Advisories

WDPN32 PGTW 270300
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: 20.4N 167.7E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 91 NM NORTHEAST OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTHWARD AT 06 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A RAPIDLY
CONSOLIDATING, BUT STILL DISORGANIZED, SYSTEM WITH A PARTIALLY EXPOSED 
LOW LEVEL CIRCULATION CENTER (LLCC) FLARING DEEP CONVECTION ALIGNED 
ALONG THE SOUTHEASTERN FLANK. A 262127Z ASCAT-B PASS REVEALED A COMPACT 
BUT WELL-DEFINED LLCC, HAVING CLOSED OFF AND CONSOLIDATED SINCE THE 
PREVIOUS SCATTEROMETER PASS 12 HOURS PRIOR. THE SCATTEROMETER DATA 
SHOWED A RELATIVELY LARGE, BUT ASYMMETRIC WIND FIELD, WITH LIGHT WINDS 
ALONG THE WESTERN FLANK AND 25-30 KNOTS ACROSS THE ENTIRE EASTERN 
HEMISPHERE OF THE CIRCULATION. THE SCATTEROMETER DATA WAS THE PRIMARY 
DATA SUPPORTING HIGH CONFIDENCE IN THE INITIAL POSITION AND INTENSITY. 
ENVIRONMENTAL CONDITIONS ARE FAVORABLE, CHARACTERIZED BY VERY WARM 
SSTS, HIGH OHC, STRONG POLEWARD OUTFLOW ALOFT AND LOW VERTICAL WIND 
SHEAR.

INITIAL WIND RADII BASIS: NOT APPLICABLE (THERE ARE NO INITIAL WIND
RADII).

CURRENT STEERING MECHANISM: WESTERN FLANK OF A LOW TO MID-LEVEL
EXTENSION OF A SUBTROPICAL RIDGE (STR) TO THE EAST. 

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

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: FAVORABLE
   VWS: 5-10 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: STRONG POLEWARD

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: TROPICAL DEPRESSION (TD) 21W IS CURRENTLY MOVING 
SLOWLY NORTHWARD ALONG THE BUILDING EXTENSION OF THE STR CENTERED TO 
THE NORTHEAST. AS THE RIDGE CONTINUES TO BUILD, AND TD 21W INTENSIFIES, 
THE GRADIENT WILL STRENGTHEN, RESULTING IN TD 21 ACCELERATING NORTHWARD 
OVER THE NEXT 12 TO 24 HOURS. A GRADUAL TURN NORTH-NORTHWESTWARD IS 
EXPECTED AFTER TAU 12, WITH THE SYSTEM MAINTAINING THIS GENERAL 
TRAJECTORY THROUGH TAU 72. BY THE END OF THE FORECAST PERIOD, THE STR 
BUILDS INTO THE SOUTH OF TD 21W, RESULTING IN A TURN BACK TO THE 
NORTHEAST BY TAU 96. OF NOTE, THE SYSTEM IS THE DOMINANT SYSTEM IN A 
BINARY INTERACTION WITH TD 20W TO THE SOUTHWEST. 20W IS FORECAST TO BE 
CAPTURED BY AND ULTIMATELY MERGE WITH THE BROAD WIND FIELD ASSOCIATED 
WITH 21W BY TAU 72. HOWEVER, DUE TO THE SIGNIFICANT SIZE DIFFERENCE, 
THE IMPACT OF THE MERGER ON THE TRACK OF 21W WILL BE MINIMAL. REGARDING 
INTENSITY, ONCE THE SYSTEM ACHIEVES SYMMETRIZATION IN THE NEXT 12 TO 24 
HOURS, IT WILL LAUNCH A PERIOD OF INTENSIFICATION FUELED BY STRONG 
POLEWARD OUTFLOW, LOW SHEAR AND WARM SSTS. HOWEVER, THE WINDOW OF 
OPPORTUNITY WILL BE SHORT-LIVED AND THE SYSTEM WILL STRUGGLE AGAINST 
DRY MID-LEVEL AIR ALONG THE WESTERN PERIPHERY. TD 21W REACHES ITS PEAK 
INTENSITY OF 50 KNOTS BY TAU 48, THEN BEGINS TO MOVE UNDER A FINGER OF 
THE SUBTROPICAL JET WHICH WILL INDUCE SHARPLY INCREASED SHEAR AND DRY 
AIR ENTRAINMENT, LEADING TO A RAPID WEAKENING PHASE. BY TAU 72, AS THE 
VORTEX DECOUPLES AND IS ENGULFED BY DRY MID-LEVEL AIR, PASSES ACROSS 
THE SUBTROPICAL JET FINGER AND MOVES UNDER AN UPPER-LEVEL LOW, IT 
BEGINS A RAPID SUBTROPICAL TRANSITION (STT). STT IS FORECAST TO BE 
COMPLETE NO LATER THAN TAU 96.  

MODEL DISCUSSION: TRACK GUIDANCE IS IN GOOD AGREEMENT THROUGH THE BULK 
OF THE FORECAST PERIOD. NAVGEM IS THE SOLE OUTLIER, SWITCHING BETWEEN 
TRACKING THE VORTEX ASSOCIATED WITH 21W AND 20W AND POSITIONED FAR TO 
THE WEST OF THE REMAINDER OF THE GUIDANCE. THE EMCWF TRACKER IS 
SIMILARLY HAVING DIFFICULTY IN MAINTAINING A LOCK ON THE VORTEX 
ASSOCIATED WITH 21W, THOUGH THE MODEL FIELDS PROVIDE A GOOD LOCK ON THE 
VORTEX AND SHOW A TRACK SIMILAR TO THE REMAINDER OF THE MODEL TRACKERS. 
THE CROSS-TRACK SPREAD AT TAU 96 IS APPROXIMATELY 150NM BETWEEN THE 
GALWEM ON THE WEST SIDE AND THE GFS ON THE EAST SIDE OF THE ENVELOPE. 
THE JTWC FORECAST IS PLACED CLOSE TO MODIFIED CONSENSUS WHICH DELETES 
THE NAVGEM AND ECMWF TRACKERS, WITH MEDIUM CONFIDENCE. INTENSITY 
GUIDANCE IS GENERALLY IN AGREEMENT ON THE OVERALL INTENSITY TREND, WITH 
A RANGE OF PEAK INTENSITIES BETWEEN 45 KNOTS ON THE LOWER END (HAFS-A 
AND GFS) AND 65 KNOTS ON THE HIGH END (CTCX AND SHIPS-NAVGEM). THE JTWC 
FORECAST IS PLACED NEAR THE MULTI-MODEL CONSENSUS MEAN WITH MEDIUM 
CONFIDENCE. 

FORECAST CONFIDENCE:
   TRACK 00-72 HR: MEDIUM
   TRACK 72-120 HR: MEDIUM
   INTENSITY 00-72 HR: MEDIUM
   INTENSITY 72-120 HR: MEDIUM//
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