WDXS31 PGTW 102100
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 17S (SEVENTEEN) WARNING 
NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 19.8S 38.5E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 205 NM EAST OF BEIRA, MOZAMBIQUE
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 05 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY SHOWS A MEDIUM
SIZED SYSTEM IN THE MOZAMBIQUE CHANNEL WITH FORMATIVE BANDS
CONSOLIDATING AND WRAPPING IN FROM THE NORTHEAST THROUGH SOUTH INTO
A PARTIALLY EXPOSED LOW-LEVEL CIRCULATION (LLC). COLD AIR
STRATOCUMULUS LINES CAN ALSO BE SEEN ALONG THE WESTERN HEMISPHERE
FEEDING INTO THE LLC. THE INITIAL POSITION IS PLACED WITH HIGH
CONFIDENCE BASED ON THE PARTIALLY EXPOSED LLC THAT LINED UP WELL
WITH A MICROWAVE LLC FEATURE IN THE 101525Z SSMIS IMAGE SUITE.
ANALYSIS INDICATES A MARGINALLY FAVORABLE ENVIRONMENT WITH WARM SST
AND STRONG POLEWARD OUTFLOW ALOFT OFFSET BY MODERATE VERTICAL WIND
SHEAR THE COLD DRY AIR INTRUSION.

INITIAL WIND RADII BASIS: PERSISTENCE. 

CURRENT STEERING MECHANISM: LOW REFLECTION OF SUBTROPICAL RIDGE
(STR) TO THE SOUTH.

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

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: COLD AIR INTRUSION AT THE LOW LEVELS. 

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: TC 17S WILL CONTINUE WESTWARD UNDER THE
STEERING INFLUENCE OF THE STR TO THE SOUTH. AFTER TAU 24, A
SECONDARY STR TO THE EAST WILL BUILD AND EXTEND WEST-NORTHWESTWARD,
ASSUME STEERING AND DRIVE THE SYSTEM AROUND ITS WESTERN PERIPHERY
SOUTHWESTWARD, INTO LANDFALL OVER CENTRAL MOZAMBIQUE SOUTH OF BEIRA
JUST BEFORE TAU 36. AFTER TAU 48, TC 17S WILL CREST THE RIDGE AXIS
THEN ACCELERATE SOUTHEASTWARD. THE MARGINALLY FAVORABLE ENVIRONMENT
WILL FUEL A MODEST INTENSIFICATION TO 50KTS BY TAU 24 BEFORE
LANDFALL. DRY AIR INTRUSION AND FRICTIONAL EFFECTS FROM THE RUGGED
MOZAMBIQUE TERRAIN WILL ERODE THE SYSTEM DOWN TO 30KTS BY TAU 48.
TC 17S WILL SPLASH BACK OVER WATER IN THE SOUTH INDIAN OCEAN AROUND
TAU 60. MOISTURE INFUSION AND INCREASED POLEWARD OUTFLOW WILL FUEL
A SECONDARY INTENSIFICATION TO A PEAK OF 50KTS AT TAU 96.
CONCURRENTLY BY TAU 72, THE SYSTEM WILL UNDERGO SUBTROPICAL
TRANSITION AND BY TAU 96, WILL ENTER THE BAROCLINIC ZONE AND
COMMENCE EXTRA-TROPICAL TRANSITION. BY TAU 120, TC 17S WILL
TRANSFORM INTO A STORM-FORCE COLD CORE LOW WITH AN EXPANDING WIND
FIELD. 

MODEL DISCUSSION: NUMERICAL FORECAST TRACK MODELS ARE SURPRISINGLY
IN TIGHT AGREEMENT FOR A DEVELOPING SYSTEM, GRADUALLY AND EVENLY
SPREADING OUT TO MERE 93NM BY TAU 72 AND 175NM BY TAU 120; HOWEVER,
GIVEN THE UNCERTAINTIES ASSOCIATED WITH A FORMATIVE SYSTEM, ITS
TRACK OVER LAND, AND ITS SUBTROPICAL AND EXTRA-TROPICAL
TRANSITIONS, THERE IS ONLY MEDIUM CONFIDENCE IN THE NEAR- TO
EXTENDED-PORTION OF THE JTWC TRACK FORECAST. 

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