Tropical Storm TEMBIN Advisory Tue Aug 21

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TYPHOON 15W (TEMBIN) WARNING NR 11//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TYPHOON (TY) 15W (TEMBIN), LOCATED APPROXIMATELY 280 NM SOUTHEAST
OF TAIPEI, TAIWAN, HAS TRACKED NORTH-NORTHWESTWARD AT 08 KNOTS OVER
THE PAST SIX HOURS. ANIMATED INFRARED (IR) SATELLITE IMAGERY, A
211146Z SSMIS IMAGE AND A 211232Z METOP-A IMAGE PROVIDE SOLID
EVIDENCE THAT TY 15W IS TURNING INCREASINGLY WESTWARD AS IT
TRANSITIONS TO THE STEERING INFLUENCE OF THE SUBTROPICAL RIDGE
(STR). ADDITIONALLY, THIS IMAGERY SUPPORTS A SIGNIFICANT WEAKENING
TREND OVER THE PAST 12 HOURS. THIS WEAKENING TREND IS LARGELY THE
RESULT OF INCREASED UPPER-LEVEL CONVERGENCE CAUSED BY A RAPIDLY
STRENGTHENING TUTT EXTENDING SOUTHWESTWARD FROM A TUTT CELL OVER
SHIKOKU, OVER THE WESTERN PERIPHERY OF THE SYSTEM AND INTO ANOTHER
TUTT CELL LOCATED SOUTH OF TAIWAN. CONSEQUENTLY, DVORAK DATA T
INTENSITY ESTIMATES HAVE DECREASED TO 77 KNOTS (T4.5) WITH A CI OF
102 KNOTS (T5.5) FROM ALL AGENCIES. THE INITIAL INTENSITY IS
ASSESSED AT 100 KNOTS BASED ON THE CI VALUES. THERE IS GOOD
CONFIDENCE IN THE CURRENT POSITION BASED ON MICROWAVE EYE FEATURES
EVIDENT IN THE AFOREMENTIONED SSMIS AND METOP-A IMAGES. TY 15W IS
TRACKING NORTH-NORTHWESTWARD TO NORTHWESTWARD ALONG THE SOUTHERN
PERIPHERY OF THE STR.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TY 15W IS FORECAST TO TRACK SLOWLY NORTHWESTWARD WITHIN A WEAK
STEERING ENVIRONMENT THROUGH TAU 24. THE 21/00Z 500MB ANALYSIS SHOWS
A DEEP MIDLATITUDE SHORTWAVE TROUGH TRACKING NORTHEASTWARD ACROSS
EASTERN CHINA, OVER THE SUBTROPICAL RIDGE (STR), WHICH IS
STRENGTHENING OVER AND SOUTH OF JAPAN. THIS TROUGH WILL TRACK NORTH
OF THE SYSTEM BY TAU 24 AND WILL THEN TRACK INTO THE SEA OF JAPAN
ALLOWING THE STR TO BUILD WESTWARD INTO EASTERN CHINA AS ALL THE
DYNAMIC MODELS INDICATE. BASED ON THE TIGHT GROUPING OF DYNAMIC
MODELS AND THEIR CONSISTENT, ACCURATE PERFORMANCE AS VERIFIED IN THE
ANALYSIS UPPER-AIR DATA, THERE IS HIGH CONFIDENCE IN THE JTWC
FORECAST TRACK ALTHOUGH THERE IS POSSIBILITY THE SYSTEM MAY TURN
MORE SHARPLY WESTWARD ACROSS SOUTHERN TAIWAN AS INDICATED BY THE
JGSM MODEL. DUE TO THE MARGINAL ENVIRONMENT, TY 15W IS FORECAST TO
WEAKEN GRADUALLY UNTIL FIRST LANDFALL OVER TAIWAN THEN WEAKEN
SIGNIFICANTLY AS IT CROSSES TAIWAN AND TRACKS OVER THE TAIWAN STRAIT.
   C. AFTER TAU 72, TY 15W IS FORECAST TO MAKE SECOND LANDFALL AND
TRACK SLOWLY WEST-SOUTHWESTWARD WHILE FURTHER WEAKENING. THE BULK OF
THE DYNAMIC MODEL GUIDANCE IS IN GOOD AGREEMENT WITH THE EXCEPTION
OF THE GFS MODEL; HOWEVER, DYNAMIC MODEL GUIDANCE IS BEGINNING TO
INDICATE A POSSIBLE STALL NORTHEAST OF HONG KONG OR PERHAPS OFF THE
COAST OF CHINA. THE GFS MODEL HAS CONTINUED TO EXCESSIVELY WEAKEN
THE SYSTEM RESULTING IN A STALL SOUTHWEST OF TAIWAN WITH BINARY
INTERACTION WITH TY 16W AFTER TAU 72. THE TWO SYSTEM CENTERS ARE
CURRENTLY SEPARATED BY 850 NM AND, BASED ON THE 21/12Z FORECASTS,
WILL APPROACH TO ABOUT 650-530 NM FROM TAU 72 TO TAU 120. UNLESS
THERE IS A SIGNIFICANT TRACK CHANGE I.E., A LONGER QUASI-STATIONARY
PERIOD EAST OF TAIWAN, BINARY INTERACTION IS UNLIKELY. OVERALL,
THERE IS LOW CONFIDENCE IN THE EXTENDED FORECAST TRACK.//
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