Tropical Storm SANBA Advisory Fri Sep 14

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR SUPER TYPHOON 17W (SANBA) WARNING NR
14//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   SUPER TYPHOON (STY) 17W (SANBA), LOCATED APPROXIMATELY 500 NM
SOUTH-SOUTHEAST OF KADENA AB, OKINAWA, JAPAN, HAS TRACKED NORTHWARD
AT 10 KNOTS OVER THE PAST SIX HOURS. ANIMATED INFRARED SATELLITE
IMAGERY CONTINUES TO DEPICT A WELL-ORGANIZED SYSTEM WITH A SYMMETRIC
190-200 NM CORE OF DEEP CONVECTION SURROUNDING AN 18 NM EYE. A
132138Z SSMIS 37 GHZ IMAGE DEPICTS CONCENTRIC EYEWALLS AND SUGGESTS
THAT THE SYSTEM MAY BE BEGINNING AN EYEWALL REPLACEMENT CYCLE, WHICH
COULD SIGNAL AN IMPENDING WEAKENING TREND. DVORAK DATA-T ESTIMATES
HAVE DECREASED SLIGHTLY TO 140 KNOTS; HOWEVER, CURRENT INTENSITY
ESTIMATES HAVE REMAINED AS HIGH AS 155 KNOTS, THEREFORE, THE INITIAL
INTENSITY REMAINS ASSESSED AT 150 KNOTS. THERE IS VERY GOOD
CONFIDENCE IN THE INITIAL POSITION BASED ON THE SMALL EYE. ANIMATED
WATER VAPOR IMAGERY INDICATES RADIAL OUTFLOW WITH AN EXCELLENT
POLEWARD CHANNEL, ENHANCED BY THE TUTT AS WELL AS A SUBTROPICAL LOW
(LOCATED SOUTHEAST OF HONSHU) WHICH IS HELPING TO PRODUCE 60-70 KNOT
UPPER-LEVEL WESTERLY FLOW NORTH OF THE SYSTEM. STY 17W HAS RAPIDLY
INTENSIFIED OVER THE PAST 24 HOURS WITH A 60 KNOT INCREASE SINCE
13/00Z. THE 13/12Z 500 MB ANALYSIS AND SATELLITE IMAGERY DEPICT A
MAJOR SHORT-WAVE TROUGH CURRENTLY DEEPENING OVER EASTERN CHINA. IT
ALSO SHOWS A NARROW SUBTROPICAL RIDGE (STR) POSITIONED OVER THE EAST
CHINA SEA AND STRONG MIDLATITUDE WESTERLY FLOW ACROSS CHINA AND THE
KOREAN PENINSULA. THE 14/00Z MINAMIDAITO-JIMA SOUNDING ALSO PROVIDES
FURTHER EVIDENCE THAT THE STR REMAINS POSITIONED NORTH OF THE SYSTEM
WITH EAST-SOUTHEASTERLY TO EASTERLY FLOW THROUGH THE LOW- TO MID-
LEVELS.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. STY 17W IS FORECAST TO CONTINUE TRACKING NORTHWARD THROUGH TAU
24 BUT SHOULD TURN NORTH-NORTHWESTWARD AS IT TRANSITIONS STEERING
INFLUENCE TO THE AFOREMENTIONED STR. THE STR IS WEAKENING IN
RESPONSE TO THE DEEP SHORTWAVE TROUGH BUT IS STILL EXPECTED TO
PROVIDE A SLIGHT WESTWARD INFLUENCE, WHICH WILL PRODUCE AN S-SHAPED
TRACK. THERE IS HIGH CONFIDENCE IN THIS PORTION OF THE TRACK BASED
ON THE TIGHT AGREEMENT IN DYNAMIC MODELS. AT TAU 48, THERE IS ONLY A
63 NM SPREAD IN SOLUTIONS. STY 17W IS EXPECTED TO MAINTAIN STY
STRENGTH THROUGH TAU 36 DUE TO THE VERY FAVORABLE OUTFLOW
CONDITIONS. AFTER TAU 48, THE SYSTEM WILL WEAKEN SIGNIFICANTLY DUE
TO INCREASING VERTICAL WIND SHEAR (VWS) ASSOCIATED WITH THE
APPROACHING MIDLATITUDE SHORTWAVE TROUGH.
   C. IN THE EXTENDED TAUS, THERE IS INCREASING CONFIDENCE IN THE
TRACK FORECAST WITH TIGHT AGREEMENT IN THE AVAILABLE NUMERICAL
GUIDANCE. OVERALL, THERE IS HIGH CONFIDENCE IN THE FORECAST TRACK,
ESPECIALLY THROUGH TAU 72. NEAR TAU 72, STY 17W WILL BEGIN TO
INTERACT WITH THE BAROCLINIC ZONE AND WILL BEGIN EXTRA-TROPICAL
TRANSITION (ETT). THE SYSTEM WILL ACCELERATE POLEWARD, WEAKEN
RAPIDLY DUE TO STRONG VWS AND LAND INTERACTION AND IS FORECAST TO
COMPLETE ETT BY TAU 96.//
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