Tropical Storm MERANTI Advisory Fri Sep 09

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 16W (SIXTEEN)
WARNING NR 01//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 16W (SIXTEEN), LOCATED APPROXIMATELY 158
NM WEST OF ANDERSEN AFB, GUAM, HAS TRACKED WEST-NORTHWESTWARD AT 03
KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED (EIR)
SATELLITE IMAGERY SHOWS DEEPENING, ALBEIT FRAGMENTED, CONVECTION
SLOWLY ORGANIZING AROUND A WEAK AND BROAD LOW LEVEL CIRCULATION. THE
INITIAL POSITION IS BASED ON THE EIR ANIMATION AND ON REFLECTIVITY
AND VELOCITY LOOPS FROM THE NWS GUAM RADAR. THE INITIAL INTENSITY IS
BASED ON THE DVORAK INTENSITY ESTIMATES FROM PGTW, KNES, AND RJTD AND
CONSISTENT WITH RECENT SCATTEROMETRY PASSES. UPPER LEVEL ANALYSIS
INDICATES THE SYSTEM IS SOUTHWEST AND NORTH OF TWO RIDGE AXES IN AN
AREA OF LOW (05-10 KNOT) VERTICAL WIND SHEAR (VWS). ADDITIONALLY,
ANIMATED WATER VAPOR SATELLITE IMAGERY SHOWS SOME EQUATORIAL OUTFLOW
THAT IS PROVIDING VENTILATION TO THE ASSOCIATED CONVECTION. THESE
FAVORABLE UPPER LEVEL DYNAMICS, IN ADDITION TO WARM SEA SURFACE
TEMPERATURES (SSTS GREATER THAN 28C), ARE ENHANCING THE CONVECTION.
HOWEVER, A TUTT CELL TO THE NORTHWEST IS CAUSING SUBSIDENCE. THE
CYCLONE IS VERY SLOWLY TRACKING ALONG THE WESTERN PERIPHERY OF A
SHALLOW LAYER OF THE NEAR EQUATORIAL RIDGE TO THE EAST.
3. FORECAST REASONING.
   A. THIS IS THE INITIAL PROGNOSTIC REASONING MESSAGE AND SETS THE
FORECAST PHILOSOPHY FOR THIS SYSTEM. .
   B. TD 16W WILL CONTINUE ON A SLOW NORTHWESTWARD TRACK IN THE NEAR
TERM. AFTER TAU 36 AND AFTER THE SYSTEM REACHES TS INTENSITY, A
BUILDING SUBTROPICAL RIDGE (STR) EAST OF JAPAN WILL ASSUME STEERING
AND ACCELERATE THE NORTHWESTWARD TRACK. INTENSIFICATION IN THE NEAR
TERM WILL BE SLOW DUE TO THE SUBSIDENT EFFECT OF THE AFOREMENTIONED
TUTT CELL. BY TAU 72, THE SYSTEM WILL REACH 55 KNOTS.
   C. IN THE EXTENDED PERIOD, TD 16W WILL REMAIN UNDER THE STEERING
INFLUENCE OF THE STR AND INITIALLY DRIFT IN AN AREA OF INCREASED VWS
(20-25 KNOTS), ONCE AGAIN TEMPERING THE INTENSIFICATION RATE DESPITE
THE DIMINISHED EFFECT OF THE TUTT CELL. HOWEVER, NEAR TAU 96, TD 16W
IS EXPECTED TO UNDERGO RAPID INTENSIFICATION AS VWS RELAXES, POLEWARD
OUTFLOW INCREASES, AND THE CYCLONE DRIFTS INTO EVEN WARMER SSTS
(GREATER THAN 30C). THE SYSTEM WILL BE A VERY STRONG TYPHOON AT 110
KNOTS BY END OF FORECAST. THE INITIAL SET OF DYNAMIC AIDS ARE SPREAD
AND FAN OUT AT APPROXIMATELY 20 DEGREES BUT MEMBERS ARE IN OVERALL
AGREEMENT WITH THE NORTHWESTWARD TRACK UP TO THE END OF FORECAST. IN
VIEW OF THE SLOW INITIAL STORM MOTION AND THE VARIABILITY OF AN
INITIAL FORECAST, THERE IS LOW CONFIDENCE IN THE JTWC WARNING TRACK.//
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