WDPN31 PGTW 010900
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 31W (THIRTYONE) 
WARNING NR 001//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 9.8N 138.1E
   INITIAL INTENSITY: 25 KTS
   GEOGRAPHIC REFERENCE: 18 NM NORTH OF YAP
   MOVEMENT PAST 6 HOURS: WEST-NORTHWESTWARD AT 10 KTS
   SIGNIFICANT WAVE HEIGHT: 12 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS TROPICAL
DEPRESSION 31W WITH COMPACT CORE OF DEEP CONVECTION OBSCURING THE
LOW-LEVEL CIRCULATION CENTER (LLCC). CLOUD TOPS ARE MEASURING
AROUND -88 C IN THE COLDEST AREAS. A 010646Z SSMIS 91 GHZ MICROWAVE
IMAGE REVEALED A RATHER COMPACT SYSTEM WITH DEEP CONVECTION
DISPLACED SLIGHTLY TO THE WEST OF THE LLCC AND FRAGMENTED BANDING
WITHIN THE NORTHERN PERIPHERY. ENVIRONMENTAL ANALYSIS INDICATES
THAT 31W IS IN A FAVORABLE ENVIRONMENT CHARACTERIZED BY MODERATE
POLEWARD AND EQUATORWARD OUTFLOW, LOW (5-10 KTS) VERTICAL WIND
SHEAR, AND WARM (29-30 C) SEA SURFACE TEMPERATURES.  THE INITIAL
POSITION IS PLACED WITH MEDIUM CONFIDENCE BASED ON THE ANIMATED
MSI. THE INITIAL INTENSITY OF 25 KTS IS ASSESSED WITH MEDIUM
CONFIDENCE BASED ON THE AGENCY DVORAK FIXES RANGING FROM T1.5-2.0.

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

CURRENT STEERING MECHANISM: SOUTHWESTERN PERIPHERY OF A SUBTROPICAL
RIDGE (STR) CENTERED TO THE NORTHEAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T1.5 - 25 KTS
   RJTD: T1.5 - 25 KTS
   KNES: T2.0 - 30 KTS

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

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: 31W IS FORECAST TO TRACK NORTHWESTWARD THROUGH
TAU 24, ALONG THE SOUTHWESTERN PERIPHERY OF THE STR. AFTER TAU 24,
ANOTHER STR CENTERED SOUTHWEST OF TAIWAN WILL BUILD AND EXTEND
EASTWARD, CAUSING 31W TO TRACK WESTWARD AS IT APPROACHES THE
PHILIPPINES. 31W IS FORECAST TO MAKE LANDFALL AROUND TAU 60 NEAR
SOUTHERN SAMAR AND LEYTE. THE TRACK WILL THEN TURN MORE
WEST-NORTHWESTWARD FROM TAU 72 ONWARD AS THE SYSTEM TRAVERSES
THROUGH THE SOUTH CHINA SEA. REGARDING INTENSITY, 31W WILL REMAIN
IN A FAVORABLE ENVIRONMENT FOR FURTHER DEVELOPMENT THROUGH
LANDFALL. AS A RESULT, THE CURRENT FORECAST CALLS FOR 70 KTS AT TAU
48. THERE WILL LIKELY BE HIGHER PEAK, JUST BEFORE LANDFALL THAT IS
UNABLE TO BE CAPTURED DUE TO TEMPORAL RESTRICTIONS. EASTERLY
VERTICAL WIND SHEAR IS EXPECTED TO RISE TO AROUND 15-20 KTS AT TAU
24, WHICH COULD POTENTIALLY KEEP THE SYSTEM FROM RAPIDLY
INTENSIFYING. HOWEVER, VERY WARM SEA SURFACE TEMPERATURES AND HIGH
OHC VALUES DEFINETELY KEEP IT AS A POSSIBILITY. THE INTENSIFICATION
TREND WILL BRIEFLY COME TO A HALT AS THE SYSTEM TRACKS THROUGH THE
VISAYAS, BUT THEN WILL CONTINUE AS THE SYSTEM TRACKS THROUGH THE
SOUTH CHINA SEA AND HEADS TOWARD VIETNAM. EQUATORWARD OUTFLOW WILL
GREATLY IMPROVE AND SEA SURFACE TEMPERATURES WILL REMAIN WARM,
ALLOWING FOR INTENSIFICATION THROUGH THE END OF THE FORECAST
PERIOD.

MODEL DISCUSSION: NUMERICAL MODEL TRACK GUIDANCE IS IN VERY GOOD
AGREEMENT THROUGH TAU 72 WITH A 60 NM CROSS TRACK SPREAD NEAR THE
TIME OF LANDFALL (TAU 60) AND A 90 NM CROSS-TRACK SPREAD AT TAU 72.
AFTER TAU 72, ALONG-TRACK SPREAD BEGINS TO INCREASE WITH MODELS
VARYING ON TRACK SPEEDS AFTER LANDFALL IN THE PHILIPPINES. ONE
MAJOR OUTLIER IS NAVGEM, WHICH TRACKS THE VORTEX UP TOWARD LUZON
RATHER THAN CONTINUING ON A DUE WEST TRACK. THE JTWC TRACK FORECAST
IS PLACED CLOSE TO THE MULTI-MODEL CONSENSUS (SLIGHTLY OFFSET DUE TO
NAVGEM) WITH HIGH CONFIDENCE THROUGH TAU 72 AND MEDIUM CONFIDENCE
THEREAFTER. INTENSITY GUIDANCE IS IN MUCH POORER AGREEMENT. RAPID
INTENSIFICATION (RI) AIDS ARE TRIGGERING THIS MODEL RUN, CAUSING
SIGNIFICANT SPREAD IN GUIDANCE THROUGH THE FIRST 48 HOURS OF THE
FORECAST PERIOD. THEN THE INTENSITY AT LANDFALL RANGES FROM 50 KTS
(HAFS-A) AND 90 KTS (HWRF). ALL MODELS SHOW INTENSIFICATION AFTER
LANDFALL, ONCE THE VORTEX EMERGES WITHIN THE SOUTH CHINA SEA, BUT
WITH LARGE VARIATIONS IN RATE. OVERALL, THE JTWC INTENSITY FORECAST
IS PLACED CLOSE TO CONSENSUS THROUGH TAU 48 AND THEN SLIGHTLY
HIGHER AFTERWARD WITH LOW CONFIDENCE. 

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