Tropical Storm NANGKA Advisory Sat Jul 11

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 11W (NANGKA) WARNING NR 31//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 11W (NANGKA), LOCATED APPROXIMATELY 855 NM EAST-
SOUTHEAST OF KADENA AB, OKINAWA, JAPAN, HAS TRACKED
WEST-NORTHWESTWARD AT 07 KNOTS OVER THE PAST SIX HOURS. ANIMATED
MULTISPECTRAL SATELLITE IMAGERY SHOWS A WEAKENING SYSTEM WITH A
CLOUD-FILLED EYE AND SPIRAL BANDING LOCATED PRIMARILY OVER THE
SOUTHERN SEMI-CIRCLE DUE TO INCREASING VWS ALONG THE NORTHERN FLANK.
THE INITIAL POSITION IS BASED ON A DIMPLE FEATURE IN THE ABOVE
ANIMATION WITH HIGH CONFIDENCE. THE INITIAL INTENSITY OF 100 KNOTS IS
SUPPORTED BY DVORAK ESTIMATES FROM PGTW AND RJTD AND REFLECTS THE
WEAKENING TREND. THE CYCLONE IS TRACKING ALONG THE SOUTHERN PERIPHERY
OF AN EXTENSION OF THE DEEP LAYERED STR TO THE NORTHEAST.
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE
PREVIOUS PROGNOSTIC REASONING MESSAGE.
   B. TY 11W WILL CONTINUE TO SLOWLY TRACK WESTWARD TO WEST-
NORTHWESTWARD UP TO TAU 36 UNDER THE INFLUENCE OF THE STEERING STR.
AFTERWARDS, THE SYSTEM WILL BEGIN TO TURN MORE POLEWARD ALONG THE
WESTERN PERIPHERY OF THE STR. MARGINALLY-FAVORABLE UPPER-LEVEL
CONDITIONS ARE EXPECTED TO CONTINUE, LEADING TO A STEADY WEAKENING
TREND THROUGH TAU 48. AFTER TAU 48, TY 11W WILL REVERT TO A
NORTHWESTWARD TRACK TOWARD THE RYUKYU ISLANDS AS THE STEERING
INFLUENCE TRANSITIONS TO A SECONDARY STR CURRENTLY POSITIONED SOUTH
OF MAINLAND JAPAN. VWS IS FORECAST TO DECREASE WITH IMPROVING
POLEWARD OUTFLOW, WHICH WILL ALLOW THE SYSTEM TO RE-INTENSIFY.
   C. IN THE EXTENDED TAUS, TY NANGKA WILL CONTINUE TO STRENGTHEN AS
FAVORABLE ENVIRONMENTAL CONDITIONS PERSIST, REACHING 95 KNOTS BY END
OF FORECAST. DYNAMIC MODEL GUIDANCE IS IN FAIR AGREEMENT WITH ALL
MODELS SHOWING A TRACK TOWARD THE RYUKYU ISLANDS. HOWEVER, THERE IS
SIGNIFICANT DIFFERENCES IN THE TRACK SPEED DURING THE INITIAL TURN IN
THE FIRST 24-36 HOURS, AND THERE REMAINS A LARGE SPREAD (ABOUT
250 NM) IN THE MODEL SOLUTIONS. IN VIEW OF THIS, THE JTWC TRACK
FORECAST CONFIDENCE LEVEL REMAINS LOW.//
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