WDIO31 PGTW 120900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL CYCLONE 03A (THREE) WARNING NR 
001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.7N 65.7E
   INITIAL INTENSITY: 35 KTS
   GEOGRAPHIC REFERENCE: 199 NM SOUTH-SOUTHWEST OF KARACHI,
PAKISTAN
   MOVEMENT PAST 6 HOURS: WESTWARD AT 07 KTS
   SIGNIFICANT WAVE HEIGHT: 16 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A SHEARED
SYSTEM, WITH DEEP FLARING CONVECTION DISPLACED TO THE WEST OF A
PARTIALLY OBSCURED LOW LEVEL CIRCULATION CENTER (LLCC). TC 03A HAS
EMERGED OFF THE GUJARAT PENINSULA AND MOVED WESTWARD UNDER A
MARGINALLY FAVORABLE UPPER-LEVEL FLOW PATTERN, WHICH ALLOWED FOR A
BRIEF BURST OF CONVECTIVE DEVELOPMENT AND ORGANIZATION. JTWC HAND
ANALYSIS OF 120000Z UPPER-LEVEL ATMOSPHERIC MOTION VECTORS (AMVS)
REVEALED A POINT SOURCE OVER TOP OF THE LLCC, WHICH HAS INDUCED
RELATIVELY LOW SHEAR, IN THE 10-15 KNOT RANGE. SUBSEQUENT AMVS
REVEAL THIS POINT SOURCE IS MOVING OFF TO THE WEST OF THE LLCC,
LEAVING IT VULNERABLE TO INCREASING EASTERLY PRESSURE AND SHEAR,
ALREADY STARTING TO INCREASE. A 120044Z SSMIS 91GHZ MICROWAVE IMAGE
REVEALS DISORGANIZED DEEP CONVECTION CURVED CYCLONICALLY IN TOWARDS
THE CENTER, THROUGHOUT THE WESTERN HEMISPHERE OF THE SYSTEM. THE
INITIAL POSITION IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON AN
EXTRAPOLATION OF THE MICROWAVE IMAGERY AND THE PGTW FIX POSITION. A
120620Z ASCAT-B PARTIAL PASS REVEALED 35-40 KNOT WINDS IN THE
WESTERN HEMISPHERE OF THE CIRCULATION, LENDING INCREASED
CONFIDENCE TO THE INITIAL INTENSITY, WHICH IS NUDGED HIGHER THAN
THE AGENCY DVORAK FIXES. THE SYSTEM LIES IN A MARGINAL ENVIRONMENT
WITH WARM (28-29C) SSTS, MODERATE VWS (20-25 KTS) AND STRONG
EQUATORWARD OUTFLOW. THE SYSTEM LIES IN A WEAK STEERING
ENVIRONMENT, CAUGHT IN A COL REGION BETWEEN A RIDGE OVER THE SAUDI
PENINSULA, A WESTWARD EXTENSION OF THE STR OVER FAR NORTHERN INDIA
AND A NEAR EQUATORIAL RIDGE (NER) JUST SOUTHWEST OF SOUTHERN INDIA.

INITIAL WIND RADII BASIS: OBJECTIVE BEST TRACK. 

CURRENT STEERING MECHANISM: WEAK STEERING PATTERN. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   KNES: T1.5 - 25 KTS
   DEMS: T1.5 - 25 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 20-25 KTS
   SST: 28-29 CELSIUS
   OUTFLOW: STRONG EQUATORWARD
   OTHER FACTORS: DRY AIR SOUTHEAST AND NORTH OVER SOUTHERN
PAKISTAN.

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

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TC 03A RAPIDLY DEVELOPED AND INTENSIFIED TO A
TROPICAL STORM STRENGTH AFTER EMERGING OVER WATER. NOW THAT ITS
MOVED INTO THE NORTH-CENTRAL NORTH ARABIAN SEA, IT HAS MOVED INTO A
WEAK STEERING PATTERN, TRAPPED BETWEEN COMPETING STEERING
INFLUENCES OF A SERIES OF DEEP-LAYER RIDGES TO THE WEST, NORTH AND
SOUTH. OVER THE NEXT 24 TO 36 HOURS, THE WESTERN EXTENSION OF RIDGE
TO THE NORTH IS EXPECTED TO RECEDE AND WEAKEN, AND THE SOUTHERN
RIDGE WILL BECOME THE DOMINANT STEERING MECHANISM, WITH THE RIDGE
OF THE SAUDI PENINSULA ACTING AS A BLOCK TO MUCH FURTHER WESTWARD
MOVEMENT. AS THE NER TO THE SOUTH MOVES DRIFTS NORTHWARD, IT WILL
PUSH TC 03A NORTH AS WELL, WITH THE SYSTEM FORECAST TO SLOWLY MOVE
NORTHWEST THEN NORTH THROUGH THE FORECAST PERIOD. THE BEST
ENVIRONMENTAL CONDITIONS ARE OCCURRING NOW OR LIKELY ALREADY COME
AND GONE, AND THUS THE SYSTEM IS LIKELY TO EXPERIENCE ONLY A VERY
SHORT WINDOW OF OPPORTUNITY FOR FURTHER INTENSIFICATION, WITH A
PEAK OF 40 KNOTS POSSIBLE BY TAU 12. THEREAFTER, AS THE UPPER-LEVEL
POINT SOURCE DISSIPATES AND MOVES AWAY, THE GENERALLY STRONG
EASTERLY FLOW ALOFT ASSOCIATED WITH THE TROPICAL EASTERLY JET (TEJ)
WILL MOVE IN ONCE MORE, AND SHEAR THE SYSTEM APART. THIS WILL BE
ACCOMPANIED BY AN INFLUX OF DRY AND DUSTY AIR FROM THE DESERTS OF
AFGHANISTAN AND PAKISTAN, WHICH WILL SMOTHER ANY REMNANT
CONVECTION, LEAVING AN EXPOSED AND STEADILY WEAKENING CIRCULATION
WHICH IS EXPECTED TO DISSIPATE AS A TROPICAL CYCLONE NO LATER THAN
TAU 36. 

MODEL DISCUSSION: DETERMINISTIC TRACK GUIDANCE IS IN GENERAL
AGREEMENT, AT LEAST THROUGH THE DURATION OF THE FORECAST, WITH ALL
CONSENSUS MEMBERS AGREEING ON A SLOW WEST-NORTHWESTWARD TRACK
THROUGH TAU 24, FOLLOWED BY A SHARP TURN TO THE NORTH-NORTHEAST BY
TAU 36. SPREAD IS ROUGHLY 140NM BETWEEN THE BULK OF THE MODELS, WITH
GFS ON THE FAR WEST AND ECMWF ON THE FAR EAST SIDE OF THE ENVELOPE.
NAVGEM MEANWHILE IS DISCOUNTED AS IT TRACKS THE CIRCULATION BACK
OVER LAND EAST OF KARACHI BY TAU 36, WHICH IS HIGHLY UNREALISTIC.
THE JTWC FORECAST LIES CLOSE TO THE CONSENSUS MEAN WITH MEDIUM
CONFIDENCE. INTENSITY GUIDANCE IS ALSO IN FAIRLY GOOD AGREEMENT,
WITH HWRF, COAMPS-TC AND DECAY-SHIPS (GFS) INDICATING A PEAK OF 40
KNOTS AT TAU 12 FOLLOWED BY STEADY WEAKENING TO 30 KNOTS BY TAU 36.
THE JTWC FORECAST LIES ON THE CONSENSUS MEAN THROUGH TAU 12 AND
DISSIPATES THE SYSTEM AT 30 KNOTS BAY TAU 36, IN LINE WITH THE BULK
OF THE GUIDANCE, WITH MEDIUM CONFIDENCE. 

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