WDPN31 PGTW 131500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) 
WARNING NR 005//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 15.7N 154.0E
   INITIAL INTENSITY: 30 KTS
   GEOGRAPHIC REFERENCE: 475 NM EAST OF SAIPAN
   MOVEMENT PAST 6 HOURS: EAST-NORTHEASTWARD AT 13 KTS
   SIGNIFICANT WAVE HEIGHT: 10 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
TD 21W IS LOCATED WITHIN THE BASE OF A BROAD, UPPER-LEVEL
SUBTROPICAL TROUGH WITH A TUTT LOW POSITIONED TO THE NORTHWEST NEAR
19N 151E, WHICH IS PRODUCING MODERATE POLEWARD OUTFLOW. OVERALL,
UPPER-LEVEL CONDITIONS ARE MARGINAL AND THE SYSTEM IS STRUGGLING TO
CONSOLIDATE. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY DEPICTS
FLARING DEEP CONVECTION OVER THE EASTERN SEMICIRCLE OF A
PARTIALLY-EXPOSED LOW-LEVEL CIRCULATION. A 131012Z AMSU-B 89GHZ
IMAGE SHOWS SHALLOW BANDING OVER THE SOUTHERN QUADRANT WRAPPING
INTO THE EASTERN QUADRANT WITH THE BULK OF THE DEEP CONVECTION
SHEARED OVER THE EASTERN SEMICIRCLE. A 131046Z ASCAT-B IMAGE SHOWS
A SMALL, DEFINED CIRCULATION WITH 25-30 KNOT WINDS OVER THE
SOUTHERN SEMICIRCLE. THE INITIAL POSITION IS PLACED WITH HIGH
CONFIDENCE BASED ON THE ASCAT-B IMAGE. THE INITIAL INTENSITY OF 30
KTS IS ASSESSED WITH HIGH CONFIDENCE BASED ON THE ASCAT-B DATA AND 
THE PGTW DVORAK FIX.

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

CURRENT STEERING MECHANISM: LOW- TO MID-LEVEL NEAR-EQUATORIAL RIDGE
(NER) POSITIONED TO THE SOUTHEAST.

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T2.0 - 30 KTS
   RJTD: T1.0 - 25 KTS
   KNES: T1.5 - 25 KTS
   CIMSS SATCON: 32 KTS AT 131134Z
   CIMSS ADT: 32 KTS AT 131140Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINAL
   VWS: 15-20 KTS
   SST: 29-30 CELSIUS
   OUTFLOW: MODERATE POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT

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

3. FORECAST REASONING.

SIGNIFICANT FORECAST CHANGES: THERE ARE NO SIGNIFICANT CHANGES TO
THE FORECAST FROM THE PREVIOUS WARNING.

FORECAST DISCUSSION: TD 21W IS FORECAST TO TRACK NORTH-NORTHEASTWARD 
TO NORTHWARD THROUGH THE FORECAST PERIOD AS IT TRANSITIONS TO THE 
STEERING INFLUENCE OF THE SUBTROPICAL RIDGE (STR) POSITIONED TO THE 
EAST. ENVIRONMENTAL CONDITIONS WILL REMAIN MARGINAL AS THE SYSTEM 
TRACKS WITHIN THE SOUTHEASTERN AND EASTERN PERIPHERY OF THE UPPER-
LEVEL SUBTROPICAL LOW WITH ROBUST POLEWARD OUTFLOW OFFSET BY DRY AIR 
ENTRAINMENT AND MODERATE VERTICAL WIND SHEAR. SLIGHT INTENSIFICATION 
IS POSSIBLE OVER THE NEXT DAY WITH GRADUAL WEAKENING AFTER TAU 24 AND 
DISSIPATION BY TAU 36. THERE IS A LOWER PROBABILITY SCENARIO THAT THE 
SYSTEM MAY TRANSITION INTO A SUBTROPICAL LOW WITH SURFACE WINDS OF 
30-35 KNOTS.

MODEL DISCUSSION: NUMERICAL MODEL GUIDANCE IS IN GOOD AGREEMENT
THROUGH TAU 24 WITH A CROSS-TRACK SPREAD OF 130NM AT TAU 24. AFTER 
TAU 24, NUMERICAL MODEL GUIDANCE DIVERGES WITH INCREASING UNCERTAINTY 
IN THE EXACT FORECAST TRACK THUS MEDIUM OVERALL CONFIDENCE. SEVERAL 
MODELS ARE INDICATING A CYCLONICALLY-SHAPED TRACK, HOWEVER, ANALYSIS 
OF THE GFS 850MB AND 500MB VORTICITY FIELDS SHOWS THAT GFS IS MERGING 
THE SYSTEM'S VORTICITY INTO A BROAD AREA OF VORTICITY ASSOCIATED WITH 
AN UPPER-LEVEL LOW. THUS, DISSIPATION IS THE LIKELIEST SCENARIO WITH
THE ENERGY GETTING ABSORBED INTO A SUBTROPICAL LOW, WHICH MAY
STRENGTHEN AS IT TRACKS POLEWARD. THE 130600Z GFS ENSEMBLE (GEFS)
ALSO REFLECTS A SIMILAR SPREADING OF PROBABILISTIC SOLUTIONS AFTER
TAU 24 WITH THE BULK OF THE SOLUTIONS IN THE 30-40 KNOT INTENSITY
RANGE. THE 130600Z COAMPS-TC ENSEMBLE ALSO INDICATES THE
DEVELOPMENT OF A 35 KNOT SUBTROPICAL LOW. FOR NOW THE JTWC FORECAST
SCENARIO REMAINS DISSIPATION BUT THE POTENTIAL FOR A GALE-FORCE
SUBTROPICAL LOW WILL BE RE-ASSESSED IN THE NEXT 12 HOURS.  

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