WDPN32 PGTW 162100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 15W (PULASAN) WARNING NR 
001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 17.6N 140.5E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 884 NM SOUTHEAST OF KADENA AB
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 21 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS AN
ASYMMETRIC SYSTEM WITH DEEP CONVECTION FLARING TO THE EAST AND
SOUTHEAST OF A BROAD AND PARTIALLY EXPOSED LOW-LEVEL CIRCULATION
CENTER (LLCC). A 161211Z ASCAT-C SCATTEROMETER PASS SHOWED A MAX
WIND BAND CONTAINING 35 KT GALES EXTENDING OVER 200 NM NORTHEAST
AND SOUTHEAST OF THE LLCC. THE RADIUS OF MAXIMUM WINDS WAS
APPROXIMATELY 130 NM WITH A BROAD CENTRAL WIND MINIMUM AROUND THE
LLCC, CHARACTERISTIC OF A MONSOON DEPRESSION. WIND SPEEDS IN THE
WESTERN SEMICIRCLE ARE WELL BELOW GALE FORCE, WITH LITTLE TO NO
CONVECTION ON THAT SIDE. AN UPPER-LEVEL TUTT CELL NORTH-NORTHWEST
OF 15W IS IMPARTING 20-25 KT OF WESTERLY VERTICAL SHEAR, WITH
UPPER-LEVEL CIRRUS SEEN STREAMING ACROSS THE LLCC FROM WEST TO
EAST. A DEEP-LAYER DRY AIR MASS IS SITUATED BENEATH THIS TUTT CELL,
ENVELOPING THE WESTERN HALF OF 15W'S CIRCULATION.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SUBTROPICAL RIDGE OVER JAPAN

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: NO CURRENT ESTIMATE
   RJTD: T2.0 - 30 KTS
   RCTP: T1.5 - 25 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD
   OTHER FACTORS: DEEP-LAYER DRY AIR MASS TO THE NORTHWEST

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.

FORECAST DISCUSSION: 15W (PULASAN) IS ASSESSED TO BE A MONSOON
DEPRESSION WITH A CHARACTERISTICALLY BROAD CIRCULATION STRUCTURE,
CONSISTING OF A MAX WIND BAND LOCATED 130 TO 230 NM EAST OF THE
CENTER. DUE TO THE POTENTIAL FOR TRANSITION INTO A TROPICAL CYCLONE
AND IMPACTS TO OKINAWA WITHIN 48 HOURS, WARNINGS ARE BEING
INITIATED ON 15W (PULASAN). THE STORM'S SHORT-TERM MOTION HAS
VARIED SOMEWHAT DUE TO ITS BROAD NATURE, BUT THE OVERALL LONG-TERM
MOTION IS NORTHWESTWARD ALONG THE SOUTHERN PERIPHERY OF THE
SUBTROPICAL RIDGE OVER JAPAN. THE TRACK FORECAST IS
STRAIGHT-FORWARD, AS THIS SAME RIDGE SHOULD DIRECT PULASAN BRISKLY
NORTHWESTWARD INTO THE EAST CHINA SEA IN 36 TO 48 HOURS. SOME TRACK
ADJUSTMENTS ARE POSSIBLE AS THE VORTEX CONSOLIDATES, AS THE BROAD
CIRCULATION CENTER IS PRONE TO GETTING YANKED AROUND BY BURSTS OF
DEEP CONVECTION. THE JTWC TRACK FORECAST IS CLOSE TO THE
MULTI-MODEL CONSENSUS AND PASSES WITHIN 20 NM OF OKINAWA IN 36 TO
48 HOURS. PULASAN IS EXPECTED TO STRUGGLE TO INTENSIFY DURING THIS
TIME, AS AN UPPER-LEVEL LOW TO ITS NORTHWEST IS IMPARTING
SIGNIFICANT WESTERLY VERTICAL SHEAR ON THE VORTEX. SHEAR VALUES MAY
DECREASE AFTER 12 TO 24 HOURS, BUT 15W WILL ALSO BE TRACKING
THROUGH A DEEP-LAYER DRY AIR MASSES TO THE NORTHWEST, WITH THE
AFOREMENTIONED UPPER-LEVEL LOW HELPING TO TRANSPORT THIS DRY AIR
INTO THE STORM CORE. PULASAN MAY THUS MAINTAIN A BROAD STRUCTURE,
WITH THE MAX WIND BAND GRADUALLY ROTATING FROM THE EASTERN SIDE OF
THE CIRCULATION TO THE NORTHERN SIDE AS IT APPROACHES OKINAWA. IT
SHOULD BE STRESSED THAT THE CPA VALUE MAY DO A POORER JOB THAN
USUAL OF CONVEYING RISK IN THIS CASE, AS THE GALE-FORCE WIND BELT
COULD REMAIN MORE THAN 100 NM NORTH OF THE STORM CENTER ACCORDING
TO CURRENT MODEL GUIDANCE. 15W'S INTENSITY IS EXPECTED TO GRADUALLY
DECREASE AFTER 24 HOURS DUE TO A SLACKENING GRADIENT BETWEEN THE
STORM AND THE SUBTROPICAL RIDGE OVER JAPAN, BUT SHOULD A COMPACT
CONVECTIVE INNER CORE DEVELOP, THE INTENSITY WOULD LIKELY BE A BIT
HIGHER THAN FORECAST. AFTER 48 HOURS, A TRACK THROUGH THE EAST
CHINA SEA IS EXPECTED TO CARRY 15W INTO MAINLAND CHINA BY 72 HOURS,
WHERE IT WOULD QUICKLY DISSIPATE INLAND.

MODEL DISCUSSION: THE JTWC TRACK FORECAST IS CLOSE TO THE
MULTI-MODEL CONSENSUS, AND TRACK SPREAD IS VERY TIGHT, LEADING TO
HIGHER-THAN-AVERAGE CONFIDENCE. THE MODEL INTENSITY GUIDANCE HAS A
LARGER SPREAD RANGING FROM TROPICAL DEPRESSION TO STRONG TROPICAL
STORM INTENSITY DURING THE FIRST 48 HOURS. THE JTWC INTENSITY
FORECAST IS SLIGHTLY ABOVE THE MULTI-MODEL CONSENSUS, BROADLY
FOLLOWING A BLEND OF THE GFS, ECMWF, AND HAFS-A.

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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