WDPN31 PGTW 210300
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 17W (SEVENTEEN) 
WARNING NR 002//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 25.5N 123.1E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 260 NM WEST-SOUTHWEST OF KADENA AB
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 11 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED PROXY VIS AND THE FIRST FEW FRAMES OF VISUAL SATELLITE
IMAGERY REVEAL AN EXPOSED BUT TIGHTLY WRAPPED LOW LEVEL CIRCULATION
TRACKING OFF THE NORTHEAST COAST OF TAIWAN. THE SYSTEM HAS MANAGED
TO GENERATE A TOUCH OF DEEP CONVECTION OVER THE SOUTHWEST QUADRANT
DURING THE PAST SIX HOURS, WITH SURFACE REPORTS FROM NEARBY ISLANDS
AND THE AGENCY DVORAKS SUPPORTING AN INTENSITY OF 25 KNOTS. THE LOW
LEVEL CIRCULATION IS TRACKING TOWARDS A NARROW BAND OF INCREASED
MOISTURE EXTENDING FROM TAIWAN TO THE EAST-NORTHEAST ALL THE WAY TO 
THE TSUSHIMA STRAIT THAT WILL SOON PROVIDE AN ADDITIONAL MOISTURE 
SOURCE. VERTICAL WIND SHEAR ALONG TRACK IS ALSO DECREASING AND THOSE 
TWO ATTRIBUTES COUPLE TO SUPPORT A DECENT CHANCE FOR THE SYSTEM TO
DEEPEN FOR A BRIEF PERIOD. STEERING FLOW FROM THE SOUTHEAST IS
EASING WHILE A NEUTRAL TO VERY WEAK NORTHEAST STEERING FLOW HAS SET
UP OVER THE TAIWAN STRAIT.

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

CURRENT STEERING MECHANISM: TD 17W IS LEAVING THE INFLUENCE OF THE
SUBTROPICAL RIDGE AND HEADING TOWARDS A NEUTRAL STEERING
ENVIRONMENT. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   RJTD: T1.0 - 25 KTS
   CIMSS ADT: 25 KTS AT 202340Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 10 KTS
   SST: 28-30 CELSIUS
   OUTFLOW: WEAK EQUATORWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TD 17W WILL SLOW AS IT TRACKS NORTH OF TAIWAN,
PAUSE, AND THEN GET SHOVED TO THE SOUTHEAST AS IT ENCOUNTERS A
RIBBON OF NORTHEASTERLIES FLOWING ALONG THE SOUTHEAST COAST OF
CHINA.  THE ISLAND OF TAIWAN WILL CUT OFF THE SOUTHERLIES OVER THE
EASTERN SEMICIRCLE, REDUCING GRADIENT LEVEL VORTICITY.  THEREFORE 
ONCE THE SYSTEM APPROACHES THE NORTHERN PASSAGE OF THE STRAIT, THE 
VORTEX IS EXPECTED TO STRETCH AND WEAKEN.  AMPLE MOISTURE WILL SUSTAIN 
THE SYSTEM THAT EXISTS IN THE TAIWAN STRAIT, BUT THERE IS NOTHING TO 
DEEPEN IT AND GUIDANCE INDICATES THAT THE SYSTEM WILL FILL GRADUALLY 
AND BECOME INDISCERNIBLE FROM ITS EXTERNAL ENVIRONMENT PRIOR TO
LANDFALL TO THE NORTHEAST OF HONG KONG.  

MODEL DISCUSSION: THE GUIDANCE IS STILL BIFURCATED SHOWING A SPLIT
AT TAU 24 BUT MORE GUIDANCE HAS FLOPPED OVER TO THE SOUTHERLY
TRACK. BOTH THE AMERICAN ENSEMBLE AND THE ECMWF DETERMINISTIC HAVE
POLEWARD SOLUTIONS, BUT THE WIDE DISPARITY IS A FUNCTION OF THE
WEAKNESS OF THE SYSTEM. THE JTWC TRACK FORECAST FOLLOWS THE LOGIC
THAT A SHALLOW SYSTEM WOULD BE KICKED SOUTHWESTWARD BY THE
NORTHEASTERLY WINDSTREAM OFF-SHORE OF CHINA. THE INTENSITY FORECAST
BALANCES THE SLIGHTLY IMPROVING ENVIRONMENT UPSTREAM AND THE
CURRENT EXPOSED STATE OF THE VORTEX AND THE MARGINAL ENVIRONMENT
OVER THE TAIWAN STRAIT. 

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