000 WTPZ41 KNHC 180232 TCDEP1 TROPICAL STORM MARTY DISCUSSION NUMBER 8 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL EP162009 800 PM PDT THU SEP 17 2009 ANALYSES FROM CIMSS AT THE UNIVERSITY OF WISCONSIN INDICATE THAT MARTY IS EXPERIENCING 10-15 KT OF SOUTHERLY VERTICAL WIND SHEAR THIS EVENING. THIS IS CAUSING THE CYCLONE TO HAVE A SOMEWHAT RAGGED APPEARANCE EVEN THOUGH THE CLOUD TOPS OF THE CENTRAL CONVECTION ARE COLDER THAN -80C. THE INITIAL INTENSITY REMAINS 40 KT FOR THIS ADVISORY...WHICH IS IN AGREEMENT WITH AN AMSU INTENSITY ESTIMATE AT 17/2031 UTC AND SLIGHTLY ABOVE THE SATELLITE INTENSITY ESTIMATES FROM TAFB AND SAB. THE FORECAST TRACK TAKES MARTY OVER GRADUALLY DECREASING SEA SURFACE TEMPERATURES AND INTO AN AREA OF STRONG SOUTHWESTERLY SHEAR. IN ADDITION...TOTAL PRECIPITABLE WATER DATA SHOWS VERY DRY AIR NOT FAR TO THE WEST OF THE STORM. THIS COMBINATION SHOULD CAUSE MARTY TO WEAKEN DURING THE PERIOD...AND THE INTENSITY FORECAST CALLS FOR IT TO DECAY TO A REMNANT LOW IN ABOUT 48 HR IN AGREEMENT WITH THE PREVIOUS FORECAST. MOST OF THE GLOBAL MODELS SHOW THE REMNANT LOW BREAKING OPEN INTO A TROUGH BEFORE 96 HR...SO THE NEW 96 HR FORECAST CALLS FOR DISSIPATION. MARTY IS CURRENTLY IN AN AREA OF LIGHT STEERING CURRENTS BETWEEN A MID/UPPER-LEVEL RIDGE OVER MEXICO AND A SECOND MID/UPPER-LEVEL RIDGE WEST OF THE CYCLONE. THE DYNAMICAL MODELS FORECAST THE RIDGE OVER MEXICO TO STEER MARTY SLOWLY TOWARD THE NORTHWEST FOR THE NEXT 24-36 HR. AFTER THAT TIME...MARTY SHOULD GRADUALLY TURN WESTWARD AS THE SYSTEM WEAKENS AND A LOW-LEVEL RIDGE TO THE NORTHWEST BECOMES THE MAIN INFLUENCE ON THE MOTION. THE NEW FORECAST TRACK IS AN UPDATE OF THE PREVIOUS TRACK. FORECAST POSITIONS AND MAX WINDS INITIAL 18/0300Z 20.0N 113.6W 40 KT 12HR VT 18/1200Z 20.5N 114.2W 35 KT 24HR VT 19/0000Z 21.4N 115.2W 35 KT 36HR VT 19/1200Z 22.2N 116.2W 30 KT 48HR VT 20/0000Z 22.6N 117.4W 25 KT...REMNANT LOW 72HR VT 21/0000Z 22.5N 119.5W 20 KT...REMNANT LOW 96HR VT 22/0000Z...DISSIPATED $$ FORECASTER BEVEN