Tropical Storm JELAWAT Advisory Sat Sep 22

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (JELAWAT) WARNING
NR 07//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 18W (JELAWAT), LOCATED APPROXIMATELY 540 NM
EAST-SOUTHEAST OF MANILA, PHILIPPINES, HAS TRACKED SOUTH-
SOUTHWESTWARD AT 03 KNOTS OVER THE PAST SIX HOURS. ANIMATED
MULTISPECTRAL SATELLITE IMAGERY (MSI) SHOWS A TIGHTLY WRAPPED
SYSTEM WITH DEEP CONVECTION WRAPPING INTO THE LOW LEVEL CIRCULATION
CENTER (LLCC). A 212132Z SSMIS 91GHZ MICROWAVE IMAGE SHOWS SEVERAL
DISPLACED DEEP CONVECTIVE BANDS OVER THE SOUTHERN SEMI-CIRCLE BUT
VERY MINIMAL CONVECTIVE ACTIVITY OVER THE NORTHERN SEMICIRCLE.
ANIMATED WATER VAPOR IMAGERY REVEALS A TUTT CELL HAS FORMED
NORTHEAST OF THE SYSTEM WHICH IS WHAT IS SUPPRESSING THE CONVECTIVE
ACTIVITY TO THE NORTH. FURTHER UPPER-LEVEL ANALYSIS DEPICTS THAT TS
18W LIES EQUATORWARD OF AN UPPER-LEVEL RIDGE AXIS WITH EXCELLENT
EQUATORWARD OUTFLOW DUE TO SPEED DIVERGENCE TO THE SOUTH AND HAS
LIMITED POLEWARD OUTFLOW DUE TO THE NEWLY FORMED TUTT CELL TO THE
NORTHEAST. THE INITIAL POSITION IS BASED ON THE AFOREMENTIONED
IMAGERY. THE INITIAL INTENSITY IS BASED ON AN AVERAGE OF DVORAK
ESTIMATES OF 45 TO 55 KNOTS FROM RJTD AND PGTW, RESPECTIVELY. TS
18W IS IN A WEAK STEERING ENVIRONMENT AS AN APPROACHING MID-
LATITUDE TROUGH HAS WEAKENED THE SUBTROPICAL RIDGE (STR) THAT WAS
THE DOMINANT STEERING INFLUENCE.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TS 18W IS EXPECTED TO DRIFT SLOWLY WEST-SOUTHWESTWARD OVER
THE NEXT 24 HOURS AS THE MID-LATITUDE TROUGH APPROACHING FROM THE
NORTHWEST WEAKENS THE CURRENT STEERING RIDGE. AFTER THE TROUGH
PASSES, THE SUBTROPICAL RIDGE WILL BEGIN TO REBUILD AND TURN TS 18W
SLOWLY ON A MORE NORTH-NORTHWESTWARD TRACK. TS 18W WILL CONTINUALLY
INTENSIFY AS IT DEVELOPS A POLEWARD OUTFLOW CHANNEL FROM THE
PASSING TROUGH WITH THE POSSIBILITY OF RAPID INTENSIFICATION BASED
ON THE FAVORABLE ENVIRONMENTAL CONDITIONS.
   C. IN THE EXTENDED TAUS, A SUBTROPICAL RIDGE THAT IS CURRENTLY
OVER EASTERN CHINA IS FORECASTED TO AMPLIFY AND BECOME TS 18W'S
DOMINANT STEERING MECHANISM, WHICH WILL TURN THE SYSTEM MORE
TOWARDS THE WEST. DUE TO FORECASTED FAVORABLE ENVIRONMENTAL
CONDITIONS, A PEAK INTENSITY OF 115 KNOTS IS FORECASTED AT TAU 96.
NUMERICAL MODEL GUIDANCE REMAINS WIDELY SPREAD. THE CURRENT
FORECAST REMAINS LEFT OF CONSENSUS TO OFFSET AN UNLIKELY SHARP
NORTHERN TRACK INTO THE STR, WHICH IS DEPICTED BY GFS, EGRR AND
JGSM. DUE TO THE LARGE SPREAD IN MODEL GUIDANCE, THERE IS LOW
CONFIDENCE IN THE CURRENT TRACK FORECAST.//
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