Tropical Storm NAMTHEUN Advisory Wed Aug 31

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 15W (FIFTEEN)
WARNING NR 01//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 15W (FIFTEEN), LOCATED APPROXIMATELY 289
NM SOUTH-SOUTHWEST OF KADENA AB, OKINAWA, JAPAN HAS TRACKED
EAST-NORTHEASTWARD AT 12 KNOTS OVER THE PAST SIX HOURS. ANIMATED
ENHANCED INFRARED SATELLITE IMAGERY DEPICTS A CONSOLIDATING LOW
LEVEL CIRCULATION CENTER (LLCC) WITH CURVED CONVECTION WRAPPING INTO
THE CENTER. A 311014Z SSMIS 91GHZ MICROWAVE IMAGE DEPICTS FRAGMENTED
BANDING WRAPPING INTO THE LLCC FROM THE EASTERN AND NORTHERN
PERIPHERIES. THE INITIAL POSITION IS BASED ON THE ABOVE IMAGERY AND
IS SUPPORTED BY SATELLITE FIXES FROM PGTW AND RJTD WITH GOOD
CONFIDENCE. THE CURRENT INTENSITY IS BASED ON DVORAK INTENSITY
ESTIMATES OF T1.5 (25 KNOTS) FROM ALL REPORTING AGENCIES. THE SYSTEM
IS LOCATED IN A FAVORABLE ENVIRONMENT WITH GOOD DIVERGENCE ALOFT AND
LOW (10-15 KNOT) VERTICAL WIND SHEAR (VWS). SEA SURFACE TEMPERATURES
ABOVE 30 CELSIUS ARE ALSO FAVORABLE FOR FURTHER DEVELOPMENT. TD 15W
IS TRACKING ALONG THE NORTHERN PERIPHERY OF AN EXTENSION OF THE NEAR
EQUATORIAL RIDGE (NER) LOCATED TO THE SOUTHEAST.
3. FORECAST REASONING.
   A. THIS IS THE INITIAL PROGNOSTIC REASONING AND SETS THE FORECAST
PHILOSOPHY.
   B. TD 15W WILL CONTINUE TO TRACK NORTHEASTWARD UNDER THE
INFLUENCE OF THE NER OVER THE NEXT 24 TO 36 HOURS. EXPECT THE
CYCLONE TO GRADUALLY INTENSIFY DUE TO CONTINUED FAVORABLE
ENVIRONMENTAL CONDITIONS; HOWEVER, DRY AIR AND SUBSIDENCE TO THE
NORTH DUE TO AN UPPER LEVEL TROUGH EXTENDING DOWN OVER THE RYUKYU
ISLANDS WILL TAPER SIGNIFICANT INTENSIFICATION. DYNAMIC MODEL
GUIDANCE IS IN GOOD AGREEMENT THROUGH THIS PORTION OF THE FORECAST.
BEYOND TAU 36, THE NER WILL RETROGRADE TO THE WEST CREATING A WEAK
STEERING ENVIRONMENT WHICH WILL CAUSE A DECELERATION OF FORWARD
MOTION AND POSSIBLY A PERIOD OF QUASI-STATIONARY OR ERRATIC MOTION.
BEYOND TAU 48, THE COMBINED EFFECTS OF A BUILDING SUB-TROPICAL RIDGE
TO THE EAST AND THE AFOREMENTIONED UPPER LEVEL TROUGH WILL FORCE THE
SYSTEM NORTHWARD INTO A REGION OF INCREASING VWS AND SUBSIDENCE
ALOFT. THESE FACTORS WILL BEGIN A WEAKENING TREND.
   C. IN THE EXTENDED FORECAST, TD 15W WILL CONTINUE TO MOVE
NORTHWARD AND RAPIDLY WEAKEN AS IT MOVES FURTHER UNDER THE UPPER
LEVEL TROUGH AND ENCOUNTERS VWS IN EXCESS OF 30 KNOTS. THE SYSTEM
WILL FULLY DISSIPATE IN THE SEA OF JAPAN BY THE END OF THE FORECAST
PERIOD. DYNAMIC MODEL GUIDANCE IS CURRENTLY STRUGGLING WITH THE LACK
OF A DOMINANT STEERING MECHANISM NEAR TAU 36. AS A RESULT, NAVGEM,
HWRF, AND THE JAPANESE ENSEMBLE SHOW A WEAKER SYSTEM TURNING
NORTHWESTWARD, AT VARYING DEGREES, ACROSS THE NORTHERN RYUKYU
ISLANDS AND INTO THE EAST CHINA SEA. THE REMAINING GUIDANCE IS IN
LINE WITH THE CURRENT FORECAST WITH SOME NOTED DIFFERENCES IN THE
FORWARD SPEED OF THE CYCLONE AS IT ACCELERATES NORTHWARD. DUE TO THE
COMPLEX STEERING ENVIRONMENT BEYOND TAU 36 AND THE LACK OF OVERALL
AGREEMENT IN THE MODELS, THERE IS LOW CONFIDENCE IN THE INITIAL JTWC
TRACK FORECAST.//
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