WDPN31 PGTW 130900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 21W (TWENTYONE) 
WARNING NR 004//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 15.2N 152.7E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 399 NM EAST OF SAIPAN
   MOVEMENT PAST 6 HOURS: EAST-NORTHEASTWARD AT 14 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 MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS
A FULL-EXPOSED, WELL-DEFINED LOW-LEVEL CIRCULATION WITH ISOLATED
DEEP CONVECTION SHEARED TO THE SOUTHEAST. A 130254Z AMSR2 89GHZ
MICROWAVE IMAGE SHOWS SHALLOW BANDING WRAPPING INTO A RAGGED
LOW-LEVEL CIRCULATION CENTER, HOWEVER, THE DEEP CONVECTIVE
STRUCTURE IS LIMITED TO THE SOUTHEAST QUADRANT WHERE WEAK CURVED
BANDING IS EVIDENT. THE INITIAL POSITION IS PLACED WITH HIGH
CONFIDENCE BASED ON THE MSI. THE INITIAL INTENSITY OF 25 KTS IS
ASSESSED WITH HIGH CONFIDENCE BASED ON THE AGENCY DVORAK FIXES.

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: T1.5 - 25 KTS
   RJTD: T1.0 - 25 KTS
   KNES: T1.5 - 25 KTS
   CIMSS SATCON: 32 KTS AT 131134Z
   CIMSS ADT: 29 KTS AT 130540Z

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: HIGH
   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 48. THERE IS A LOWER PROBABILITY SCENARIO THAT THE 
SYSTEM MAY TRANSITION INTO A SUBTROPICAL LOW WITH SURFACE WINDS OF 
30-35 KNOTS. 

MODEL DISCUSSION: WITH THE EXCEPTION OF AEMN (GFS ENSEMBLE MEAN),
THE MODEL GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU 36 WITH A
CROSS-TRACK SPREAD OF 95NM AT TAU 36. AFTER TAU 36, NUMERICAL MODEL
GUIDANCE DIVERGES WITH INCREASING UNCERTAINTY IN THE EXACT FORECAST
TRACK THUS MEDIUM OVERALL CONFIDENCE. THE 130000Z GFS ENSEMBLE
(GEFS) ALSO REFLECTS A SIMILAR SPREADING OF PROBABILISTIC SOLUTIONS
AFTER TAU 36 WITH THE BULK OF THE SOLUTIONS IN THE 30-40 KNOT
INTENSITY RANGE. THE 130000Z COAMPS-TC ENSEMBLE IS MORE SUPPORTIVE
OF A 35 KNOT PLUS SUBTROPICAL LOW DEVELOPING WITH A FEW SOLUTIONS
INDICATING A 50 KNOT PLUS SYSTEM. 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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