000 WTPZ43 KNHC 090230 TCDEP3 TROPICAL STORM LOWELL DISCUSSION NUMBER 9 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL EP132008 800 PM PDT MON SEP 08 2008 THE OBLIGATORY EVENING CONVECTIVE BURST HAS ONCE AGAIN DEVELOPED OVER THE LOW-LEVEL CENTER LIKE IT HAS THIS TIME THE PAST 2 DAYS... EXCEPT THIS EVENING A CURVED BAND HAS FORMED IN THE WESTERN SEMICIRCLE. THIS CURVED BAND PATTERN HAS YIELDED AN OFFICIAL DVORAK INTENSITY ESTIMATE OF T3.0/45 KT FROM BOTH TAFB AND SAB...WHEREAS A SHEAR PATTERN YIELDS A DATA T-NUMBER OF T3.5/55 KT. THE INITIAL INTENSITY OF 50 KT IS A BLEND OF THESE TWO T-NUMBER ESTIMATES. THE INITIAL MOTION ESTIMATE IS 310/07. IT APPEARS THAT THE FORWARD SPEED OF LOWELL HAS DECREASED DURING THE PAST 6 HOURS...WHICH IS IN LINE WITH WHAT THE GLOBAL AND REGIONAL MODELS HAVE BEEN ADVERTISING THE PAST 2 DAYS. A MID- TO UPPER-LEVEL TROUGH CONTINUES TO DROP SOUTHWARD ALONG THE U.S. CALIFORNIA COAST...AND THIS FEATURE IS EXPECTED TO GRADUALLY ERODE THE SUBTROPICAL RIDGE TO THE NORTH OF LOWELL BY 24-36 HOURS. A WEAKNESS IS EXPECTED TO DEVELOP IN THE RIDGE AXIS ALLOWING LOWELL TO MOVE NORTHWARD THROUGH THE RIDGE AND EVENTUALLY RECURVE NORTHEASTWARD ACROSS BAJA CALIFORNIA AND NORTHWESTERN MEXICO. NHC MODEL GUIDANCE IS IN REASONABLE AGREEMENT ON THIS TRACK SCENARIO EVOLVING THROUGH 72 HOURS...BUT THEN DIVERGE IN BOTH TRACK AND FORWARD SPEED AFTER THAT. IF LOWELL IS AS WEAK AS FORECAST BY THE TIME IT REACHES BAJA CALIFORNIA...THE LOW-LEVEL CIRCULATION WILL LIKELY GET STRIPPED AWAY BY THE MOUNTAINOUS TERRAIN AND BE LEFT BEHIND OVER THE PACIFIC. AS A MINIMUM... HOWEVER...THE SIERRA MADRE MOUNTAINS OVER NORTHWESTERN MEXICO WILL KEEP THE LOW-LEVEL CENTER FROM RACING NORTHEASTWARD TO SOUTHERN MINNESOTA LIKE THE HWRF MODEL IS PREDICTING. THE OFFICIAL FORECAST IS JUST AN UPDATE OF THE PREVIOUS TRACK...AND IS CLOSE TO THE NHC MODEL CONSENSUS. SIMILAR TO THE PAST TWO EVENINGS...A LARGE CONVECTIVE BURST WITH CLOUD TOPS AS COLD AS -85C HAS DEVELOPED OVER AND SOUTHWEST OF THE LOW-LEVEL CENTER. LAST NIGHT'S INTENSIFICATION PROCESS WAS DISRUPTED BY ENHANCED VERTICAL SHEAR CREATED BY EASTERLY OUTFLOW EMANATING FROM A LARGE MESOSCALE CONVECTIVE SYSTEM THAT DEVELOPED ALONG THE SIERRA MADRE MOUNTAINS TO THE EAST OF LOWELL. THE FEATURE THAT TRIGGERED THAT MCS HAS SINCE MOVED NORTHWARD OVER THE GULF OF CALIFORNIA AND SHOULD NOT BE A FACTOR TONIGHT. HOWEVER...DESPITE WEAKENING SHEAR CONDITIONS FORECAST BY ALL THE MODELS...ALONG WITH VERY MOIST MID-TROPOSPHERIC CONDITIONS...THE STATISTICAL SHIPS INTENSITY MODEL AND THE VARIOUS DYNAMICAL MODELS ARE ALL PREDICTING LOWELL WILL WEAKEN THROUGHOUT THE FORECAST PERIOD. THE OFFICIAL INTENSITY FORECAST FOLLOWS THE PREVIOUS FORECAST TREND...BUT REMAINS ABOVE THE SHIPS INTENSITY FORECAST...WHICH DISSIPATES LOWELL IN ABOUT 72 HOURS. FORECAST POSITIONS AND MAX WINDS INITIAL 09/0300Z 20.2N 112.5W 50 KT 12HR VT 09/1200Z 20.9N 113.1W 45 KT 24HR VT 10/0000Z 21.7N 113.4W 45 KT 36HR VT 10/1200Z 22.7N 113.2W 40 KT 48HR VT 11/0000Z 23.7N 112.7W 35 KT 72HR VT 12/0000Z 24.9N 111.8W 35 KT...INLAND OVER SRN BAJA 96HR VT 13/0000Z 26.0N 111.0W 30 KT...OVER GULF OF CALIF 120HR VT 14/0000Z 27.3N 109.9W 30 KT...INLAND OVER MEXICO $$ FORECASTER STEWART