Tropical Storm MEGI Advisory Fri Sep 23

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 20W (MEGI) WARNING
NR 03//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 20W (MEGI), LOCATED APPROXIMATELY 960 NM EAST-
SOUTHEAST OF TAIPEI, TAIWAN, HAS TRACKED WESTWARD AT 15 KNOTS OVER
THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE IMAGERY
SHOWS AN EXPANSIVE SYSTEM WITH MONSOON DEPRESSION CHARACTERISTICS
THAT IS SLOWLY CONSOLIDATING. FORMATIVE BANDS HAVE DEEPENED BUT
REMAIN FRAGMENTED. THE INITIAL POSITION IS BASED ON A LOW
REFLECTIVITY CIRCULATION FEATURE IN THE 230935Z AMSU-B MICROWAVE
IMAGE. THE INITIAL INTENSITY OF 35 KNOTS IS BASED ON AN OVERALL
ASSESSMENT OF AGENCY DVORAK ESTIMATES RANGING FROM T2.0 TO T3.0.
THERE IS LOW CONFIDENCE IN THE INITIAL POSITION GIVEN THE FORMATIVE
NATURE OF THE CYCLONE AND THE POSSIBILITY OF MULTIPLE CIRCULATIONS.
UPPER LEVEL ANALYSIS INDICATES THE STORM IS IN THE VICINITY OF AN
ANTICYCLONIC POINT SOURCE WITH LOW VERTICAL WIND SHEAR AND EXCELLENT
DUAL OUTFLOW. ADDITIONALLY, SEA SURFACE TEMPERATURES AT 30C ARE
EXCEPTIONALLY WARM AND CONDUCIVE FOR FURTHER DEVELOPMENT. THE CYCLONE
IS TRACKING ALONG THE SOUTHWESTERN PERIPHERY OF A MID-LEVEL
REFLECTION OF THE SUBTROPICAL RIDGE (STR) TO THE NORTH-NORTHEAST.
3. FORECAST REASONING.
    A. THERE IS NO CHANGE TO FORECAST PHILOSOPHY FROM THE PREVIOUS
PROGNOSTIC REASONING BULLETIN.
    B. TS 20W IS EXPECTED TO CONTINUE ON A WEST-NORTHWESTWARD
TRAJECTORY THROUGHOUT ITS LIFESPAN, STEERED BY THE STR THAT IS
FORECAST TO BUILD. ENVIRONMENTAL CONDITIONS WILL REMAIN FAVORABLE FOR
STEADY INTENSIFICATION, REACHING A PEAK OF 90 KNOTS BY TAU 72 AS THE
SYSTEM DRIFTS ACROSS THE WARM PHILIPPINE SEA.
    C. IN THE EXTENDED PERIOD, TS MEGI WILL CROSS SOUTHERN TAIWAN AND
THE TAIWAN STRAIT PRIOR TO MAKING LANDFALL IN EASTERN CHINA SHORTLY
AFTER TAU 96. LAND INTERACTION WILL BE THE PRIMARY CAUSE OF ITS RAPID
WEAKENING DURING THIS STAGE. SANS THE UNCERTAINTY IN THE INITIAL
POSITION, MODEL GUIDANCE REMAINS IN GENERALLY GOOD AGREEMENT, LENDING
HIGH CONFIDENCE TO THE JTWC TRACK FORECAST.//
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