WDPN31 PGTW 121500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 28W (TWENTYEIGHT) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 21.3N 165.1E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 147 NM NORTHWEST OF WAKE ISLAND
   MOVEMENT PAST 6 HOURS: NORTH-NORTHWESTWARD AT 08 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS A
COMPACT CIRCULATION WHICH HAS TUCKED UNDER A FLARE-UP IN CONVECTIVE
ACTIVITY, AFTER HAVING BEEN PARTIALLY EXPOSED OVER THE PREVIOUS FEW
HOURS. THE RELATIVELY SMALL LOW LEVEL CIRCULATION CENTER (LLCC) IS
EMBEDDED WITHIN A LARGER REGION OF FLARING CONVECTION, PARTICULARLY
ON THE NORTHEASTERN PERIPHERY AS WESTERLY VERTICAL WIND SHEAR (VWS)
REMAINS STEADY AT 15-20 KNOTS. THE INITIAL POSITION IS PLACED WITH
MEDIUM CONFIDENCE BASED ON EIR AS WELL AS AN EARLIER PARTIAL 121026Z
ASCAT-B IMAGE, WHICH SHOWS A DEFINED CENTER WITH 20-25 KNOT WINDS
OVER THE EASTERN PERIPHERY. THE INITIAL INTENSITY OF 25 KTS IS 
ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE PGTW DVORAK ESTIMATE OF 
T2.0, AS WELL AS THE KNES DVORAK ESTIMATE OF T2.5 ALONG WITH THE 
RECENT ASCAT DATA. ENVIRONMENTAL ANALYSIS INDICATES MARGINALLY 
FAVORABLE CONDITIONS, WITH AN UPPER-LEVEL ANTICYCLONE POSITIONED OVER 
TOP OF THE SYSTEM INCREASING MODERATE OUTFLOW ALOFT, ALONG WITH WARM
(27-29C) SEA SURFACE TEMPERATURES (SSTS), OFFSET BY STEADY MODERATE
(15-20KT) VERTICAL WIND SHEAR (VWS).

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

CURRENT STEERING MECHANISM: WESTERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR) TO THE EAST-NORTHEAST. 

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T2.0 - 30 KTS
   KNES: T2.5 - 35 KTS

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 15-20 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE POLEWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: NOT APPLICABLE

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TD 28W IS CURRENTLY TRACKING 
NORTHWESTWARD ALONG THE WEST SIDE OF A DEEP-LAYER SUBTROPICAL RIDGE
(STR) CENTERED NEAR 25N 177E. OVER THE NEXT 24-36 HOURS THE STR IS 
EXPECTED TO REORIENT TO A MORE MERIDIONAL NORTH-SOUTH ORIENTATION. AS 
THIS OCCURS THE SYSTEM WILL THEN TRANSIT NORTHWARD IN A MARGINALLY 
FAVORABLE ENVIRONMENT WHICH WILL FUEL A WEAK INTENSIFICATION TREND UP 
TO 35-40KTS. THEREAFTER, AN APPROACHING AMPLIFIED UPPER-LEVEL TROUGH 
TO THE WEST OF THE SYSTEM AND WILL BEGIN EXTRA-TROPICAL TRANSITION 
WHILE STEERING THE SYSTEM NORTHEASTWARD BY TAU 48. BETWEEN TAU 48 AND 
TAU 72 THE SYSTEM WILL ENTER THE BAROCLINIC ZONE AND COMPLETE 
EXTRATROPICAL TRANSITION WHILE TRANSFORMING INTO A GALE-FORCE LOW 
PRESSURE SYSTEM WITH AN EXPANDING WIND FIELD. THE MAXIMUM WINDS 
WITHIN THE CIRCULATION MAY IN FACT INCREASE DURING THE BAROCLINIC 
INTERACTION PROCESS.

MODEL DISCUSSION: WITH THE EXCEPTION OF THE AFUM TRACKER, WHICH IS
THE EASTERN OUTLIER. THE NUMERICAL MODEL GUIDANCE IS IN TIGHT
AGREEMENT WITH A GRADUAL SPREAD TO 175NM BY TAU 72, LENDING HIGH
CONFIDENCE TO THE JTWC FORECAST TRACK. ADDITIONALLY, BOTH THE GFS
ENSEMBLE (GEFS) MEMBERS AND ECMWF ENSEMBLE (EPS) MEMBERS ARE IN
OVERALL GOOD AGREEMENT THROUGH OUT THE FORECAST PERIOD. RELIABLE
INTENSITY GUIDANCE IS IN FAIR AGREEMENT AND SUPPORTS A STEADY
INTENSIFICATION TO 40 KNOTS. THERE IS THE POSSIBILITY FOR FURTHER
INTENSIFICATION BEYOND THE FORECAST TIMEFRAME, HOWEVER THE SYSTEM
IS FORECAST TO BE FULLY EXTRATROPICAL BEYOND TAU 72. OVERALL THE
JTWC INTENSITY FORECAST IS ASSESSED WITH MEDIUM CONFIDENCE.  

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