WDPN32 PGTW 270900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 18W (NORU) WARNING NR 022//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 15.6N 111.2E
   INITIAL INTENSITY: 125 KTS
   GEOGRAPHIC REFERENCE: 169 NM EAST OF DA NANG, VIETNAM
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 50 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
HAVING RECENTLY UNDERGONE ANOTHER ROUND OF RAPID INTENSIFICATION,
WHICH PEAKED AT 270000Z AT 130 KNOTS, TYPHOON (TY) 18W HAS RECENTLY
STARTED TO WEAKEN.  ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI)
AND ENHANCED INFRARED (EIR) IMAGERY SHOWS THAT AFTER PEAKING AT
0000Z, THE EYE HAS SUBSEQUENTLY FILLED IN, WHILE CLOUD TOP TEMPS
HAVE WARMED CONSIDERABLY, LEADING TO MUCH LOWER DVORAK CURRENT
INTENSITY ESTIMATES. HOWEVER, THE SYSTEM HAS NOT GIVEN UP THE GHOST
ENTIRELY JUST YET, WITH CONVECTIVE HOT TOWERS CONTINUING TO FIRE UP
ON THE SOUTHERN SIDE OF THE CORE AND WRAP UPSHEAR, SO THE ONSET OF
RAPID WEAKENING MAY STILL LIE A BIT IN THE FUTURE. 270609Z GMI AND
270611Z AMSR-2 37GHZ MICROWAVE IMAGES REVEALED A VERY SMALL INNER
CORE AND WEAK 9NM EYE FEATURE, WHICH LENT HIGH CONFIDENCE TO THE
INITIAL POSITION. THE MICROWAVE IMAGERY SUGGESTED DEVELOPMENT OF A
WEAK OUTER EYEWALL OR CONVECTIVE BAND ABOUT 30-40NM OUT FROM THE
CENTER AS WELL. THE INITIAL INTENSITY IS ASSESSED WITH MEDIUM
CONFIDENCE, WELL ABOVE THE SUBJECTIVE AND OBJECTIVE DVORAK CURRENT
INTENSITY ESTIMATES, WHICH ARE DECEIVINGLY LOW DUE TO THE
WELL-KNOWN LOW BIASES IN THE DVORAK TECHNIQUE WITH VERY SMALL
SYSTEMS. EVEN THOUGH SHEAR IS QUITE HIGH (25-30 KNOTS), THE SYSTEM
SO FAR HAS BEEN ABLE TO SUSTAIN ENOUGH UPSHEAR CONVECTION TO PUSH
BACK AGAINST THE STRONG SHEAR. OTHER THAN THE SHEAR, THE OVERALL
ENVIRONMENT IS FAVORABLE WITH WARM SSTS BETWEEN THE STORM AND THE
COAST AND STRONG WESTWARD AND MODERATE EQUATORWARD OUTFLOW.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SOUTHERN SIDE OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) ENTRENCHED OVER SOUTHERN CHINA. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T6.5 - 127 KTS
   RJTD: T5.5 - 102 KTS
   KNES: T6.0 - 115 KTS
   CIMSS SATCON: 85 KTS AT 270513Z
   CIMSS ADT: 99 KTS AT 270540Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 25-30 KTS
   SST: 30-31 CELSIUS
   OUTFLOW: STRONG WESTWARD

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: OVER THE PAST 12 HOURS, TY NORU TOOK A BIT OF
A JOG TO THE SOUTH BUT HAS RECENTLY TURNED BACK TO THE NORTH AND
NOW SEEMS TO HAVE STEADIED UP ON A WESTWARD HEADING. AS THE
SYSTEM WOBBLED IT ALSO SLOWED SIGNIFICANTLY, CUTTING TRACK SPEED IN
HALF, DOWN TO JUST 8 KNOTS SINCE 0000Z. WHILE SOME ADDITIONAL
WOBBLE IS CERTAINLY A POSSIBILITY GOING FORWARD, THE OVERALL TRACK
MOTION IS FORECAST TO REMAIN JUST A SHADE NORTH OF DUE WEST THROUGH
LANDFALL, AS THE SYSTEM TRACKS ALONG THE SOUTHERN SIDE OF THE DEEP
STR CENTERED TO THE NORTH OF HONG KONG. EVEN THOUGH THE SYSTEM
CONTINUES TO FIGHT MIGHTILY AGAINST THE STRONG AND PERSISTENT
EASTERLY SHEAR, IT WILL NOT BE ABLE TO SUSTAIN THAT FIGHT MUCH
LONGER, AND WILL BEGIN TO WEAKEN FROM NOW THROUGH LANDFALL UNDER THE 
INFLUENCE OF THE SHEAR AND POSSIBLE EYEWALL REPLACEMENT CYCLE (ERC). 
HOWEVER, THE SYSTEM IS STILL ANTICIPATED TO BE A VERY STRONG TYPHOON 
WHEN IT CROSSES THE COAST IN THE VICINITY OF DA NANG AROUND TAU 18. 
ONCE ASHORE THE SYSTEM WILL RAPIDLY WEAKEN DUE TO FRICTIONAL EFFECTS 
OF THE RUGGED TERRAIN AND ULTIMATELY DISSIPATE OVER NORTHEAST THAILAND 
BY TAU 48.
 

MODEL DISCUSSION: NUMERICAL GUIDANCE IS IN VERY GOOD AGREEMENT ON
THE TRACK THROUGH LANDFALL, WITH ALL CONSENSUS MEMBERS CONTAINED
WITHIN A 25NM ENVELOPE WHICH IS CENTERED JUST SOUTH OF DA NANG.
AFTER LANDFALL THE GUIDANCE SPREADS A BIT BUT REMAINS IN A
RELATIVELY SMALL ENVELOPE. THE JTWC FORECAST LIES ON THE NORTHERN
SIDE OF THE ENVELOPE IN LIGHT OF THE RECENT NEAR-TERM MOTION, WITH
HIGH CONFIDENCE. INTENSITY GUIDANCE IS IN GOOD AGREEMENT AS WELL,
WITH ALL CONSENSUS MEMBERS AGREEING ON WEAKENING FROM NOW TO
LANDFALL, FOLLOWED BY RAPID DISSIPATION THEREAFTER. AGAIN THE JTWC
FORECAST LIES ON THE HIGHER END OF THE GUIDANCE THROUGH THE
MAJORITY OF THE FORECAST PERIOD, WITH HIGH CONFIDENCE.  

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