Tropical Storm JELAWAT Advisory Wed Sep 26

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR SUPER TYPHOON 18W (JELAWAT)
WARNING NR 20//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   SUPER TYPHOON (STY) 18W (JELAWAT), LOCATED APPROXIMATELY 390 NM
EAST-NORTHEAST OF MANILA, PHILIPPINES, HAS TRACKED
NORTH-NORTHWESTWARD AT 08 KNOTS OVER THE PAST SIX HOURS. ANIMATED
MULTISPECTRAL SATELLITE IMAGERY SHOWS THE SYSTEM HAS MAINTAINED
EXTREMELY DEEP CONVECTIVE BANDS WRAPPED TIGHTLY INTO A 12-NM EYE,
LENDING HIGH CONFIDENCE TO THE INITIAL POSITION. THE INITIAL
INTENSITY WAS BASED ON CONGRUENT DVORAK ESTIMATES OF 140 KNOTS FROM
PGTW, RJTD, AND KNES. UPPER LEVEL ANALYSIS INDICATES THE SYSTEM HAS
SUSTAINED A HIGHLY EFFICIENT MESOSCALE ANTICYCLONE ALOFT IN AN AREA
OF LOW (05-10 KNOT) VERTICAL WIND SHEAR (VWS). THESE DYNAMICS HAVE
BEEN PRIMARILY RESPONSIBLE FOR THE ROBUST RADIAL OUTFLOW, AS
EVIDENCED ON ANIMATED WATER VAPOR IMAGERY. THE CYCLONE IS TRACKING
ALONG THE WESTERN PERIPHERY OF A DEEP-LAYERED NEAR-EQUATORIAL
STEERING RIDGE (NER) TO THE EAST.
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. STY 18W IS FORECAST TO TRACK SLOWLY NORTH-NORTHWESTWARD UNDER
THE CONTINUED STEERING INFLUENCE OF THE NER. A SHORTWAVE TROUGH TO
THE NORTH OF THE SYSTEM IS EXPECTED TO CONTINUE MOVING EASTWARD OVER
THE NEXT 12 TO 24 HOURS. IN THE WAKE OF THE TROUGH, THE NER WILL
BUILD AND STEER STY 18W MORE NORTHWESTWARD. DUE TO THE ADJUSTMENT IN
THE SHORT WAVE PATTERN AND IN THE STORM MOTION, VWS IS EXPECTED TO
INCREASE, ACCOUNTING FOR THE GRADUAL WEAKENING OF THE SYSTEM. THE
NUMERICAL MODELS CONTINUE TO COME INTO BETTER AGREEMENT THROUGH TAU
72, ALTHOUGH WBAR CONTINUES TO STRAY RIGHT OF THE MODEL ENVELOPE INTO
AN UNLIKELY TRACK TOWARDS THE STEERING RIDGE. THERE IS HIGH
CONFIDENCE IN THE FORECAST TRACK THROUGH TAU 72 WHICH LIES NEAR THE
CONSENSUS.
    C. STY 18W IS EXPECTED TO ROUND THE STEERING RIDGE AXIS AND MOVE
NORTHEASTWARD AFTER TAU 72 AHEAD OF A SECOND MID-LATITUDE TROUGH THAT
IS EXPECTED TO DEVELOP OVER EAST ASIA DURING THE NEXT FEW DAYS.
INCREASING VWS AHEAD OF THE TROUGH WILL CONTINUE TO WEAKEN THE SYSTEM
DURING THIS PERIOD. ALTHOUGH THE NUMERICAL MODEL GUIDANCE BECOMES
TIGHTER IN THE EXTENDED TAUS, THERE ARE SIGNIFICANT DIFFERENCES IN
LATERAL SPEED AMONG THE TRACKERS. THE GFDN AND GFS MODELS SHOW A
RAPID TRACK TO THE NORTHEAST PRESUMABLY ASSOCIATED WITH EXTRATROPICAL
TRANSITION, WHICH IS ANTICIPATED TO BEGIN BY TAU 120. THE CURRENT
EXTENDED FORECAST IS SLIGHTLY SLOWER THAN THE MODEL CONSENSUS TO
OFFSET THE GFDN AND GFS MODEL SOLUTIONS. GIVEN UNCERTAINTY IN THE
POST-RECURVATURE TRACK SPEED, THERE IS LOW CONFIDENCE IN THE EXTENDED
TRACK FORECAST.//
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