Tropical Storm SIX Advisory Thu Aug 13

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 06W (SIX) WARNING
NR 012//
RMKS//
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION (TD) 06W (SIX), LOCATED APPROXIMATELY 411 NM
EAST OF KADENA AB, OKINAWA, HAS TRACKED WEST-SOUTHWESTWARD AT 11
KNOTS OVER THE PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE
IMAGERY DEPICTS EXPOSED LOW-LEVEL CLOUD LINES WRAPPING INTO A BROAD,
RAGGED CENTER. A 120646Z SSMIS 91GHZ MICROWAVE IMAGE SHOWS ISOLATED
CONVECTIVE BURSTS OVER THE WEST AND SOUTHWEST QUADRANTS BUT OVERALL
WEAK CONVECTIVE STRUCTURE. RECENT ASCAT IMAGERY AND THE SSMIS WIND
PRODUCT INDICATE A HIGHLY ASYMMETRIC WIND FIELD WITH A PATCH OF 20-
25 KNOT WINDS OVER THE NORTHWEST QUADRANT AND A BROADENING CENTER
REFLECTING A GRADUAL WEAKENING TREND. HOWEVER, THE INITIAL INTENSITY
IS HELD AT 25 KNOTS BASED ON THE ASCAT DATA AND DVORAK CURRENT
INTENSITY ESTIMATES OF 1.0-1.5 (25 KNOTS). THE SYSTEM REMAINS
EMBEDDED WITHIN AN EXPANSIVE REGION OF ENHANCED EASTERLIES BETWEEN
THE UPPER-LEVEL RIDGE AND A TUTT CELL TO THE SOUTHEAST WITH LOW TO
MODERATE (15 TO 20 KNOTS) VERTICAL WIND SHEAR, DRY AIR ENTRAINMENT
AND WEAK DIFFLUENCE. TD 06W IS TRACKING WEST-SOUTHWESTWARD UNDER THE
STEERING INFLUENCE OF A LOW-LEVEL SUBTROPICAL RIDGE (STR) POSITIONED
TO THE NORTH AND NORTHWEST.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. DUE TO THE WSW-ENE ORIENTATION OF THE LOW-LEVEL STR, TD 06W IS
FORECAST TO CONTINUE TRACKING WEST-SOUTHWESTWARD THROUGH TAU 24. AS
IT APPROACHES THE RYUKYU ISLANDS, THE SYSTEM SHOULD TURN WESTWARD TO
WEST-NORTHWESTWARD AS IT TRACKS ALONG THE SOUTHWEST PERIPHERY OF THE
STR. ENVIRONMENTAL CONDITIONS ARE EXPECTED TO REMAIN MARGINAL WITH
LOW TO MODERATE VWS AND DRY AIR ENTRAINMENT (700-500MB RH VALUES OF
50-55%), THEREFORE, THE SYSTEM SHOULD REMAIN SHALLOW AND WEAK WITH
SOME POTENTIAL FOR DISSIPATION DURING THE FORECAST PERIOD. NUMERICAL
MODEL GUIDANCE SUPPORTS THE JTWC FORECAST TRACK AND REMAINS IN FAIR
AGREEMENT WITH MINOR VARIATIONS IN TRACK ORIENTATION THROUGH TAU 48.
MODEL SPREAD AT TAU 36 TO 48 RANGES FROM 50 TO 110NM, THEREFORE, THE
JTWC FORECAST TRACK IS POSITIONED CLOSE TO THE MULTI-MODEL CONSENSUS
WITH MODERATE CONFIDENCE.//
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