Advisories

WDPN31 PGTW 071500
MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 15E (NOLO) WARNING NR 068//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.

SUMMARY:
   INITIAL POSITION: 25.8N 160.3E
   INITIAL INTENSITY: 65 KTS
   GEOGRAPHIC REFERENCE: 1770 NM WEST-NORTHWEST OF JOHNSTON ISLAND
   MOVEMENT PAST 6 HOURS: WESTWARD AT 15 KTS
   SIGNIFICANT WAVE HEIGHT: 33 FEET

SATELLITE ANALYSIS, INITIAL POSITION AND INTENSITY DISCUSSION:
ANIMATED ENHANCED INFRARED (EIR) SATELLITE IMAGERY DEPICTS
SIGNIFICANT DEGRADATION OF THE CENTRAL DENSE OVERCAST (CDO), WHICH
NOW LACKS DISCERNIBLE SPIRAL BANDING. A SHARP EASTERN EDGE TO THE
CIRRUS CANOPY IS CONSISTENT WITH 25-35 KTS OF EASTERLY VERTICAL
WIND SHEAR (VWS) ASSOCIATED WITH A STRONG UPPER-LEVEL RIDGE
CENTERED TO THE NORTH. DESPITE MARGINALLY FAVORABLE SEA SURFACE
TEMPERATURES (SST) OF 27-28 DEGREES CELSIUS, THE ENVIRONMENT IS
UNFAVORABLE. A 070815Z SSMIS PASS REVEALED MODEST VORTEX TILT WITH
HEIGHT. A 070933Z ASCAT-B PASS INDICATED THAT THE WIND FIELD HAS
CONTRACTED AND BECOME ASYMMETRIC, WITH THE HIGHEST WINDS CONFINED
TO THE EASTERN SEMICIRCLE. THE INITIAL POSITION IS PLACED WITH
MEDIUM CONFIDENCE BASED ON EXTRAPOLATED ASCAT-B DATA, HEDGED
WESTWARD OF THE OVERSHOOTING CLOUD TOPS. THE INITIAL INTENSITY HAS
BEEN LOWERED TO 65 KTS WITH MEDIUM CONFIDENCE, CONSISTENT WITH THE
MAJORITY OF THE OBJECTIVE ESTIMATES FROM CIMSS AND THE T4.0 DVORAK
FIXES FROM PGTW AND RCTP.

INITIAL WIND RADII BASIS: SCATTEROMETER DATA 

CURRENT STEERING MECHANISM: SOUTHERN PERIPHERY OF A DEEP-LAYER
SUBTROPICAL RIDGE (STR)

AGENCY DVORAK AND AUTOMATED FIXES: 
   PGTW: T4.0 - 65 KTS
   RJTD: T4.5 - 77 KTS
   RCTP: T4.0 - 65 KTS
   PHFO: T4.5 - 77 KTS
   CIMSS SATCON: 66 KTS AT 070940Z
   CIMSS ADT: 57 KTS AT 071140Z
   CIMSS AIDT: 68 KTS AT 071140Z
   CIMSS D-MINT: 70 KTS AT 071149Z
   CIMSS D-PRINT: 68 KTS AT 071140Z

FORECASTER ASSESSMENT OF CURRENT ENVIRONMENT: UNFAVORABLE
   VWS: 25-30 KTS
   SST: 27-28 CELSIUS
   OUTFLOW: MODERATE WESTWARD

ANALYSIS CONFIDENCE:
   INITIAL POSITION: MEDIUM
   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: TYPHOON (TY) 15E HAS BEEN MOVING NEARLY DUE
WEST OVER THE PAST 12 HOURS ALONG THE SOUTHERN FLANK OF A
DEEP-LAYER STR CENTERED TO THE NORTHEAST. THE SYSTEM WILL GRADUALLY
TURN POLEWARD AND DECELERATE THROUGH TAU 24 AS IT APPROACHES A WEAK
WESTERN EXTENSION OF THE STR. A TROUGH PASSING TO THE NORTH WILL
ERODE THE STR, ALLOWING TY 15E TO TURN SHARPLY POLEWARD, CROSS THE
RIDGE AXIS NEAR TAU 60, AND ACCELERATE EAST-NORTHEASTWARD MOTION AS
IT ENCOUNTERS DEEP-LAYER MID-LATITUDE WESTERLY FLOW. VWS WILL EASE
TO 15-20 KTS OVER THE NEXT 12 HOURS, WHICH WILL SLOW THE RATE OF
WEAKENING. THE FORECAST CALLS FOR TY 15E TO BOTTOM OUT AT 60 KTS AT
TAU 12 AND TAU 24, THEN BRIEFLY REINTENSIFY WHEN IT TURNS NORTHWARD
AND TAPS INTO A POLEWARD OUTFLOW CHANNEL CONNECTED TO A JET STREAM
TO THE NORTH. THEREAFTER, STEADY WEAKENING WILL COMMENCE AS SHEAR
INCREASES, THE SST FALLS BELOW 26 DEGREES CELSIUS, AND THE VORTEX
MOVES BENEATH THE JET STREAM. INCREASING BAROCLINICITY TOWARD THE
END OF THE FORECAST WILL LEAD TO EXTRATROPICAL TRANSITION, WHICH
WILL COMPLETE BY TAU 120.

MODEL DISCUSSION: TRACK GUIDANCE IS IN GOOD AGREEMENT THROUGH TAU
48. BOTH CROSS-TRACK AND ALONG-TRACK SPREAD INCREASE SIGNIFICANTLY
THEREAFTER, REFLECTING UNCERTAINTY IN THE TIMING AND IMPACT OF THE
TROUGH ON THE STR. DYNAMICAL MODELS FAVOR A MORE WESTWARD AND
SLOWER SCENARIO, WHILE THE AI MODELS GENERALLY DEPICT AN EARLIER
AND SHARPER RECURVATURE. ECMWF IS THE SLOWEST TRACKER. NOTABLY,
SEVERAL OF THE DYNAMICAL SOLUTIONS EVENTUALLY CATCH UP TO THE
FASTER AI MODELS BY TAU 120. THE JTWC FORECAST TRACK IS SET USING A
BLEND OF THE AI-MODEL MEAN AND THE MULTI-MODEL CONSENSUS, LEANING
MORE STRONGLY TOWARD THE AI SOLUTIONS. WHILE THE OVERALL TREND OF
THE INTENSITY GUIDANCE HAS NOT CHANGED SIGNIFICANTLY, THE RELIABLE
AIDS HAVE TRENDED STRONGER BOTH AT THE FORECAST MINIMUM AT TAU 12
AND TAU 24 AND DURING THE BRIEF REINTENSIFICATION PHASE. GFS CALLS
FOR REINTENSIFICATION TO 65 KTS AT TAU 36-48, WHILE DEEPMIND AND
HAFS-A REACH 70 KTS AND 90 KTS, RESPECTIVELY. MODERATE SHEAR ACTING
ON THE SYSTEM WILL LIKELY PREVENT SIGNIFICANT INTENSIFICATION.
THEREFORE, THE JTWC INTENSITY FORECAST IS SET ON THE LOWER END OF
THE GUIDANCE ENVELOPE. TOWARD THE END OF THE FORECAST PERIOD, THE
INTENSITY FORECAST REFLECTS A BLEND OF GFS AND DEEPMIND.

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