Tropical Storm NAMTHEUN Advisory Fri Sep 02

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 15W (NAMTHEUN)
WARNING NR 04//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM (TS) 15W (NAMTHEUN), LOCATED APPROXIMATELY 144 NM
SOUTH-SOUTHEAST OF KADENA AB, HAS TRACKED EAST-NORTHEASTWARD AT
14 KNOTS OVER THE PAST SIX HOURS. THE CURRENT POSITION IS BASED,
WITH HIGH CONFIDENCE, ON RECENT SATELLITE FIXES FROM PGTW AND RTJD,
RADAR FIXES FROM THE JMA, AND A 010550Z SSMI IMAGE. THE INTENSITY OF
50 KNOTS IS SET SLIGHTLY ABOVE DVORAK T-NUMBER ESTIMATES BASED ON
THE RECENT APPEARANCE OF A LOW LEVEL EYE FEATURE IN BOTH
MULTISPECTRAL AND MICROWAVE SATELLITE IMAGERY. THE RELATIVELY
COMPACT CIRCULATION OF TS 15W HAS BECOME CONSIDERABLY MORE ORGANIZED
AND RAPIDLY INTENSIFIED OVER THE PAST SIX HOURS, SUPPORTED BY LOW
VERTICAL WIND SHEAR, STRONG POLEWARD OUTFLOW ALOFT, AND PASSAGE OVER
VERY WARM WATER. THE SYSTEM IS TRACKING EAST-NORTHEASTWARD ALONG THE
PERIPHERY OF A NARROW STEERING RIDGE POSITIONED TO THE SOUTH AND
EAST.
3. FORECAST REASONING.
   A. FORECAST INTENSITIES HAVE INCREASED SIGNIFICANTLY FROM THE
PREVIOUS WARNING BASED ON RECENT TRENDS AND GENERALLY SUPPORTIVE
ANTICIPATED ENVIRONMENTAL CONDITIONS. THE FORECAST TRACK HAS
SHIFTED SLIGHTLY TO THE EAST FROM THE PREVIOUS WARNING.
EXTRATROPICAL TRANSITION IS NOW ANTICIPATED, BEGINNING AROUND TAU 96.
   B. TS 15W IS EXPECTED TO CONTINUE TRACKING GENERALLY
NORTHEASTWARD FOR THE NEXT 12 HOURS UNDER THE INFLUENCE OF THE
CURRENT STEERING RIDGE. THEREAFTER, THE SYSTEM SHOULD TURN POLEWARD
AND SLIGHTLY WESTWARD AS THE CURRENT STEERING RIDGE REORIENTS AND A
SECOND RIDGE POSITIONED TO THE NORTHEAST OF THE CYCLONE EXERTS AN
INCREASING INFLUENCE ON THE STORM TRACK THROUGH TAU 72. CONTINUED
RAPID INTENSIFICATION IS ANTICIPATED IN THE NEAR-TERM AS THE SYSTEM
RESPONDS TO FAVORABLE UPPER-LEVEL SUPPORT AND VERY HIGH ALONG-TRACK
OCEAN HEAT CONTENT. THE INTENSIFICATION TREND IS EXPECTED TO END BY
TAU 48 AS THE FLOW AHEAD OF A TROUGH APPROACHING FROM THE NORTHWEST
INCREASES VERTICAL SHEAR OVER THE SYSTEM. FURTHER WEAKENING IS
EXPECTED WHILE TS 15W TRACKS OVER LAND BETWEEN TAU 48 AND TAU 72.
THERE IS CONSIDERABLE SPREAD IN THE NUMERICAL MODEL TRACK FORECAST
GUIDANCE. THE CURRENT OFFICIAL TRACK FORECAST IS CONSISTENT WITH THE
MAJORITY OF CONSENSUS MODEL GUIDANCE AND LIES TO THE WEST OF A FEW
EASTERN OUTLIERS INCLUDING THE NAVGEM, COAMPS-TC, AND HWRF MODELS,
WHICH DEPICT A SHARP RIGHT TURN OVER OR TO THE SOUTH OF HONSHU AFTER
TAU 72. THE ANTICIPATED SYNOPTIC PATTERN DOES NOT CURRENTLY SUPPORT
THE SCENARIO DEPICTED BY THESE OUTLIERS. HOWEVER, GIVEN THE NOTED
SPREAD IN NUMERICAL MODEL GUIDANCE, THERE IS LOW CONFIDENCE IN THE
CURRENT JTWC TRACK FORECAST THROUGH TAU 72.
    C. TS 15W IS EXPECTED TO CONTINUE POLEWARD AND WEAKEN AS IT
BEGINS EXTRATROPICAL TRANSITION OVER THE SEA OF JAPAN IN THE
EXTENDED FORECAST PERIOD. ALTHOUGH THE MAJORITY OF CONSENSUS TRACK
FORECAST MODEL GUIDANCE IS FAIRLY CONSISTENT REGARDING THE
ORIENTATION OF THE TRACK DURING THIS PERIOD, THERE IS CONSIDERABLE
DISAGREEMENT REGARDING TRACK SPEED. THEREFORE, THERE IS LOW
CONFIDENCE IN THE CURRENT JTWC EXTENDED PERIOD TRACK FORECAST.//
NNNN
NNNN