Tropical Storm SANBA Advisory Sat Sep 15

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 17W (SANBA) WARNING NR
18//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 17W (SANBA), LOCATED APPROXIMATELY 250 NM SOUTH-
SOUTHEAST OF KADENA AB, OKINAWA, JAPAN, HAS TRACKED NORTH-
NORTHWESTWARD AT 10 KNOTS OVER THE PAST SIX HOURS. ANIMATED
MULTISPECTRAL SATELLITE IMAGERY (MSI) DEPICTS A VERY LARGE TIGHTLY
WRAPPED SYSTEM WITH A REDEVELOPING EYE. CONVECTION IS FULLY WRAPPED
AROUND THE REDEVELOPING EYE WITH THE DEEPEST CONVECTION CONCENTRATED
IN THE EASTERN SEMICIRCLE. THE NARROW WEDGE OF DRY AIR THAT BEGAN
WRAPPING INTO THE SYSTEM FROM THE SOUTHWEST SIX HOURS AGO HAS NOW
ENCIRCLED THE NORTHERN PERIPHERY OF THE SYSTEM. THIS IS LIKELY
RESULTING IN A SLIGHT DECREASE IN CONVECTION IN THIS REGION. A
150103Z AMSUB 89GHZ IMAGE SHOWS A MICROWAVE EYE WITH DEEP CONVECTION
ALONG THE EASTERN SEMICIRCLE. THE INITIAL INTENSITY OF 110 KNOTS IS
BASED ON DVORAK ESTIMATES FROM PGTW, RJTD AND KNES RANGING FROM 102
TO 127 KNOTS. THERE IS VERY GOOD CONFIDENCE IN THE INITIAL POSITION
BASED ON THE SMALL EYE DEPICTED IN THE MSI. UPPER LEVEL ANALYSIS
SHOWS TY 17W HAS CONTINUED TO MAINTAIN RADIAL OUTFLOW. A SMALL
TROPICAL UPPER TROPOSPHERIC TROUGH (TUTT) CELL TO THE NORTHWEST OF
17W BRIEFLY RESULTED IN A SLIGHT DECREASE IN POLEWARD OUTFLOW,
HOWEVER THE DIFFLUENT REGION TO THE EAST OF THE SHORTWAVE TROUGH
OVER EASTERN CHINA IS BEGINNING TO ENHANCE POLEWARD OUTFLOW.
VERTICAL WIND SHEAR (VWS) HAS INCREASED SLIGHTLY TO 15-25 KNOTS,
RESULTING IN A MODERATELY FAVORABLE UPPER LEVEL ENVIRONMENT. THE
SUBTROPICAL (STR) RIDGE TO THE EAST OF TY 17W REMAINS THE
PREDOMINANT STEERING MECHANISM, AND THIS IS EVIDENT IN THE RECENT
NORTHWESTWARD MOTION OF THE SYSTEM.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TY 17W IS FORECAST TO CONTINUE TRACKING NORTH-NORTHWESTWARD
ALONG THE WESTERN PERIPHERY OF THE STR TO THE EAST. THE APPROACHING
SHORTWAVE TROUGH WILL LIKELY RESULT IN AN INCREASED FORWARD TRACK
SPEED IN A MORE NORTHWARD DIRECTION BETWEEN TAUS 24 AND 36. AS THE
SHORTWAVE TROUGH BEGINS TO SLOWLY ERODE THE STR, TY 17W WILL TAKE ON
A MORE NORTHEASTWARD TRACK AND UNDERGO EXTRATROPICAL TRANSITION (ET)
BY TAU 72. DUE TO INCREASING VERTICAL WIND SHEAR, DECREASING SEA
SURFACE TEMPERATURES, AND AN INCREASINGLY UNFAVORABLE UPPER LEVEL
ENVIRONMENT, 17W IS EXPECTED TO GRADUALLY DECREASE IN INTENSITY.
THERE IS HIGH CONFIDENCE IN THE FORECAST UNTIL TAU 48, AT WHICH
POINT NUMERICAL MODEL GUIDANCE DIVERGES. JGSM AND WBAR ARE WEST OF
THE GUIDANCE ENVELOPE WITH A TRACK EXTENDING THROUGH THE CENTRAL
KOREAN PENINSULA WHILE NOGAPS AND GFDN FAVOR A MORE EASTWARD TRACK
NEAR THE SOUTHEASTERN COAST OF SOUTH KOREA. BY TAU 72 GUIDANCE IS
SPREAD UP TO 175 NM AS THE DYNAMIC MODELS STRUGGLE TO RESOLVE THE
EROSION OF THE STR.
   C. BEYOND TAU 72, TY 17W IS EXPECTED TO BEGIN EXTRATROPICAL
TRANSITION AS IT INTERACTS WITH THE BAROCLINIC ZONE. THIS WILL
ACCELERATE THE SYSTEM POLEWARD AND WEAKEN IT RAPIDLY AS IT
ENCOUNTERS STRONG VWS AND INTERACTS WITH LAND. TY 17W IS FORECAST TO
COMPLETE ETT BY TAU 96. IN THE EXTENDED TAUS, FORECAST TRACK
CONFIDENCE REMAINS LOW.//
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Storm tracks Sat Sep 15

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Atlantic Ocean
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2012

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