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 04//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 16W (SIXTEEN), LOCATED APPROXIMATELY 175
NM WEST-NORTHWEST OF ANDERSEN AFB, GUAM HAS TRACKED NORTHWESTWARD AT
07 KNOTS OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED
SATELLITE IMAGERY DEPICTS INCREASED CONVECTION BUILDING JUST SOUTH
OF A CONSOLIDATING LOW LEVEL CIRCULATION CENTER (LLCC) DUE TO
NORTHERLY VERTICAL WIND SHEAR (VWS). A 091301Z GPM 89GHZ MICROWAVE
IMAGE SHOWS CONVECTIVE BANDING WRAPPING FROM THE SOUTH INTO A
DEFINED LLCC WHICH SUPPORTS THE CURRENT POSITION WITH GOOD
CONFIDENCE. THE CURRENT INTENSITY HAS BEEN HELD AT 30 KNOTS BASED ON
AN ASSESSMENT OF DVORAK INTENSITY ESTIMATES RANGING FROM 25 TO 30
KNOTS AND IS SUPPORTED BY A PARTIAL 091121Z ASCAT PASS SHOWING THE
EASTERN SEMI-CIRCLE WITH 25 KNOT WINDS TO THE SOUTHEAST; HOWEVER,
THE BULK OF DEEP CONVECTION IS ON THE SOUTHWESTERN SIDE.
ENVIRONMENTAL ANALYSIS INDICATES A MARGINALLY-FAVORABLE ENVIRONMENT
WITH LOW (5-10 KNOT) VWS, WEAK RADIAL OUTFLOW, AND WARM (29 CELSIUS)
SEA SURFACE TEMPERATURES (SSTS). THE SYSTEM HAS STARTED TO TRACK
NORTHWESTWARD UNDER THE INFLUENCE OF A BUILDING LOW LEVEL SUB-
TROPICAL RIDGE (STR) TO THE NORTHEAST.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TD 16W WILL CONTINUE TRACKING NORTHWESTWARD UNDER THE BUILDING
STR THROUGHOUT THE FORECAST PERIOD. THE FAVORABLE ENVIRONMENTAL
CONDITIONS WILL ALLOW FOR A STEADY INTENSIFICATION OVER THE NEXT 36
HOURS. AFTERWARDS, INCREASED VWS DUE TO NORTHEASTERLY FLOW
ASSOCIATED WITH THE EASTERN EDGE OF AN UPPER LEVEL STR POSITIONED
OVER CHINA WILL TEMPER THE RATE OF INTENSIFICATION. BEYOND TAU 48,
THIS RIDGE WILL WEAKEN, DECREASING THE VWS ONCE AGAIN. DYNAMIC MODEL
GUIDANCE REMAINS IN FAIRLY GOOD AGREEMENT DURING THIS PORTION OF THE
FORECAST, WITH ONLY SMALL DIFFERENCES IN THE FORWARD SPEED OF THE
CYCLONE.
   C. IN THE EXTENDED FORECAST, TD 16W WILL REMAIN UNDER THE
STEERING INFLUENCE OF THE STR AND BY TAU 120, WILL REACH THE
SOUTHWESTERN PERIPHERY OF THE STR AND TURN POLEWARD TOWARDS A BREAK
IN THE STEERING RIDGE. BEYOND TAU 72, IMPROVED VWS AND OUTFLOW
ASSOCIATED WITH A MID-LATITUDE TROUGH APPROACHING FROM THE WEST,
COMBINED WITH INCREASED SSTS AND HIGH OCEAN HEAT CONTENT, WILL
SUPPORT AN INCREASED INTENSIFICATION RATE WITH THE POSSIBILITY OF
RAPID INTENSIFICATION. DYNAMICAL MODEL GUIDANCE IS IN FAIR AGREEMENT
IN THE EXTENDED PORTION OF THE FORECAST WITH AN 125NM SPREAD AT TAU
120. HWRF CONTINUES TO BE THE ONLY OUTLIER OF THE GROUP WITH A TRACK
INTO TAIWAN. THE JTWC TRACK FORECAST IS LAID NEAR THE MULTI-MODEL
CONSENSUS (CONW) THROUGH TAU 72 AND THEN RIGHT OF CONW FAVORING ALL
OF THE GLOBAL MODELS WHICH SHOW A RECURVE SCENARIO. DUE TO IMPROVING
MODEL TRACKERS WITH A SMALL SPREAD IN THE EXTENDED FORECAST, THERE
IS HIGH CONFIDENCE IN THE JTWC TRACK FORECAST.//
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