Tropical Storm MERANTI Advisory Sat Sep 10

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 16W (SIXTEEN)
WARNING NR 03//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 16W (SIXTEEN), LOCATED APPROXIMATELY 141
NM WEST OF ANDERSEN AFB, GUAM HAS TRACKED NORTH-NORTHWESTWARD AT 06
KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE
IMAGERY DEPICTS A CONSOLIDATING LOW LEVEL CIRCULATION CENTER (LLCC)
WITH FLARING CONVECTION BEGINNING TO BUILD BACK OVER THE CENTER. A
090235Z AMSR-2 MICROWAVE IMAGE AND THE GUAM RADAR LOOP SHOW A WELL-
DEFINED LLCC WHICH SUPPORTS THE CURRENT POSITION WITH GOOD
CONFIDENCE. ADDITIONALLY, THE AMSR-2 MICROWAVE IMAGE SHOWS BROKEN
CONVECTIVE BANDING WRAPPING IN FROM ALL QUADRANTS WITH THE MOST
INTENSE CONVECTION ALONG THE SOUTHERN PERIPHERY OF THE LLCC. THE
CURRENT INTENSITY HAS BEEN INCREASED BASED ON THE IMPROVEMENT OF THE
LLCC AND BANDING SEEN IN THE MICROWAVE AND RADAR IMAGERY.
ENVIRONMENTAL ANALYSIS INDICATES A MARGINALLY-FAVORABLE ENVIRONMENT
WITH LOW (5-10 KNOT) VERTICAL WIND SHEAR (VWS), WEAK RADIAL OUTFLOW,
AND WARM (29 CELSIUS) SEA SURFACE TEMPERATURES (SSTS). THE SYSTEM IS
CURRENTLY WITHIN A WEAKNESS IN THE LOW LEVEL SUB-TROPICAL RIDGE
(STR) WHICH HAS LED TO QUASI-STATIONARY MOTION OVER THE PAST 12-18
HOURS.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. AS THE STR REBUILDS OVER THE NEXT 6 HOURS, TD 16W WILL RETURN
TO A NORTHWESTWARD TRACK. THIS STEERING MECHANISM IS EXPECTED TO
REMAIN IN PLACE THROUGHOUT THE FORECAST PERIOD, LEADING TO
PERSISTENT NORTHWESTWARD MOTION. 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 IS IN GOOD AGREEMENT DURING THIS PORTION OF THE FORECAST,
WITH ONLY SMALL DIFFERENCES IN THE NEAR-TERM DUE TO THE RECENT
ERRATIC STORM MOTION.
   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. BEYOND TAU 72,
IMPROVED VWS AND OUTFLOW, COMBINED WITH INCREASED SSTS AND 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 A
200NM SPREAD AT TAU 120. THE ONLY OUTLIER OF THE GROUP IS HWRF WHICH
TAKES THE SYSTEM 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. HOWEVER, DUE TO THE RECENT ERRATIC STORM MOTION THERE IS
AN OVERALL LOW CONFIDENCE IN THE JTWC TRACK FORECAST.//
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