Tropical Storm NAMTHEUN Advisory Thu Sep 01

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 15W (NAMTHEUN) WARNING NR 05//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 15W (NAMTHEUN), LOCATED APPROXIMATELY 158 NM
SOUTHEAST OF KADENA AB, HAS TRACKED EAST-NORTHEASTWARD AT 12 KNOTS
OVER THE PAST SIX HOURS. TY 15W IS CLASSIFIED AS A MIDGET SYSTEM
WITH A VERY SMALL, COMPACT LOW-LEVEL CIRCULATION CENTER AND HAS
RAPIDLY INTENSIFIED OVER THE PAST 24 HOURS FROM 25 KNOTS TO THE
CURRENT INTENSITY OF 65 KNOTS. THIS ASSESSMENT IS BASED ON DVORAK
CURRENT INTENSITY ESTIMATES RANGING FROM 55 (KNES) TO 77 KNOTS
(PGTW). ADDITIONALLY, THE LATEST SATCON ESTIMATE IS 60 KNOTS BUT HAD
PEAKED AT 64 KNOTS AT 010815Z. ANIMATED ENHANCED INFRARED (EIR)
SATELLITE IMAGERY EARLIER INDICATED A PINHOLE EYE BUT THIS EYE HAS
SINCE BECOME CLOUD-FILLED. RECENT EIR IMAGERY SHOWS A 110-NM CENTRAL
DENSE OVERCAST FEATURE OBSCURING THE CENTER, HOWEVER, RADAR FIXES
FROM THE JMA SUPPORT THE CURRENT POSITION WITH HIGH CONFIDENCE.
ENVIRONMENTAL CONDITIONS REMAIN FAVORABLE WITH 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 SOUTHEAST.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TY 15W IS EXPECTED TO TURN POLEWARD 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 36 AS THE FLOW AHEAD
OF A TROUGH APPROACHING FROM THE NORTHWEST INCREASES VERTICAL SHEAR
OVER THE SYSTEM. FURTHER WEAKENING IS EXPECTED WHILE TY 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 GFDN 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. IN THE EXTENDED PERIOD, TY 15W IS EXPECTED TO RECURVE AND
WEAKEN RAPIDLY AS IT BEGINS EXTRATROPICAL TRANSITION (ETT) OVER THE
EAST SEA. TY 15W SHOULD COMPLETE ETT BY TAU 120 AS IT BECOMES
EMBEDDED WITHIN THE MIDLATITUDE WESTERLIES AND GAINS FRONTAL
CHARACTERISTICS. 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.//
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