Tropical Storm MALAKAS Advisory Tue Sep 20

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 18W (MALAKAS) WARNING
NR 36//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 18W (MALAKAS), LOCATED APPROXIMATELY 49 NM
WEST OF YOKOSUKA, JAPAN, HAS TRACKED EAST-NORTHEASTWARD AT 24 KNOTS
OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED (EIR) SATELLITE
IMAGERY AND RADAR IMAGERY DEPICT A BROAD, ILL-DEFINED LOW-LEVEL
CIRCULATION CENTER (LLCC) WITH DEEP CONVECTION SHEARED TO THE
NORTHEAST DUE TO STRONG (50 TO 60 KNOTS) WESTERLY VERTICAL WIND
SHEAR (VWS). BOTH EIR AND RADAR IMAGERY SUGGEST A WEAK FRONTAL
STRUCTURE, HOWEVER, TS 18W IS CLEARLY WEAKENING RAPIDLY AS IT
CROSSES HONSHU SO THE SYSTEM IS UNLIKELY TO DEVELOP INTO AN EXTRA-
TROPICAL LOW. SURFACE OBSERVATIONS ACROSS CENTRAL HONSHU GENERALLY
INDICATE SUSTAINED WINDS LESS THAN 20 KNOTS WITH NO DEFINED
CIRCULATION EVIDENT. A 201145Z METOP-A 89GHZ IMAGE DEPICTS SHALLOW
CURVED BANDING OVER THE SOUTH QUADRANT (OVER WATER) OF THE LLCC BUT
NO CONVECTIVE STRUCTURE ELSEWHERE. DUE TO THE WEAK NATURE OF THE
LLCC, THERE IS LOW CONFIDENCE IN THE CURRENT POSITION. THE CURRENT
INTENSITY IS DIFFICULT TO ACCURATELY ASSESS OVER THE ROUGH TERRAIN,
HOWEVER, IT IS CONSERVATIVELY ESTIMATED AT 40 KNOTS BUT COULD BE
LOWER. CURRENTLY TS 18W IS ACCELERATING NORTHEASTWARD ALONG THE
NORTHWESTERN PERIPHERY OF THE DEEP-LAYERED SUBTROPICAL RIDGE, WITHIN
THE MIDLATITUDE WESTERLIES.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TS 18W IS FORECAST TO TRACK RAPIDLY NORTHEASTWARD TO EAST-
NORTHEASTWARD THROUGH THE FORECAST PERIOD. DYNAMIC MODEL GUIDANCE IS
IN EXCELLENT AGREEMENT, THEREFORE, THERE IS HIGH CONFIDENCE IN THE
JTWC FORECAST TRACK. THE SYSTEM WILL CONTINUE TO WEAKEN RAPIDLY DUE
TO LAND INTERACTION, PERSISTENT STRONG VWS AND COOLER SST, AND IS
EXPECTED TO DISSIPATE BY TAU 24.//
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