WDPN31 PGTW 121500 MSGID/GENADMIN/NAVPACMETOCCEN PEARL//JTWC// SUBJ/PROGNOSTIC REASONING FOR TROPICAL DEPRESSION 31W WARNING NR 04// RMKS/ 1. FOR METEOROLOGISTS. 2. PROGNOSTIC REASONING FOR 121200Z6 TO 151200Z9 DEC 01. A. TROPICAL DEPRESSION (TD) 31W, LOCATED APPROXIMATELY 120 NM SOUTHEAST OF POHNPEI, HAS BEEN TRACKING NORTH-NORTHEASTWARD AT 07 KNOTS DURING THE PAST SIX HOURS. THE WARNING POSITION IS BASED ON 121130Z8 INFRARED SATELLITE IMAGERY AND NEARBY SURFACE DATA. THE WARNING INTENSITY IS BASED ON SATELLITE CURRENT INTENSITY ESTIMATES OF 25 AND 30 KNOTS. ANIMATED SATELLITE IMAGERY INDICATES THAT OVERALL ORGANIZATION OF THE SYSTEM HAS INCREASED OVER THE PAST SIX HOURS. A 121038Z5 SPECIAL SENSOR MICROWAVE IMAGE (SSM/I) PASS REVEALS A WELL ORGANIZED LOW LEVEL CIRCULATION CENTER (LLCC) WITH CONVECTION SHEARED SLIGHTLY WEST OF THE LLCC. B. TD 31W IS A WEAK SYSTEM EMBEDDED IN THE MONSOON TROUGH, THEREFORE IT HAS BEEN TRACKING NORTH-NORTHEASTWARD UNDER THE STEERING INFLUENCE OF THE LOWER LEVEL SOUTHWESTERLIES ASSOCIATED WITH THE TROUGH. AS TD 31W INTENSIFIES, THE MID LEVEL EASTERLIES ASSOCIATED WITH A BUILDING MID-LEVEL HIGH HEIGHT CENTER NORTH OF THE REGION SHOULD BECOME THE DOMINANT STEERING MECHANISM. THUS, A WESTWARD TURN IS EXPECTED MIDWAY THROUGH THE FORECAST PERIOD. THE STATISTICAL AIDS CONSISTING OF CLIPR AND CSUM AGREE WITH THIS SCENARIO. NCEP AVIATION (AVN) AND NOGAPS (NGPS) MODELS TRACK THE DEPRESSION EASTWARD FOR ABOUT 24 TO 36 HOURS, AND THEN GRADUALLY TURN IT BACK TO THE WEST THEREAFTER. GEOPHYSICAL FLUID DYNAMICS NAVY (GFDN) MODEL TRACKS THE SYSTEM EASTWARD THROUGH THE ENTIRE PERIOD. JTWC ATTRIBUTES THE EASTWARD MOTION DEPICTED IN THE MODELS TO THEIR POOR RESOLUTION OF THE SYSTEM FROM THE TROUGH IN THAT IT IS EMBEDDED. THEREFORE, THE FORECAST TRACK IS BASED ON A BLEND OF CLIMATOLOGY, PERSISTENCE, CLIPR, CSUM, AND THE MEDIUM LAYER BETA ADVECTION MODELS. C. TD 31W IS EXPECTED INTENSIFY THROUGH THE PERIOD AS FAVORABLE VERTICAL SHEAR AND STRONG DIFFLUENCE CHARACTERIZE THE UPPER-LEVEL FLOW PATTERN. D. CURRENTLY, TD 31W DOES NOT MEET WIND RADII CRITERIA. FORECAST WIND RADII ARE BASED ON CLIMATOLOGY FOR AN AVERAGE SIZED SYSTEM. FORECAST TEAM: VILPORS/WAUGAMAN/NUTONGLA// NNNN