Tropical Storm NAMTHEUN Advisory 02 Σεπ.

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 15W (NAMTHEUN) WARNING NR 06//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON 15W (NAMTHEUN), LOCATED APPROXIMATELY 157 NM EAST-
SOUTHEAST OF KADENA AB, HAS TRACKED NORTH-NORTHEASTWARD AT 09 KNOTS
OVER THE PAST SIX HOURS. ANIMATED ENHANCED INFRARED SATELLITE
IMAGERY AND A 011002Z 91 GHZ SSMIS IMAGE DEPICTS DEEP FLARING
CENTRAL CONVECTION EMANATING FROM A VERY COMPACT LOW LEVEL
CIRCULATION CENTER. THE INITIAL POSITION IS BASED ON RECENT RADAR
DATA FROM JMA WITH GOOD CONFIDENCE. THE INITIAL INTENSITY IS
ASSESSED AT 65 KNOTS AND IS BASED ON PERSISTENCE IN THE CONVECTIVE
STRUCTURE AND RECENT DVORAK ESTIMATES FROM ALL AGENCIES REPORTING
T4.0 (65 KNOTS). UPPER-LEVEL ANALYSIS SHOWS THE PRESENCE OF A STRONG
POLEWARD OUTFLOW CHANNEL INTO A TROUGH JUST OFF TO THE NORTHWEST AND
COMPETING SOUTHWESTERLY FLOW WHICH IS HINDERING ANY SUBSTANTIAL
EQUATORWARD OUTFLOW. SEA SURFACE TEMPERATURES ARE FAVORABLE, NEAR 30
CELSIUS, WITH HIGH OCEAN HEAT CONTENT. CURRENTLY TY 15W IS TRACKING
NORTHEAST AND DEACCELERATING INTO A WEAK STEERING ENVIRONMENT AS THE
NEAR-EQUATORIAL RIDGE MOVES OFF TO THE SOUTHWEST.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. IN THE SHORT-TERM FORECAST TY 15W WILL REMAIN IN A WEAK
STEERING ENVIRONMENT OVER THE NEXT 48 HOURS AS A DEEP-LAYERED
SUBTROPICAL RIDGE TO THE NORTHEAST SLOWLY BUILDS AND GENTLY GUIDES
TY 15W TO THE NORTH-NORTHWEST. INTENSIFICATION IS EXPECTED OVER THE
NEXT 48 HOURS REACHING A PEAK INTENSITY OF 100 KNOTS.
INTENSIFICATION IS PARTICULARLY DIFFICULT GAUGE AS WESTERLY WINDS
ALOFT WILL CONTINUE TO COMPETE WITH THE STRONG POLEWARD CHANNEL INTO
THE TROUGH, AND FAVORABLE WATERS WILL PERSIST. BEYOND TAU 48 THE
STEERING RIDGE WILL BUILD SUFFICIENTLY TO STEER TY 15W INTO THE HIGH
WIND SHEAR OF THE UPPER-LEVEL TROUGH AND BEGIN TO WEAKEN
CONSIDERABLY.
    C. IN THE EXTENDED TAUS TY 15W WILL SHIFT BACK TO A
NORTHEASTWARD TRACK AS IT ROUNDS THE RIDGE AXIS RIDING INTO THE
TROUGH. LANDFALL IS EXPECTED NEAR SASEBO, JAPAN AROUND TAU 72 AS A
STRONG TROPICAL STORM. THE LAND INTERACTION WILL LEAD TO FURTHER
WEAKENING AND BY TAU 120 TY 15W WILL START TRANSITIONING INTO AN
EXTRATROPICAL SYSTEM. DYNAMIC MODEL GUIDANCE HAS BEEN STRUGGLING DUE
TO THE SMALL STRUCTURE OF THE STORM, BUT IS STARTING TO COME INTO
BETTER AGREEMENT AND OVERALL SUPPORTING THE FORECAST PHILOSOPHY.
NAVGEM, COAMPS-TC, AND GFDN ARE STILL SHOWING AN EARLIER TURN TO THE
NORTHEAST UNREALISTICALLY ERODING THE STEERING RIDGE. DUE TO THE
UNCERTAINTY AND WEAK STEERING ENVIRONMENT THERE IS LOW CONFIDENCE IN
THE FORECAST TRACK.//
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