Tropical Storm KALMAEGI Advisory Fri Sep 12

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 15W (FIFTEEN)
WARNING NR 06//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL DEPRESSION 15W (FIFTEEN), LOCATED APPROXIMATELY 417 NM
NORTH-NORTHWEST OF KOROR, HAS TRACKED WESTWARD AT 17 KNOTS OVER THE
PAST SIX HOURS. ANIMATED MULTISPECTRAL SATELLITE IMAGERY SHOWS THE
DEEP CONVECTIVE BANDING ALONG THE PERIPHERY OF THE LOW LEVEL
CIRCULATION CENTER (LLCC) HAS BEEN STEADILY IMPROVING OVER THE PAST
FEW HOURS. A RECENT 112057Z SSMIS MICROWAVE PASS SHOWS THE POSSIBLE
EARLY STAGES OF AN EYE WALL DEVELOPING. THE CURRENT INTENSITY
REMAINS AT 30 KNOTS AS DVORAK ESTIMATES FROM PGTW AND KNES HAVE BEEN
PERSISTENT OVER THE PAST SIX HOURS. UPPER LEVEL ANALYSIS INDICATES
TD 15W REMAINS SOUTH OF A SUBTROPICAL RIDGE (STR) AND IS IN A WEAKLY
DIVERGENT REGION, PROVIDING MARGINAL SOUTHWESTERLY OUTFLOW. VERTICAL
WIND SHEAR (VWS) HAS BEEN SLOWLY WEAKENING, CURRENTLY AT LOW TO
MODERATE LEVELS (10 TO 15 KNOTS), AS THE TRACK SPEED FOR TD 15W
REMAINS BETWEEN 18 TO 23 KNOTS AND IS IN PHASE WITH THE UPPER LEVEL
FLOW. TD 15W CONTINUES TRACKING WESTWARD UNDER THE STEERING
INFLUENCE OF THE DEEP LAYERED REFLECTION OF THE STR LOCATED TO THE
NORTH OF THE SYSTEM.
3. FORECAST REASONING.
   A. NO CHANGE TO THE FORECAST PHILOSOPHY SINCE THE PREVIOUS
PROGNOSTIC REASONING MESSAGE.
   B. TD 15W IS FORECAST TO TRACK WESTWARD TO WEST-NORTHWESTWARD
UNDER THE STEERING INFLUENCE OF THE STR, BUT WILL UNDERGO A PERIOD
OF DECREASED SPEEDS AS A MIGRATORY UPPER-LEVEL TROUGH PASSES TO THE
NORTH OF THE RIDGE BETWEEN TAU 36 AND 48. FAVORABLE ENVIRONMENTAL
CONDITIONS ARE EXPECTED TO PERSIST WITH HIGH SEA SURFACE
TEMPERATURES, IMPROVING UPPER LEVEL OUTFLOW, AND DECREASING VWS
ALLOWING TD 15W TO GRADUALLY INTENSIFY, REACHING TO TYPHOON STRENGTH
BY TAU 48. TD 15W IS FORECAST TO MAKE LANDFALL IN EASTERN LUZON
AROUND TAU 60, LEADING TO A TEMPORARY WEAKENING TREND IN THE LATER
TAUS.
   C. TD 15W IS EXPECTED TO MAINTAIN A WEST-NORTHWESTWARD TRACK
THROUGH THE REMAINDER OF THE FORECAST PERIOD AS THE STR REBUILDS TO
THE NORTH. BY TAU 96 THE FAVORABLE ENVIRONMENT WILL CAUSE TD 15W TO
INTENSIFY AS IT APPROACHES THE COAST OF CHINA, MAKING LANDFALL TO
THE NORTH OF HAINAN AROUND TAU 120. AVAILABLE MODEL GUIDANCE REMAINS
IN GOOD AGREEMENT. DUE TO THE UNCERTAINTY OF INTENSITY AND TRACK
SPEEDS IN THE EXTENDED TAUS, THERE IS LOW CONFIDENCE IN THE JTWC
FORECAST.//
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