WDPN31 PGTW 160300
MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//
SUBJ/PROGNOSTIC REASONING FOR TROPICAL STORM 15W WARNING NR 31//
RMKS/
1. FOR METEOROLOGISTS.
2. 12 HOUR SUMMARY AND ANALYSIS.
   A. TROPICAL STORM (TS) 15W HAS SLOWLY WEAKENED OVER THE PAST
12 HOURS UNDER THE COMPETING INFLUENCES OF MODERATE WESTERLY
VERTICAL WIND SHEAR AND FAVORABLE POLEWARD OUTFLOW. VERTICAL WIND
SHEAR HAS RELAXED SLIGHTLY AS RIDGING BUILDS IN THE UPPER LEVELS,
AND AN INFUSION OF DRY AIR TO THE WEST OF THE SYSTEM IS NOTED IN
RECENT ANIMATED WATER VAPOR SATELLITE IMAGERY.
   B. THE CURRENT POSITION IS BASED ON A SERIES OF RECENT MICROWAVE
IMAGES AND SATELLITE FIXES FROM PGTW AND RJTD. THE CURRENT INTEN-
SITY IS SET CONSERVATIVELY ABOVE A 3.0 DVORAK T-NUMBER OBTAINED
BY PGTW AT 152330Z BASED ON THE STRONG LOW LEVEL STRUCTURE
EXHIBITED IN A 152134Z CORIOLIS PASS. TS 15W IS TRACKING ALONG THE
NORTHWESTERN PERIPHERY OF AN EXTENSIVE SUBTROPICAL STEERING RIDGE
TO THE SOUTHEAST. THE LOW LEVEL CIRCULATION CENTER IS NEARLY FULLY-
EXPOSED, WITH DEEP CONVECTION CONFINED TO A CONVERGENT BAND ON THE
SOUTHERN PERIPHERY. 
3. FORECAST REASONING
   A. THE TAU 0 TO TAU 36 TRACK FORECAST HAS BEEN ADJUSTED SLIGHTLY
TO THE EAST AND SOUTH SINCE THE PREVIOUS ISSUANCE OF THE PROGNOSTIC
REASONING MESSAGE. IN ADDITION, EXTRATROPICAL TRANSITION IS NOW
EXPECTED TO BEGIN PRIOR TO TAU 72 AND BE COMPLETE BY TAU 96. THE
OVERALL FORECAST PHILOSOPHY REMAINS UNCHANGED.
   B. TS 15W WILL CONTINUE TO TRACK GENERALLY EASTWARD TO EAST-
NORTHEASTWARD OVER THE NEXT 24 HOURS ALONG THE NORTHERN PERIPHERY
OF A SUBTROPICAL RIDGE EXTENSION TO THE SOUTH AND EAST. THIS RIDGE
IS EXPECTED TO AMPLIFY POLEWARD AHEAD OF A DEVELOPING MID-LATITUDE
TROUGH BETWEEN TAU 36 AND TAU 72, INDUCING A NORTHWARD TRACK TOWARD
THE BAROCLINIC ZONE. THE TROPICAL STORM WILL ENCOUNTER INCREASING
WESTERLY VERTICAL WIND SHEAR AND A DRIER, COOLER MID-LATITUDE AIR-
MASS AHEAD OF THE APPROACHING TROUGH AFTER TAU 48. THESE FACTORS
WILL INDUCE THE FIRST STAGE OF EXTRATROPICAL TRANSITION BY TAU 72.
TS 15W IS EXPECTED TO NEARLY MAINTAIN INTENSITY OVER THE NEXT 48
HOURS AS STRONG UPPER LEVEL DIFFLUENCE AND PASSAGE OVER WATERS WITH
HIGH OCEAN HEAT CONTENT VIES WITH MODERATE VERTICAL WIND SHEAR AND
POTENTIAL DRY AIR ENTRAINMENT TO SET THE INTENSITY. WEAKENING
SHOULD BEGIN AFTER TAU 48 AS THE SYSTEM ACCELERATES INTO THE
BAROCLINIC ZONE.
   C. TS 15W WILL COMPLETE EXTRATROPICAL TRANSITION IN THE TAU 96
TO TAU 120 EXTENDED FORECAST PERIOD. NUMERICAL MODEL GUIDANCE
SUGGESTS THAT TS 15W WILL WEAKEN ONLY SLOWLY AS IT INTERACTS
FAVORABLY WITH THE MID-LATITUDE FLOW PATTERN DURING EXTRATROPICAL
TRANSITION. THEREFORE, THE CURRENT FORECAST ANTICIPATES A 40 KNOT
POST-TRANSITION CIRCULATION.
FORECAST TEAM: ALPHA//
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