Tropical Storm RAMMASUN Advisory dom, 13-07

MSGID/GENADMIN/JOINT TYPHOON WRNCEN PEARL HARBOR HI//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 09W (RAMMASUN) WARNING
NR 11//
RMKS/
1. FOR METEOROLOGISTS.
2. 6 HOUR SUMMARY AND ANALYSIS.
   TROPICAL STORM 09W (RAMMASUN), LOCATED APPROXIMATELY 247 NM NORTH
OF YAP, HAS TRACKED WESTWARD AT 20 KNOTS OVER THE PAST SIX HOURS.
ANIMATED MULTISPECTRAL SATELLITE IMAGERY SHOWS TIGHTLY CURVED BANDS
FLOWING INTO THE LOW LEVEL CIRCULATION CENTER (LLCC) BUT IN PART DUE
TO THE DEVELOPING CENTRAL CONVECTION, THE EXACT POSITIONING OF THE
LLCC IS DIFFICULT. TRACK SPEEDS HAVE SEEN A MARKED INCREASE FROM THE
PREVIOUS 13 KNOTS TO 20 KNOTS IN THE LAST SIX HOURS. THE DEEP-
LAYERED SUB-TROPICAL RIDGE (STR) TO THE NORTH OF TS 09W REMAINS AN
ANCHORED FEATURE, BUT AS THE LLCC CONTINUES TO COALESCE, THERE
APPEARS TO HAVE BEEN A JUMP IN THE POSITIONING OF THE CIRCULATION.
CURRENT INTENSITY HAS BEEN MAINTAINED AT 35 KNOTS AS THE DEEP
CENTRAL CONVECTION REMAINS A FRESH FEATURE. UPPER-LEVEL ANALYSIS
REVEALS A SLIGHT IMPROVEMENT IN THE SOUTHWESTERN OUTFLOW CHANNEL,
BUT CONTINUES TO STRUGGLE UNDER THE INFLUENCE OF THE LOW TO MODERATE
(10 TO 15 KNOTS) VERTICAL WIND SHEAR (VWS).
3. FORECAST REASONING.
   A. THERE IS NO CHANGE TO THE FORECAST TRACK PHILOSOPHY FROM THE
PREVIOUS PROGNOSTIC REASONING BULLETIN.
   B. TS 09W IS FORECAST TO TRACK WESTWARD FOR THE NEXT 72 HOURS
ALONG THE SOUTHERN PERIPHERY OF THE ENTRENCHED STR. ENVIRONMENTAL
CONDITIONS WILL REMAIN MARGINAL FOR THE NEXT 36, LEADING TO SLOW
INTENSIFICATION THROUGH TAU 36. NEAR TAU 48, VWS WILL BEGIN TO
DECREASE AND OUTFLOW WILL IMPROVE, ALLOWING FOR INCREASED
INTENSIFICATION AS THE SYSTEM APPROACHES THE SOUTHWESTERN EDGE OF
THE STR.
   C. AFTER TAU 48, A MID-LATITUDE TROUGH IS FORECAST TO DIG OVER
EASTERN CHINA, WHICH WILL MODIFY THE STR, LEADING TO A MORE WEST-
NORTHWEST TRACK TOWARDS LUZON. FAVORABLE SEA SURFACE TEMPERATURES IN
THE PHILIPPINE SEA ALONG WITH INCREASED OUTFLOW WILL LEAD TO A
MAXIMUM INTENSITY AS THE SYSTEM APPROACHES LAND, REACHING 95 KNOTS
BEFORE LANDFALL. AFTER LANDFALL, THE TERRAIN OF LUZON WILL SIGNIFI-
CANTLY WEAKEN THE SYSTEM AS IT MOVES INTO THE SOUTH CHINA SEA.
BY TAU 96 THE LAND INFLUENCE WILL DECREASE THE STORM STRENGTH TO
70 KNOTS, BUT AS THE SYSTEM REORIENTS, INTENSITIES WILL AGAIN START
TO INCREASE THROUGH TAU 120, CURRENTLY FORECAST TO BE 85 KNOTS.
MODEL GUIDANCE IS IN GOOD AGREEMENT; HOWEVER, DUE TO THE LOW
CONFIDENCE IN THE UPPER LEVEL ENVIRONMENT AND THE RECENT TRACK SPEED
CHANGES, THE CONFIDENCE IN THE FORECAST REMAINS LOW.//
NNNN
NNNN