Tropical Storm JELAWAT Advisory Thu Sep 27

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR SUPER TYPHOON 18W (JELAWAT) WARNING NR
27//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   SUPER TYPHOON (STY) 18W (JELAWAT), LOCATED APPROXIMATELY 430 NM
SOUTH-SOUTHWEST OF KADENA AIR BASE, OKINAWA, JAPAN, HAS TRACKED
NORTHWESTWARD AT 09 KNOTS OVER THE PAST SIX HOURS. ANIMATED
MULTISPECTRAL IMAGERY (MSI) REVEALS A VERY SYMMETRIC SYSTEM WITH A
THICK RING OF DEEP CONVECTION AND A NOW LARGER 40 NM EYE. A 262326Z
SSMIS 91GHZ MICROWAVE IMAGE REVEALS A TIGHTLY WRAPPED AND WELL
DEFINED SYSTEM. THE INITIAL POSITION IS BASED ON THE LARGE EYE IN
THE MSI ANIMATION WITH HIGH CONFIDENCE. THE INITIAL INTENSITY IS AN
AVERAGE OF DVORAK ESTIMATES FROM PGTW, KNES, AND RJTD RANGING FROM
140 TO 127 KNOTS. UPPER LEVEL ANALYSIS INDICATES THE CYCLONE HAS
CONTINUED TO MAINTAIN EXCELLENT RADIAL OUTFLOW AND IS APPROACHING A
RIDGE AXIS TO THE NORTH IN AN AREA OF LOW (05-10 KNOT) VERTICAL WIND
SHEAR. STY 18W IS TRACKING ALONG SOUTHWESTERN PERIPHERY OF AN
EXTENSION OF THE NEAR-EQUATORIAL RIDGE (NER) TO THE EAST.
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING BULLETIN.
   B. STY 18W WILL CONTINUE TO TRACK GENERALLY NORTHWESTWARD OVER
THE NEXT 24 HOURS ALONG THE SOUTHWESTERN PERIPHERY OF THE NER.
AFTERWARDS, A SECONDARY MID-LATITUDE TROUGH, CURRENTLY EXITING FROM
EASTERN CHINA, WILL APPROACH FROM THE NORTHWEST AND WEAKEN THE NER
CAUSING THE SYSTEM TO RECURVE NORTHEASTWARD. AFTER TAU 48, THE
SUBTROPICAL RIDGE (STR) EAST OF JAPAN, ALSO WEAKENED BY THE SAME
TROUGH, WILL ASSUME STEERING AND CAUSE 18W TO ACCELERATE
NORTHEASTWARD. THE SYSTEM WILL GRADUALLY WEAKEN AS THE UPPER LEVEL
CONDITIONS BECOME MORE UNFAVORABLE.
   C. IN THE EXTENDED PERIOD, STEERED BY THE STR, STY 18W WILL
CONTINUE TO ACCELERATE NORTHEASTWARD INTO THE BAROCLINIC ZONE,
BEGINNING EXTRATROPICAL TRANSITION AT TAU 96, AND MAKING LANDFALL ON
HONSHU, JAPAN. ALTHOUGH THE MODELS REMAIN IN GOOD AGREEMENT IN TRACK
POSITION, THEY VARY IN FORWARD TRANSLATION SPEED AFTER RECURVATURE.
IN THE EXTENDED TAUS, MODELS ALSO CONTINUALLY VARY IN FORWARD
TRANSLATION SPEED, WITH ECMWF CONTINUALLY SHOWING A VERY SLOW TRACK.
ALL MODELS NOW TRACK ACROSS JAPAN AND INTO THE SEA OF JAPAN, BUT
HISTORICALLY STORMS ARE NOT ABLE TO CROSS JAPAN'S MOUNTAINOUS
GEOGRAPHY, THEREFORE THE OFFICIAL FORECAST MAKES LANDFALL IN JAPAN
BUT STAYS EAST. DUE THE UNCERTAINTIES IN MODEL SPEED, FORECAST
CONFIDENCE REMAINS LOW.//
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