Tropical Storm MEGI Advisory mar, 27.09.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 20W (MEGI) WARNING NR 17//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 20W (MEGI), LOCATED APPROXIMATELY 152 NM SOUTHEAST OF
TAIPEI, TAIWAN, HAS TRACKED NORTHWESTWARD AT 13 KNOTS OVER THE PAST
SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY (MSI) SHOWS A
35-NM RAGGED EYE WITH TIGHTLY-CURVED BANDING WRAPPING INTO THE
LOW-LEVEL CIRCULATION CENTER (LLCC). THE INITIAL POSITION IS BASED ON
THE EYE IN THE MSI LOOP WHICH LINES UP WELL WITH A LOW REFLECTIVITY
MICROWAVE EYE FEATURE IN THE 262231Z 36GHZ GPM PASS. THE INITIAL
INTENSITY IS ASSESSED AT 100 KNOTS AND IS BASED ON DVORAK CURRENT
INTENSITY ESTIMATES OF T5.5 FROM ALL AGENCIES. UPPER-LEVEL ANALYSIS
INDICATES EXCELLENT RADIAL OUTFLOW WITH ENHANCED POLEWARD OUTFLOW
INTO A MIDLATITUDE SHORTWAVE TROUGH TO THE NORTH. HOWEVER, MOST
RECENT WATER VAPOR SATELLITE IMAGERY INDICATES SUBSIDENCE ALONG THE
NORTHWEST QUADRANT, INDICATING THE SYSTEM IS APPROACHING AN AREA OF
INCREASING VERTICAL WIND SHEAR (VWS). TY MEGI IS TRACKING
WEST-NORTHWESTWARD ALONG THE PERIPHERY OF A DEEP-LAYERED SUBTROPICAL
RIDGE (STR) POSITIONED TO THE NORTHEAST.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING BULLETIN.
   B. TY 20W IS FORECAST TO CONTINUE TRACKING WEST-NORTHWESTWARD,
DRAG THROUGH CENTRAL TAIWAN SHORTLY AFTER TAU 06, THEN CROSS THE
TAIWAN STRAIT BEFORE MAKING A FINAL LANDFALL IN EASTERN CHINA JUST
BEFORE TAU 24. AFTERWARDS, A SECONDARY STR OVER NORTHEASTERN CHINA
WILL ASSUME STEERING AND CAUSE THE CYCLONE TO TRACK ON A FLATTER
WESTWARD TRAJECTORY. A GRADUAL WEAKENING IS EXPECTED OVER THE NEXT FEW
HOURS AS INCREASED VERTICAL WIND SHEAR (VWS) ASSOCIATED WITH THE
LEADING EDGE OF THE SECONDARY STR IMPACTS THE SYSTEM. AFTER LANDFALL
INTO TAIWAN, RAPID WEAKENING IS EXPECTED AS LAND INTERACTION,
COMBINED WITH INCREASING VWS, ERODES THE SYSTEM. TY MEGI WILL
EVENTUALLY DISSIPATE INLAND NORTH OF HONG-KONG BY TAU 48. THE DYNAMIC
GUIDANCE REMAINS IN TIGHT AGREEMENT WITH SOME SPREADING AFTER
LANDFALL IN CHINA. OVERALL, THERE IS HIGH CONFIDENCE IN THE JTWC
TRACK FORECAST.//
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