000 WTPZ45 KNHC 140859 TCDEP5 TROPICAL STORM ISELLE DISCUSSION NUMBER 4 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL EP102008 200 AM PDT THU AUG 14 2008 THE CONVECTIVE PATTERN HAS BECOME SLIGHTLY MORE SYMMETRIC...BUT MICROWAVE IMAGERY INDICATES THE LOW-LEVEL CENTER IS STILL BENEATH THE NORTHEASTERN PORTION OF THE DEEP CONVECTION. THE DVORAK INTENSITY ESTIMATES ARE UNCHANGED FROM THE PREVIOUS CYCLE AT 35-45 KT...BUT MANUAL ANALYSIS OF THE AMBIGUITIES FROM A QUIKSCAT PASS AT AROUND 01Z SUGGESTS MAXIMUM WINDS OF ABOUT 40 KT...SO THE ADVISORY INTENSITY IS NUDGED UPWARD TO 40 KT. THE CYCLONE IS STRUGGLING IN THE FACE OF EASTERLY SHEAR...AND DYNAMICAL MODELS FORECAST THE SHEAR TO PERSIST THROUGHOUT THE FIVE-DAY FORECAST PERIOD...WITH PERHAPS EVEN A SLIGHT INCREASE IN SHEAR IN THE SHORT TERM. AS A RESULT...DESPITE WARM WATERS ALONG THE FORECAST TRACK FOR THE NEXT TWO TO THREE DAYS...NONE OF THE OBJECTIVE INTENSITY MODELS FORECAST MUCH STRENGTHENING IF AT ALL. THE OFFICIAL FORECAST CLOSELY FOLLOWS THE SHIPS AND LGEM GUIDANCE IN CALLING FOR VERY LITTLE CHANGE IN STRENGTH THROUGH ABOUT 72 HOURS...FOLLOWED BY WEAKENING OVER COOLER WATERS. GIVEN THE PERSISTENT SHEAR...AND THE LESS-THAN-IMPRESSIVE DEPICTIONS OF THE CYCLONE IN THE DYNAMICAL MODEL FIELDS...IT REMAINS POSSIBLE THAT ISELLE WILL NOT LAST THE ENTIRE FIVE-DAY PERIOD. THE 01Z QUIKSCAT PASS...AND PASSIVE MICROWAVE IMAGERY FROM SSMIS AND TRMM AT AROUND THAT SAME TIME...INDICATE THAT THE CENTER OF ISELLE IS MOVING A TINY BIT SOUTH OF THE PREVIOUS TRACK...AND THE INITIAL MOTION ESTIMATE IS ABOUT 290/9. ISELLE IS BEING STEERED BY A MID- TO UPPER-LEVEL RIDGE OVER NORTHERN MEXICO...AND ALL OF THE DYNAMICAL MODELS FORECAST THE CYCLONE TO CONTINUE MOVING GENERALLY WEST-NORTHWESTWARD OR NORTHWESTWARD FOR THE NEXT TWO TO THREE DAYS ALONG THE SOUTHWESTERN PERIPHERY OF THAT RIDGE. THE SPREAD IN THE TRACK GUIDANCE IS SIGNIFICANT THROUGH 72 HOURS AND BECOMES PARTICULARLY LARGE BEYOND THAT TIME...SEEMINGLY IN PART BECAUSE THE MODELS DO NOT AGREE ON THE STRENGTH AND DEPTH OF ISELLE. SOME MODELS INCLUDING THE GFS AND UKMET FORECAST THE CYCLONE TO HAVE DISSIPATED BY DAY 3. PRESUMING A WEAKENED ISELLE DOES LAST INTO THE 3-5 DAY TIME FRAME...IT WILL PROBABLY NOT BE MOVING VERY FAST SINCE IT WILL LIKELY BE TOO FAR EAST TO GET CAUGHT UP IN THE LOW-LEVEL EASTERLY TRADES. THE NEW OFFICIAL TRACK FORECAST IS VERY SIMILAR TO THE PREVIOUS ONE...AND IS CLOSE TO THE CONSENSUS THROUGH ABOUT 48 HOURS. FORECAST POSITIONS AND MAX WINDS INITIAL 14/0900Z 17.6N 108.6W 40 KT 12HR VT 14/1800Z 18.3N 109.8W 45 KT 24HR VT 15/0600Z 19.1N 110.9W 45 KT 36HR VT 15/1800Z 19.9N 112.0W 45 KT 48HR VT 16/0600Z 20.5N 113.1W 45 KT 72HR VT 17/0600Z 21.0N 114.5W 40 KT 96HR VT 18/0600Z 21.5N 116.0W 30 KT 120HR VT 19/0600Z 21.5N 117.5W 25 KT $$ FORECASTER KNABB