Tropical Storm TWENTYONE Advisory Mi, 12.10.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE)
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
SUMMARY:
   INITIAL POSITION: 15.2N 151.3E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 318 NM EAST OF SAIPAN
   MOVEMENT PAST 6 HOURS: SOUTH-SOUTHEASTWARD AT 04 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET
SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
COMPACT, SYMMETRICAL AREA OF CONVECTION DISPLACED SLIGHTLY
SOUTHEASTWARD OF A PARTIALLY EXPOSED LOW LEVEL CIRCULATION CENTER
(LLCC). THE LATEST AVAILABLE MICROWAVE IMAGE WAS A SSMIS 91GHZ PASS
FROM 120705Z AND SHOWED WEAK LOW-LEVEL BANDING FEATURES TO THE
NORTHWEST OF AN AREA OF DEEP, BUT RELATIVELY DISORGANIZED
CONVECTION. RECENT SCATTEROMETER PASSES FROM THE 121100Z HOUR
INDICATE A WELL-DEFINED LLCC, WHICH ALONG WITH MULTIPLE AGENCY FIX
POSITIONS, PROVIDED HIGH CONFIDENCE TO THE INITIAL POSITION. THE
INITIAL INTENSITY OF 25 KNOTS IS ASSESSED WITH HIGH CONFIDENCE
BASED ON DVORAK CURRENT INTENSITY ESTIMATES OF T1.5 (25 KNOTS) FROM
BOTH PGTW AND KNES AS WELL AS THE AFOREMENTIONED SCATTEROMETER
PASSES WHICH SHOWED 25-28 KNOT WINDS IN THREE QUADRANTS OF THE
CIRCULATION. THE OVERALL ENVIRONMENT IS MARGINALLY FAVORABLE, WITH
THE SYSTEM MOVING SLOWLY OVER VERY WARM, HIGH OHC WATERS, PROVIDING
AMPLE TCHP. A TROPICAL UPPER TROPOSPHERIC TROUGH (TUTT) CELL IS
CURRENTLY SITTING DUE NORTH OF 21W, IMPARTING A MODERATE
NORTHWESTERLY SHEAR OVER THE SYSTEM AS EVIDENCED BY THE SATELLITE
DEPICTION. FINALLY, OUTFLOW IS MODERATE TOWARDS THE SOUTHWEST, BUT
IS STRONGLY DIVERGENT RIGHT OVER THE SYSTEM, PROVIDING SUPPORT FOR
THE CONVECTION, WHICH HAS PERSISTED FOR THE PAST SIX HOURS EVEN IN
THE FACE OF THE MODERATE SHEAR.
INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).
CURRENT STEERING MECHANISM: WEAK STEERING PATTERN
AGENCY DVORAK AND AUTOMATED FIXES:
   PGTW: T1.5 - 25 KTS
   KNES: T1.5 - 25 KTS
FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 20-25 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE EQUATORWARD
ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: HIGH
   INITIAL WIND RADII: NOT APPLICABLE
3. FORECAST REASONING.
SIGNIFICANT FORECAST CHANGES: THIS INITIAL PROGNOSTIC REASONING
MESSAGE ESTABLISHES THE FORECAST PHILOSOPHY.
FORECAST DISCUSSION: TD 21W IS CURRENTLY DRIFTING SLOWLY TOWARDS
THE EAST-SOUTHEAST IN A WEAK AND COMPETING STEERING PATTERN DEFINED
BY A STRONG SUBTROPICAL RIDGE (STR) ENTRENCHED TO THE NORTHEAST AND
AN EXTENSION OF A NEAR EQUATORIAL RIDGE (NER) TO THE SOUTH. OVER
THE NEXT 24 HOURS THE STR SLIDES EAST AND EXTENDS TO SOUTHWARD TO
NEAR THE EQUATOR, WITH ITS WESTERN PERIPHERY BECOMING ORIENTED TO
A NORTH-SOUTH AXIS IN THE PROCESS. SIMULTANEOUSLY A MODERATELY
STRONG UPPER-LEVEL TROUGH WILL MOVE ACROSS JAPAN AND INTO THE
NORTHWEST PACIFIC AND STEADILY ERODE THE STR FROM THE WEST. THE
COMBINED EFFECTS WILL PUSH 21W ONTO AN ACCELERATING NORTHWARD
TRAJECTORY AFTER TAU 24. BY TAU 72 THE SYSTEM WILL BEGIN TO SLOW
DOWN AS IT APPROACHES THE TAIL END OF A COLD FRONT TRAILING THE
EXTRA-TROPICAL LOW THAT DEVELOPS AHEAD OF THE AFOREMENTIONED
TROUGH. BY AROUND TAU 72, THE SYSTEM WILL TAKE A HARD RIGHT TURN,
MOVING EASTWARD ALONG THE REMNANT FRONTAL BOUNDARY WHILE
TRANSITIONING TO A EXTRATROPICAL CYCLONE AS IT BECOMES EMBEDDED
WITHIN THE BAROCLINIC ZONE, IS ENGULFED BY MID-LEVEL DRY AIR AND
MOVES INTO THE RIGHT ENTRANCE REGION OF A STRONG 200MB JET MAX.
WHILE THE SYSTEM IS CURRENTLY IN A RATHER UNFAVORABLE POSITION
RELATIVE TO THE TUTT CELL TO THE NORTH, THE TUTT IS EXPECTED TO
MOVE OFF TO THE WEST BY TAU 24, WHICH WILL PUT TD 21W IN A MORE
FAVORABLE POSITION TO TAKE ADVANTAGE OF THE DIFFLUENT POLEWARD
OUTFLOW ON THE EAST SIDE OF THE TUTT. THE SYSTEM WILL BE SLOW TO
INTENSITY AT FIRST BUT ONCE THE TUTT MOVES TO THE WEST, THE RATE OF
INTENSIFICATION WILL INCREASE, BUT THE SYSTEM WILL CONTINUE TO
FIGHT AGAINST MODERATE SHEAR AND INCREASINGLY DRY AIR, PUTTING A
CAP ON THE PEAK INTENSITY. STRONG DRY AIR ENTRAINMENT, COOLER SSTS
AND INCREASED SHEAR WILL CONSPIRE TO WEAKEN THE SYSTEM AFTER TAU
72, AS IT UNDERGOES ETT.
MODEL DISCUSSION: DETERMINISTIC AND ENSEMBLE GLOBAL MODEL FIELDS
ARE IN GENERAL AGREEMENT ON THE OVERALL TRACK SCENARIO. THE
CONSENSUS TRACKERS ARE CONFINED TO A 150NM ENVELOPE THROUGH TAU 48,
BUT THEREAFTER BEGIN TO DIVERGE SIGNIFICANTLY BOTH IN TERMS OF
ALONG AND ACROSS-TRACK. GLOBAL MODEL FIELDS SHOW DEVELOPMENT OF A
COMPLEX LOW PRESSURE SYSTEM WITH MULTIPLE CENTERS OF CIRCULATION
AROUND TD 21W, WHICH COMPLICATES THE OVERALL TRACK AND INTENSITY
FORECAST. THE GFS, GEFS AND NAVGEM HOLD THE SYSTEM BACK FURTHER TO
THE WEST THROUGH TAU 72, WHILE THE ECMWF, ECENS, GALWEM AND UKMET
TAKE THE SYSTEM MUCH FURTHER TO THE EAST. THIS TREND CONTINUES
THROUGH TAU 120, WITH THE GFS AND GEFS TURNING THE SYSTEM SOUTHWARD
WHILE THE REMAINDER OF THE GUIDANCE RACES OFF TO THE EAST ALONG THE
FRONTAL BOUNDARY. THE JTWC TRACK IS PLACED ON THE EAST SIDE OF THE
GUIDANCE ENVELOPE THROUGH TAU 24 IN LIGHT OF THE CURRENT MOTION
VECTOR, THEN LIES CLOSE TO THE CONSENSUS MEAN TO TAU 72 AND
SLIGHTLY NORTH OF IT THROUGH TAU 120. CONFIDENCE IS MEDIUM TO TAU
72 AND LOW THEREAFTER. INTENSITY GUIDANCE IS MIXED, WITH THE COTC
AND CTCX PEAKING THE SYSTEM NEAR 70 KNOTS DURING AND AFTER ETT,
WHILE HWRF RAMPS UP FAST BUT PEAKS NEAR 50 KNOTS AROUND TAU 48.
MEANWHILE THE GFS AND DECAY SHIPS NAVGEM DRAG THE CONSENSUS MEAN
DOWN TO A PEAK NEAR 45 KNOTS. THE JTWC FORECAST IS BELOW ALL THE
GUIDANCE IN THE FIRST 24 HOURS DUE TO THE FORECAST CONTINUANCE OF
THE MODERATE SHEAR, THEN TRACKS THE CONSENSUS MEAN THEREAFTER, WITH
LOW CONFIDENCE THROUGH THE FORECAST PERIOD.
FORECAST CONFIDENCE:
   TRACK 0 - 72 HR: MEDIUM
   TRACK 72-120 HR: LOW
   INTENSITY 0 - 72 HR: LOW
   INTENSITY 72-120 HR: LOW//
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