WDXS31 PGTW 152100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 13S (DJOUNGOU) WARNING 
NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 15.2S 64.9E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 660 NM SOUTHWEST OF DIEGO GARCIA
   MOVEMENT PAST 6 HOURS: EASTWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 14 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
RAPIDLY ORGANIZING CYCLONE WITH DEEP CONVECTIVE BURSTS AS COLD AS
-90 DEGREES CELSIUS PULSING ON ALL SIDES OF THE LOW-LEVEL
CIRCULATION CENTER (LLCC). A 1718Z GMI MICROWAVE PASS SHOWED A
FORMATIVE EYEWALL WITH A RADIUS OF ABOUT 20 NM. SINCE THAT TIME, A
WARM SPOT HAS OCCASIONALLY APPEARED NEAR THE LLCC IN EIR IMAGERY.
BASED ON THE STRUCTURAL EVOLUTION OVER THE PAST SIX HOURS, IT IS
LIKELY THAT THE CYCLONE IS BEGINNING A PERIOD OF RAPID
INTENSIFICATION. THE INITIAL INTENSITY IS SET AT 35 KT BASED ON A
1736Z ASCAT-C PASS SHOWING 35 KT WINDS IN THE NORTHEAST QUADRANT,
SUPPORTED BY AGENCY DVORAK INTENSITY ESTIMATES. HOWEVER, AS NOTED,
QUICK INTENSIFICATION HAS LIKELY BEEN OCCURRING SINCE THE 1800Z
WARNING TIME.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: NEAR-EQUATORIAL RIDGE TO THE NORTH

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.5 - 35 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: HIGHLY FAVORABLE
   VWS: 0-5 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD AND EQUATORWARD

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TROPICAL CYCLONE 13S (DJOUNGOU) APPEARS TO BE
RAPIDLY FORMING A COMPACT INNER CORE STRUCTURE WITHIN AN
ENVIRONMENT CONSISTING OF VERY WARM SEA SURFACE TEMPERATURES, LIGHT
SHEAR, AND DEEP MOISTURE. THESE CONDITIONS ARE EXPECTED TO PERSIST
FOR THE NEXT 36-48 HOURS AS DJOUNGOU TRACKS EASTWARD, GUIDED BY A
MID-LATITUDE LONGWAVE TROUGH WELL TO THE SOUTHEAST. ADDITIONALLY,
DJOUNGOU WILL PASS OVER AN EDDY OF LOCALLY HIGHER OCEAN HEAT
CONTENT VALUES (IN EXCESS OF 60 KJ PER SQUARE CM) DURING THE 24-48
HOUR PERIOD. THIS ENVIRONMENT IS LIKELY TO FACILITATE RAPID
INTENSIFICATION (RI) IN THE NEAR TERM, AND STATISTICAL RI AIDS
SUPPORT THIS WITH 40-60 PERCENT PROBABILITIES OF RI DURING THE NEXT
48 HOURS. THE JTWC INTENSITY FORECAST BRINGS DJOUNGOU TO TYPHOON
INTENSITY (65 KT) IN 24 HOURS AND TO 100 KT IN 48 HOURS. PEAK
INTENSITY IS LIKELY TO OCCUR BETWEEN 48 AND 72 HOURS AS THE CYCLONE
APPROACHES THE SUBTROPICAL JET, BENEFITING FROM CONSTRUCTIVE
INTERFERENCE WITH A 130 KT JET STREAK TO THE SOUTHEAST THAT WILL
PROVIDE SIGNIFICANT UPPER-LEVEL DIVERGENT FORCING OVER THE CYCLONE.
BY 72 HOURS, VERTICAL SHEAR WILL INCREASE DRAMATICALLY AS DJOUNGOU
NEARS THE JET AXIS, AND THE CYCLONE'S ACCELERATING SOUTHEASTWARD
MOTION ALONG THE JET WILL BRING IT OVER COOLER WATERS, ULTIMATELY
LOWER THAN 26 DEGREES CELSIUS BY 120 HOURS, LEADING TO RAPID
WEAKENING. MOST DYNAMICAL MODELS PREDICT THAT DJOUNGOU WILL BEGIN
EXTRATROPICAL TRANSITION AROUND 96 HOURS, ULTIMATELY BECOMING
ABSORBED INTO A FRONTAL BOUNDARY BY 120 HOURS. THERE IS SIGNIFICANT
ALONG-TRACK SPREAD DURING THE 72-120 HOUR PERIOD DUE TO UNCERTAINTY
IN HOW QUICKLY THE CYCLONE WILL VERTICALLY DECOUPLE AND WHETHER THE
MID-LATITUDE TROUGH WILL FULLY ENTRAIN IT.

MODEL DISCUSSION: DYNAMICAL MODEL GUIDANCE HAS FAIRLY TIGHT
CROSS-TRACK SPREAD THROUGH THE FORECAST PERIOD, BUT ALONG-TRACK
SPREAD GROWS DRAMATICALLY DURING THE 72-120 HOUR PERIOD. THE JTWC
FORECAST IS CLOSE TO THE MULTI-MODEL CONSENSUS, BUT LEANS TOWARDS
THE SLOWER ECMWF, ECMWF ENSEMBLE MEAN, AND GFS ENSEMBLE MEAN BEYOND
72 HOURS DUE TO BETTER RUN-TO-RUN CONSISTENCY. THE JTWC INTENSITY
FORECAST IS A BIT ABOVE THE MULTI-MODEL CONSENSUS, TAKING INTO
ACCOUNT THE EXPECTATION FOR RAPID INTENSIFICATION DURING THE FIRST
48 HOURS.

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