY ABOVE THE PGTW DVORAK
ESTIMATE OF T3.0 (45 KNOTS) IN LIGHT OF THE HIGHER RJTD ESTIMATE
AND A 110018Z ASCAT-B PASS WHICH DEPICTED A BAND OF 45-49 KNOT
WINDS ON THE SOUTHERN SIDE OF THE CIRCULATION CENTER. TS 18W HAS
SLOWED SIGNIFICANTLY OVER THE PAST SIX HOURS, NOW MOVING AT ONLY 4
KNOTS. THE SYSTEM LIES IN A MARGINAL ENVIRONMENT, WITH MODERATE VWS
OFFSET BY RELATIVELY STRONG WESTWARD OUTFLOW. SSTS ARE WARM BUT OHC
VALUES ARE LOW, WHICH BECOMES IMPORTANT NOW THAT THE SYSTEM HAS
SLOWED DOWN.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: LOW TO MID-LEVEL SUBTROPICAL RIDGE
(STR) TO THE NORTH.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   RJTD: T3.5 - 55 KTS
   CIMSS ADT: 37 KTS AT 110550Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE WESTWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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: PREVIOUS FORECASTS HAD TS CONSON MOVING AT A
STEADY CLIP WESTWARD, MAKING LANDFALL IN SHORT ORDER. OVER THE
PREVIOUS SIX HOURS, THE SYSTEM HAS SLOWED SIGNIFICANTLY, AS THE
STEERING RIDGE TO THE NORTH HAS WEAKENED SLIGHTLY, SLACKENING THE
OVERALL GRADIENT THAT WAS STEERING THE SYSTEM WESTWARD. WHILE THE
FORECAST TRACK HAS NOT CHANGED SIGNIFICANTLY, IN LIGHT OF THE
RECENT SPEED CHANGES, THE SYSTEM IS FORECAST TO REMAIN OFFSHORE FOR
ANOTHER 24 HOURS BEFORE MAKING LANDFALL ALONG THE CENTRAL COAST OF
VIETNAM, SOUTH OF DA NANG. DUE TO THE PERSISTENT MODERATE EASTERLY
VWS, THE PROXIMITY TO LAND AND THE LOW OHC OF THE OFFSHORE WATERS,
TS 18W IS FORECAST TO STEADILY WEAKEN OVER THE NEXT 24 HOURS, WITH
AN EXPECTED LANDFALL INTENSITY OF 40 KNOTS. ONCE INLAND, THE SYSTEM
WILL RAPIDLY WEAKEN DUE TO TERRAIN INTERACTION, AND IS EXPECTED TO
FULLY DISSIPATE NO LATER THAN TAU 72. AN ALTERNATE SCENARIO HAS
DEVELOPED, WHICH THE GFS AND HWRF ARE NOW SHOWING. THEY INDICATED A
QUASI-STATIONARY MOTION IN THE NEAR-TERM, FOLLOWED BY A SHARP TURN
NORTHWARD, WITH LANDFALL NEAR TAU 48 WELL UP THE VIETNAMESE COAST,
NORTH OF HUE. WHILE NOT CONSIDERED A HIGH PROBABILITY SCENARIO, THE
RECENT MOTION SEEN IN THE RADAR AND MSI SUPPORTS AT LEAST THE EARLY
PORTION OF THE ALTERNATE SCENARIO, AND THUS IT CANNOT BE DISMISSED
ENTIRELY AT THIS TIME.  

MODEL DISCUSSION: TRACK GUIDANCE SHOWS A WIDE SPREAD, DIVERGING
EARLY IN THE FORECAST PERIOD. NAVGEM AND EGRR SHOW THE SYSTEM
DIVING SOUTHWESTWARD TO NORTHEAST CAMBODIA. MEANWHILE HWRF, GFS AND
THE GFS ENSEMBLE AS MENTIONED TURN THE SYSTEM NORTHWEST PARALLELING
THE COAST UNTIL LANDFALL NORTH OF HUE. THE REMAINDER OF THE
CONSENSUS MEMBERS REMAIN TIGHTLY CLUSTERED ABOUT THE CONSENSUS MEAN
WHICH REMAINS CONSISTENT WITH THE PREVIOUS FORECASTS. THE JTWC
TRACK LIES CLOSE TO THE TRACK CONSENSUS BUT DEPARTS FROM THE MEAN
TRACK SPEED, KEEPING THE SYSTEM MOVING MUCH SLOWER THAN THE MEAN.
IN LIGHT OF THE UNCERTAINTY IN THE MODELS AND THE POSSIBILITY OF AN
ALTERNATE SCENARIO, THERE IS LOW CONFIDENCE IN THE JTWC FORECAST
TRACK. INTENSITY GUIDANCE HOWEVER REMAINS IN GOOD AGREEMENT THROUGH
THE DURATION OF THE FORECAST PERIOD. AGAIN HOWEVER, IN LIGHT OF THE
POSSIBILITY OF A TRACK SHIFT TO THE NORTH, THERE IS LOW CONFIDENCE
IN THE INTENSITY FORECAST. 

FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: LOW
   INTENSITY 0 - 72 HR: LOW//
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