Advisories

WDPN31 PGTW 031500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 30W (IN-FA) WARNING NR 045//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 23.3N 138.8E
   INITIAL INTENSITY: 75 KTS
   GEOGRAPHIC REFERENCE: 164 NM WEST-SOUTHWEST OF IWO TO
   MOVEMENT PAST 6 HOURS: NORTHWESTWARD AT 03 KTS
   SIGNIFICANT WAVE HEIGHT: 28 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS TYPHOON
30W (IN-FA) WITH A FURTHER IMPROVED APPEARANCE, INDICATED BY THE
PRESENCE OF A RAGGED 28 NM EYE AND DEEP CONVECTION BEGINNING TO
SURROUND THE EYE IN ALL QUADRANTS. THE WESTERN PORTION OF THE
SYSTEM CONTINUES TO BE THE WEAK POINT, LIKELY HINDERED BY THE DRY
AIR STREAMING DOWN FROM THE NORTH INTO THE VORTEX. A TIMELY 031156Z
METOP-B ASCAT PASS REVEALED AN ASYMMETRY IN THE SOUTHWESTERN
QUADRANT OF THE CORE, WHERE THE WINDS ARE WEAKER COMPARED TO THE
OTHER 3 QUADRANTS. THE INITIAL POSITION AND WIND RADII ARE PLACED
WITH HIGH CONFIDENCE BASED ON THE ASCAT DATA. THE INITIAL INTENSITY
OF 75 KTS IS ASSESSED WITH MEDIUM CONFIDENCE BASED ON THE
COMBINATION OF THE T4.5 AGENCY DVORAK FIXES AND CIMSS INTENSITY
ESTIMATES RANGING FROM 69 TO 79 KTS. ENVIRONMENTAL ANALYSIS
INDICATES THAT 30W IS IN A MARGINALLY FAVORABLE ENVIRONMENT
CHARACTERIZED BY STRONG POLEWARD OUTFLOW ALOFT, LOW (5-10 KTS)
VERTICAL WIND SHEAR, AND WARM (27-28 C) SEA SURFACE TEMPERATURES
SLIGHTLY OFFSET BY DRY AIR ENTRAINMENT FROM THE WEST.

INITIAL WIND RADII BASIS: 031156Z METOP-B ASCAT SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: PRIMARILY DRIVEN BY A DEEP-LAYER
SUBTROPICAL RIDGE (STR) POSITIONED TO THE EAST

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.5 - 77 KTS
   RJTD: T4.5 - 77 KTS
   RCTP: T3.5 - 55 KTS
   CIMSS SATCON: 74 KTS AT 030930Z
   CIMSS ADT: 77 KTS AT 031200Z
   CIMSS AIDT: 69 KTS AT 031200Z
   CIMSS D-MINT: 79 KTS AT 031154Z
   CIMSS D-PRINT: 69 KTS AT 031200Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: MARGINALLY FAVORABLE
   VWS: 5-10 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: STRONG POLEWARD
   OTHER FACTORS: DRY AIR ENTRAINMENT FROM THE NORTHWEST

ANALYSIS CONFIDENCE:
   INITIAL POSITION: HIGH
   INITIAL INTENSITY: MEDIUM
   INITIAL WIND RADII: HIGH

3. FORECAST REASONING.

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

FORECAST DISCUSSION: TY 30W HAS STARTED TO GAIN LATITUDE OVER THE
PAST 6 HOURS AS THE STR TO THE EAST BECOMES THE DOMINANT STEERING
MECHANISM. THE STR TO THE WEST, CENTERED SOUTH OF TAIWAN, HAS
STARTED TO WEAKEN, ALLOWING THE STEERING PATTERN TO BECOME MORE
DEFINED. AS A RESULT, THE SPEED OF ADVANCE OF 30W IS LIKELY TO
BEGIN STEADILY INCREASING IN SHORT ORDER. THE SYSTEM IS FORECAST TO
TRACK GENERALLY NORTHWARD, ALONG THE WESTERN PERIPHERY OF THE STR,
THROUGH TAU 24. FOLLOWING TAU 24, TRACK SPEEDS WILL SIGNIFICANTLY
INCREASE AS 30W ROUNDS THE RIDGE AXIS AND ACCELERATES NORTHEASTWARD
THROUGH THE REMAINDER OF THE FORECAST PERIOD. IN TERMS OF
INTENSITY, 30W IS FORECAST TO CONTINUE INTENSIFYING THROUGH TAU 24
IN RESPONSE TO THE ROBUST POLEWARD OUTFLOW AND LOW TO MODERATE
VERTICAL WIND SHEAR. BY TAU 36, WESTERLY SHEAR IS EXPECTED TO
INCREASE TO OVER 30 KTS, WHICH WILL INITIATE A WEAKENING TREND. BY
TAU 48, 30W WILL HAVE CROSSED THE 26 C SST ISOTHERM, FURTHER AIDING
IN WEAKENING THE SYSTEM. HOWEVER, THE EXCEPTIONAL POLEWARD OUTFLOW
CHANNEL AND BAROCLINIC FORCING IS FORECAST TO ALLOW 30W TO MAINTAIN
TYPHOON-INTENSITY THROUGHOUT THE TRANSITION INTO AN EXTRATROPICAL
CYCLONE. EXTRATROPICAL TRANSITION IS FORECAST TO BEGIN AT TAU 60
WITH COMPLETION AROUND TAU 72, AS THE VORTEX BECOMES EMBEDDED
UNDERNEATH THE STRONG MID-LATITUDE WESTERLIES.

MODEL DISCUSSION: MODEL TRACK GUIDANCE CONTINUES TO BE IN GOOD
AGREEMENT IN THE CROSS-TRACK DIRECTION WITH THE EXCEPTION OF A
COUPLE OF OUTLIERS. NAVGEM REMAINS A SIGNIFICANT EAST-OF-TRACK AND
SLOW OUTLIER, AND GALWEM IS NOW A WEST-OF-TRACK OUTLIER.
DISREGARDING THESE EXCEPTIONS, CROSS-TRACK SPREAD IS MEASURED TO BE
ABOUT 60 NM AT TAU 48 AND 130 NM AT TAU 60. THE ALONG-TRACK SPREAD
REMAINS QUITE LARGE WITH A CLEAR SEPARATION BETWEEN THE SLOW
PHYSICS-BASED MODELS AND THE FAST AI-BASED MODELS. THE ALONG-TRACK
SPREAD INCREASES DRAMATICALLY AFTER TAU 48, WITH A MEASUREMENT AT
TAU 60 OF NEARLY 400 NM. THE JTWC TRACK FORECAST IS PLACED NEAR THE
MODIFIED CONSENSUS (REMOVING THE TWO OUTLIERS) THROUGH TAU 36 AND
THEN HEDGED CLOSER TO THE AI-CONSENSUS THROUGH TAU 60 WITH OVERALL
MEDIUM CONFIDENCE. INTENSITY GUIDANCE GENERALLY AGREES ON SLIGHT
INTENSIFICATION THROUGH TAU 24 FOLLOWED BY WEAKENING THROUGH TAU
60. INTERESTINGLY, TWO RAPID INTENSIFICATION (RI) AIDS HAVE
TRIGGERED THIS MODEL RUN, AND ARE THE ONLY AIDS DEPICTING MAJOR
REDEVELOPMENT OF 30W. THE JTWC INTENSITY FORECAST FOLLOWS THE
MULTI-MODEL CONSENSUS THROUGHOUT THE FORECAST PERIOD WITH MEDIUM
CONFIDENCE.


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