WDPN33 PGTW 171500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 23W (NESAT) WARNING NR 012//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.0N 115.4E
   INITIAL INTENSITY: 90 KTS
   GEOGRAPHIC REFERENCE: 209 NM EAST-SOUTHEAST OF HONG KONG
   MOVEMENT PAST 6 HOURS: WESTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 36 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TY 23W IS ACTING LIKE THE LITTLE ENGINE THAT COULD, CONTINUES TO
FACE DOWN PERSISTENT SHEAR AND REFUSES TO GIVE UP THE GHOST.
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY INDICATES A
PINHOLE HAS DEVELOPED, MEASURED AT JUST 7NM, COCOONED IN A 200NM
WIDE CENTRAL DENSE OVERCAST (CDO). DEEP CONVECTION IS LIMITED TO A
VERY THIN BAND ON THE EAST SIDE, WITH THE CDO EXHIBITING A VERY
SHARP EDGE FACING THE SHEAR, WHICH IS BEING SLOWLY BUT STEADILY
ERODED. THE MOST RECENT SSMIS 91GHZ MICROWAVE IMAGE FROM 170924Z
SHOWED THAT THE BULK OF THE DEEP CONVECTION IS CONSTRAINED TO WEST
AND SOUTH, WITH AN OPEN EYEWALL ON THE EAST SIDE AND LIMITED
CONVECTION IN THE MAJORITY OF THE EASTERN HEMISPHERE OF THE SYSTEM.
THE SYSTEM HAS HOWEVER BECOME MORE VERTICALLY STACKED SINCE 12
HOURS AGO BASED ON THE LOW LEVEL EYE FEATURE IN THE 36GHZ IMAGE.
THE INITIAL POSITION IS ASSESSED WITH HIGH CONFIDENCE BASED ON
EXTRAPOLATION OF THE MICROWAVE EYE AND A TIGHT GROUPING OF AGENCY
FIXES CENTERED ON THE PINHOLE EYE. THE INITIAL INTENSITY OF 90
KNOTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE PREPONDERANCE
OF SUBJECTIVE AND OBJECTIVE INTENSITY ESTIMATES AND A 170957Z SMAP
PASS WHICH SHOWED A MAX WIND OF 85 KNOTS WHICH CONVERTS TO A 91
KNOT 1-MIN WIND. THE ENVIRONMENT REMAINS FAVORABLE FOR THE MOMENT,
WITH SHEAR HAVING PICKED UP A BIT TO MODERATE LEVELS, GOOD RADIAL
OUTFLOW AND WARM SSTS. DRY AIR CONTINUES TO BE AN INHIBITING FACTOR,
PARTICULARLY IN THE EAST AND NORTHEAST QUADRANTS.

INITIAL WIND RADII BASIS: SMAP DATA.

CURRENT STEERING MECHANISM: EASTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) CENTERED NEAR MYANMAR. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T5.0 - 90 KTS
   RJTD: T4.5 - 77 KTS
   KNES: T5.0 - 90 KTS
   CIMSS ADT: 85 KTS AT 171210Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 15-20 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: MODERATE RADIAL
   OTHER FACTORS: DRY AIR INTRUSION IN THE EAST AND NORTHEAST
QUADRANTS.

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: CONSISTENT WITH THE PREVIOUS FEW FORECASTS,
THERE HAS BEEN NO SIGNIFICANT CHANGE IN THE FORECASTED STEERING
PATTERN. TY 23W WILL CONTINUE TO TRACK WEST-SOUTHWEST ALONG THE
SOUTH PERIPHERY OF THE DEEP STR, TURNING SLIGHTLY MORE
SOUTHWESTWARD, THROUGH THE 24 HOURS. THE SYSTEM REACHES AN
INFLECTION POINT SOUTHEAST OF HAINAN AT TAU 36 WHEN THE STR CENTER
IS EXPECTED TO REPOSITION TO NEAR TAIWAN, PLACING TY 23W NOW ON THE
SOUTHWEST SIDE OF THE STEERING RIDGE. IN RESPONSE, THE SYSTEM WILL
SLOW DOWN A BIT, AND TURN ONTO A MORE WEST-NORTHWEST TRACK THROUGH
THE END OF THE FORECAST. THE SYSTEM HAS LIKELY REACHED ITS PEAK,
AND EVEN THOUGH IT HAS BEEN SUCCESSFUL IN FIGHTING BACK AGAINST THE
SHEAR, THE EROSION OF THE EASTERN SIDE OF THE CDO INDICATES THAT
ITS DAYS ARE NUMBERED. EVEN WHEN IT BEGINS TO SHEAR APART, THE
SYSTEM SHOULD MAINTAIN INTENSITY FOR ABOUT 12 HOURS BUT THEN WILL
EMBARK UPON A FAIRLY RAPID WEAKENING TREND AFTER TAU 12. AFTER THE
SYSTEM ROUNDS SOUTHERN HAINAN, UPPER-LEVEL CONVERGENCE, MODERATE
SOUTHERLY SHEAR AND AN INFLUX OF DRY AIR WILL QUICKEN THE PACE OF
WEAKENING, WITH THE SYSTEM FORECAST TO BE BELOW TROPICAL STORM
STRENGTH BY TAU 72. AS THE SYSTEM RAPIDLY WEAKENS SOUTH OF HAINAN,
THE LOWER-LEVEL FLOW WILL IMPART A LARGER STEERING INFLUENCE, AND
THE SYSTEM WILL TURN WEST AS IT MAKES LANDFALL ALONG THE NORTHERN
VIETNAMESE COAST JUST AFTER TAU 72. THE SYSTEM WILL RAPIDLY
DISSIPATE OVER THE RUGGED TERRAIN BY TAU 96.  

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN GENERAL
AGREEMENT ON THE TRACK SCENARIO, THROUGH THE CONSENSUS MEAN
CONTINUES TO SHIFT SOUTHWARD A LITTLE MORE WITH EACH RUN. THE
NAVGEM CONTINUES TO BE NORTHERN MARKER AND UKMET ENSEMBLE THE
SOUTHERN, DEFINING A 120NM ENVELOPE AT TAU 72. BY TAU 96 AS THE
SYSTEM RAPIDLY WEAKENS, MODEL GUIDANCE GETS SQUIRRELY, WITH SOME
MODELS LOSING THE VORTEX OR TRACKING IT ERRATICALLY. HOWEVER THE
BULK OF THE GUIDANCE IS TIGHTLY PACKED NEAR THE FORECAST TRACK
THROUGH THE FORECAST PERIOD, PROVIDING HIGH CONFIDENCE THROUGH TAU
72 AND MEDIUM THEREAFTER. INTENSITY GUIDANCE IS IN GOOD AGREEMENT
THAT THE SYSTEM HAS PEAKED AND WILL BEGIN RAPID WEAKENING AFTER TAU
12. THE JTWC FORECAST LIES ON THE HIGH SIDE OF THE ENVELOPE WITH
MEDIUM CONFIDENCE. 

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