Tropical Storm SANBA Advisory Tue Sep 11

MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 17W (SEVENTEEN)
WARNING NR 02//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
   A. TROPICAL DEPRESSION (TD) 17W (SANBA), LOCATED APPROXIMATELY
120 NM NORTH-NORTHWEST OF PALAU, HAS TRACKED NORTH-NORTHWESTWARD AT
08 KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE
IMAGERY AND A 101612Z AMSU-B MICROWAVE IMAGE SHOW A LOW LEVEL
CIRCULATION CENTER EMBEDDED WITHIN AN AREA OF PERSISTENT, DEEP
CONVECTION. CONVECTIVE BANDING HAS CONSOLIDATED ALONG WITH INCREASED
SYMMETRY. THE CURRENT INTENSITY IS BASED ON DVORAK ESTIMATES RANGING
FROM 30 TO 35 KNOTS FROM RJTD AND PGTW. UPPER LEVEL ANALYSIS
INDICATES 17W IS DIRECTLY BENEATH A POINT SOURCE ANTICYCLONE IN A
REGION OF WEAK (5-10 KNOT) VERTICAL WIND SHEAR. RECENT WATER VAPOR
SATELLITE IMAGERY DEPICTS GOOD RADIAL OUTFLOW WITH AN ENHANCED
OUTFLOW INTO A TROPICAL UPPER TROPOSPHERIC TROUGH (TUTT). THIS
CYCLONE IS BEING STEERED BY THE SOUTHWESTERN EXTENT OF A SUBTROPICAL
RIDGE (STR) TO THE EAST-NORTHEAST.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TD 17W IS EXPECTED TO TRACK MORE POLEWARD ALONG THE SOUTH-
SOUTHWESTERN PERIPHERY OF THE STR, COMMENCING OVER THE NEXT SIX
HOURS AS THE STR BECOMES MORE POLEWARD-ORIENTED. THIS STR SHOULD
REMAIN THE PRIMARY STEERING INFLUENCE THROUGHOUT THE FORECAST
PERIOD. TD 17W IS EXPECTED TO GRADUALLY INTENSIFY DUE TO FAVORABLE
SEA SURFACE TEMPERATURES (26-30 DEGREES CELSIUS) AND CONTINUED
FAVORABLE UPPER LEVEL ENVIRONMENT.
   C. IN THE EXTENDED FORECAST, TD SANBA IS EXPECTED TO CONTINUE
ALONG ITS AFOREMENTIONED TRACK. DUE TO FAVORABLE ALONG-TRACK
ENVIRONMENTAL CONDITIONS, 17W WILL CONTINUE TO INTENSIFY THROUGH TAU
120 IN THE VICINITY OF OKINAWA. SOME UNCERTAINTY REMAINS WITH 17W
INTERACTION WITH THE NARROW EAST-WEST ORIENTED TUTT EXTENDING FROM
NEAR TAIWAN TO NORTHEAST OF GUAM. IF A TUTT CELL BECOMES CUT OFF TO
THE WEST OF 17W TRACK, THE POTENTIAL EXISTS FOR INCREASED OUTFLOW
AND A GREATER INCREASE IN INTENSITY FROM THE CURRENT FORECAST.
NUMERICAL MODEL GUIDANCE REMAINS SPREAD OUT DURING THE FIRST 72
HOURS, WITH JGSM FAVORING A MORE EASTWARD TRACK AND WBAR, ECMWF AND
GFS A MORE NORTH-NORTHWESTWARD TRACK. THIS FORECAST IS SIMILAR TO
THE PREVIOUS FORECAST AND FAVORS THE MORE NORTH-NORTHWESTWARD TRACK
WITH LOW CONFIDENCE.//
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