WDPN31 PGTW 191500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 30W (FENGSHEN) 
WARNING NR 007//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 16.1N 118.8E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 156 NM NORTHWEST OF MANILA, PHILIPPINES
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 16 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
FENGSHEN HAS BEEN DOWNGRADED TO A TROPICAL DEPRESSION, AND EARLIER
POSITIONS HAVE BEEN REANALYZED TO REFLECT A LOWER INTENSITY.
PRIMARILY, THESE CHANGES HAVE BEEN MADE TO REFLECT THE 191234Z
ASCAT-B BULLSEYE PASS, WHICH REVEALED A VERY SMALL ROTATION WEST OF
THE LINGAYEN GULF, WITH MAXIMUM ASSOCIATED WINDS OF JUST 25 KNOTS,
THESE BEING RESTRICTED TO THE EASTERN SECTIONS OF THE ROTATION. THE
WESTERN PORTION SHOWED WINDS OF LESS THAN 15 KNOTS, WHILE WINDS OF
30-35 KNOTS WERE PRESENT FAR TO THE NORTH, STREAMING SOUTHWESTWARD
OUT OF THE LUZON STRAIT. ANIMATED ENHANCED INFRARED (EIR) SATELLITE
IMAGERY SHOWS COLLAPSING CONVECTION TO THE SOUTH, AND ISOLATED
POCKETS OF MODERATE CONVECTION STARTING TO FORM TO THE NORTHWEST
AND WEST OF THE LOW LEVEL CIRCULATION CENTER (LLCC). THE INITIAL
POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE SMALL
CIRCULATION CENTER IN THE ASCAT DATA. THE INITIAL INTENSITY IS
ASSESSED WITH HIGH CONFIDENCE USING THE ASCAT DATA AS WELL, AND
CONSISTENT WITH THE RJTD T2.0 FIX AS WELL AS THE D-MINT AND D-PRINT
ESTIMATES. THE ENVIRONMENT REMAINS FAVORABLE OVERALL, THOUGH A
REDUCTION IN UPPER-LEVEL OUTFLOW, PARTICULARLY THE CHOKING OFF OF
MOST OF THE EQUATORWARD COMPONENT AND THE INFLUX OF SOME ELEVATED
MID-LEVEL EASTERLY SHEAR, HAS PUT A DAMPER ON THINGS FOR THE TIME
BEING.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) FAR TO THE NORTHEAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T3.0 - 45 KTS
   RJTD: T2.0 - 30 KTS
   RCTP: T3.0 - 45 KTS
   KNES: T3.0 - 45 KTS
   DEMS: T3.0 - 45 KTS
   CIMSS ADT: 37 KTS AT 191140Z
   CIMSS AIDT: 35 KTS AT 191140Z
   CIMSS D-MINT: 30 KTS AT 191021Z
   CIMSS D-PRINT: 28 KTS AT 191300Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 10-15 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: MID-LEVEL EASTERLY SHEAR. 

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THE INITIAL INTENSITY HAS BEEN
LOWERED TO TROPICAL DEPRESSION STRENGTH AND THE FORECAST HAS BEEN
TRUNCATED TO 96 HOURS. 

FORECAST DISCUSSION: TROPICAL DEPRESSION (TD) 30W IS FORECAST TO
TRACK NORTHWESTWARD ALONG THE SOUTHWESTERN PERIPHERY OF THE STR TO
THE NORTHEAST OVER THE NEXT 12 TO 24 HOURS, THEN TURN SHARPLY WEST
AS THE RIDGE EXTENDS WESTWARD INTO SOUTHERN CHINA. AFTER TAU 24,
THE SYSTEM SLOWS QUICKLY AS IT MOVES INTO A WEAK AND COMPETING
STEERING PATTERN BETWEEN RIDGING TO THE WEST AND EAST IN THE
UPPER-LEVELS, WHILE STRONG NORTHEASTERLY FLOW ASSOCIATED WITH A
COLD SURGE WILL SERVE AS A BUFFER ALONG THE COAST OF SOUTHERN CHINA
AND BLOCK FURTHER WESTWARD MOVEMENT. BY TAU 36, THE RIDGE OF
NORTHERN LAOS BUILDS, SETTING UP A DEEP NORTHEASTERLY FLOW THROUGH
THE ATMOSPHERIC COLUMN, FORCING TD 30W TO MAKE A RAPID TURN TOWARDS
THE SOUTHWEST. THE SYSTEM WILL THEN ACCELERATE SOUTHWESTWARD
THROUGH THE REMAINDER OF THE FORECAST PERIOD, MAKING LANDFALL ALONG
THE CENTRAL COAST OF VIETNAM AROUND TAU 84. THE RECENT MOTION OF TD
30W HAS KEPT IT WITHIN FAIRLY CLOSE PROXIMITY TO WESTERN LUZON,
WHICH HAS DISRUPTED THE VORTEX CONSOLIDATION PROCESS. COMBINED WITH
THE LOSS OF THE EQUATORWARD OUTFLOW CHANNEL, THIS HAS LED TO AN
OVERALL WEAKENING OF THE SYSTEM. HOWEVER, THE MOST RECENT FRAMES OF
WATER VAPOR IMAGERY SHOW UPPER-LEVEL POLEWARD OUTFLOW IS ALREADY
STARTING TO IMPROVE, FLOWING INTO THE BASE OF A JET MAX EAST OF
SHANGHAI. AND THE LATEST FRAMES OF SHORTWAVE INFRARED (SWIR)
SUGGEST THE LLCC MAY BE RAPIDLY CONSOLIDATING, NOW THAT IT'S MOVE
OFFSHORE ENOUGH TO ESTABLISH UNINTERRUPTED FLOW ON THE EASTERN
FLANK. GLOBAL MODELS INDICATE THE OUTFLOW WILL STEADILY IMPROVE,
WHILE SHEAR REMAINS RELATIVELY LOW, THROUGH TAU 36, ALLOWING THE
SYSTEM TO STEADILY INTENSIFY TO A PEAK OF 60 KNOTS BY TAU 36. AFTER
THIS POINT IN THE FORECAST, SHEAR IS EXPECTED TO INCREASE, AND THE
SYSTEM WILL SUFFER FROM DRY AIR ENTRAINED INTO THE CORE, RESULTING
IN RAPID WEAKENING THROUGH TAU 72.  THE SYSTEM WILL MAKE LANDFALL
AS A WEAK TROPICAL STORM, AND DISSIPATE OVER THE MOUNTAINS OF
SOUTHERN VIETNAM BY TAU 96. 

MODEL DISCUSSION: MODEL GUIDANCE IS IN FAIR AGREEMENT. ALL MEMBERS
OF THE TRACK CONSENSUS AGREE ON THE OVERALL TRACK, BUT DISAGREE ON
HOW FAR NORTH THE TRACK WILL GET BEFORE TURNING BACK TO THE
SOUTHWEST. THE FNV3 AND ECMWF ARE POSITIONED ON THE NORTHERN EDGE
OF THE GUIDANCE ENVELOPE, WHILE THE GALWEM AND GEFS MEAN ARE
LOCATED ON THE SOUTHERN EDGE, WITH A 125NM SPREAD BETWEEN THEM AT
TAU 48. AFTER TAU 48, CROSS-TRACK SPREAD DOES NOT CHANGE MUCH, BUT
THE DISPERSION AXIS SHIFTS TO THE ALONG-TRACK AXIS. BY TAU 72, THE
DISTANCE BETWEEN THE FAST JGSM AND THE SLOW FNV3 INCREASES TO 225NM
AND INCREASES FURTHER TO 300NM BY THE END OF THE FORECAST PERIOD.
THE JTWC FORECAST IS PLACED ON THE NORTHERN EDGE OF THE GUIDANCE
THROUGH TAU 36, CONSISTENT WITH THE FNV3 AND ECMWF. THEREAFTER, THE
TRACK SHIFTS TO HEDGE CLOSE TO THE ECMWF THROUGH THE END OF THE
FORECAST PERIOD. INTENSITY GUIDANCE IS ALSO SHOWING INCREASED
SPREAD, WITH THE SHIPS MEMBERS SHOWING A PEAK OF JUST 40 KNOTS,
WHILE THE HAFS-A PEAKS NEAR 60 KNOTS BUT AT TAU 12, WHILE HWRF AND
COAMPS-TC REACH A PEAK OF 55-65 KNOTS AT TAU 48. THE JTWC FORECAST
IS CONSISTENT WITH THE HWRF SOLUTION THROUGH THE FORECAST PERIOD. 

FORECAST CONFIDENCE:
   TRACK 00-72 HR: MEDIUM
   TRACK 72-120 HR: LOW
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: LOW//
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