Tropical Storm SANBA Advisory Fri Sep 14

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 17W (SANBA) WARNING NR 11//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 17W (SANBA), LOCATED APPROXIMATELY 650 NM SOUTH OF
KADENA, OKINAWA, JAPAN, HAS TRACKED NORTH-NORTHWESTWARD AT 09 KNOTS
OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY
(MSI) REVEALS A PERSISTENT 18 NM EYE COMPLETELY ENCIRCLED BY DEEP
CONVECTION OVER THE PAST SIX HOURS AS THE SYSTEM CONTINUES TO
UNDERGO RAPID INTENSIFICATION. A 130638Z TRMM 85 GHZ MICROWAVE
IMAGE DEPICTS A VERY SYMMETRIC SYSTEM WITH A SMALL EYE AND DEEP
CONVECTION SURROUNDING THE SYSTEM WITH SOME BANDING TO THE NORTH
AND TO THE SOUTH. THE CURRENT POSITION IS BASED ON THE VISIBLE EYE
IN THE MSI WITH HIGH CONFIDENCE. ANIMATED WATER VAPOR IMAGERY
INDICATES RADIAL OUTFLOW WITH AN EXCELLENT POLEWARD AND EQUATORWARD
CHANNEL. THIS POLEWARD FLOW IS BEING ENHANCED BY A TUTT CELL TO THE
NORTHEAST AS WELL AS THE SUBTROPICAL LOW LOCATED SOUTHEAST OF
HONSHU. THE INITIAL INTENSITY OF 115 KNOTS IS BASED ON DVORAK
ESTIMATES FROM PGTW AND RJTD AND THE CONTINUED EXCELLENT SATELLITE
PRESENTATION. TY 17W SHOULD CONTINUE RAPID INTENSIFICATION AS THERE
IS LOW (<5 KNOTS) OF VERTICAL WIND SHEAR ACTING UPON THE SYSTEM,
FAVORABLE UPPER LEVEL ENVIRONMENT AND FAVORABLE SEA SURFACE
TEMPERATURES (29-30 DEGREES CELSIUS.)
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TY 17W IS TRACKING NORTH-NORTHWESTWARD, ALONG THE WESTERN
PERIPHERY OF THE NEAR-EQUATORIAL RIDGE (NER). THE SYSTEM IS
EXPECTED TO CONTINUE THIS MOTION THROUGH TAU 72. THERE IS HIGH
CONFIDENCE IN THIS PORTION OF THE TRACK BASED ON THE TIGHT
AGREEMENT AMONG DYNAMIC MODELS. GFDN CONTINUES TO BE THE SOLE
EASTERN OUTLIER, DIVERGING THE SYSTEM TO THE EAST OF OKINAWA AFTER
TAU 48, AND ECMWF IS THE WESTERMOST OUTLIER; HOWEVER, IT DOES STAY
WITH THE MODEL PACKING UNTIL TAU 72. GIVEN THE RECENT RAPID
INTENSIFICATION, THE FAVORABLE ENVIRONMENTAL CONDITIONS AND AN
EXPECTED ENHANCEMENT TO POLEWARD OUTFLOW AS A SHORTWAVE TROUGH
APPROACHES FROM EASTERN CHINA, TY 17W IS FORECAST TO CONTINUE
INTENSIFYING OVER THE NEXT 48 HOURS. A PEAK INTENSITY OF 140 KNOTS
IS FORECAST BETWEEN TAU 36 AND TAU 48.
   C. IN THE EXTENDED TAUS, THERE IS LOW CONFIDENCE IN THE TRACK
FORECAST DUE TO UNCERTAINTY IN THE DEPTH AND ORIENTATION OF THE A
MAJOR SHORTWAVE TROUGH AS IT DIGS INTO THE YELLOW SEA AND EAST
CHINA SEA FROM EASTERN CHINA. DYNAMIC MODEL GUIDANCE BEGINS TO
DIVERGE AT TAU 96 BUT ALL DYNAMIC MODELS NOW AGREE ON A TURN INTO
SOUTH KOREA WITH THE EXCEPTION AFOREMENTIONED GFDN, WHICH TAKES THE
SYSTEM INTO SOUTHERN JAPAN,AND ECMWF WHICH KEEPS THE SYSTEM OVER
THE YELLOW SEA. BOTH THE GFDN AND ECMWF SOLUTIONS SEEM UNLIKELY AS
A RIDGE IS FORECASTED TO BE WELL ESTABLISHED OVER SOUTHERN JAPAN,
AND THE TROUGH APPROACHING FROM EASTERN CHINA. AFTER TAU 96, TY 17W
WILL START TO RAPIDLY WEAKEN DUE TO INCREASING VWS ASSOCIATED WITH
THE SHORTWAVE TROUGH AND WILL BEGIN EXTRA-TROPICAL TRANSITION AT
TAU 120 AS IT INTERACTS WITH THE BAROCLINIC ZONE.//
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