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| #297961 (Received by flhurricane at: 10:34 AM 26.Sep.2009) TCDAT3 TROPICAL DEPRESSION EIGHT DISCUSSION NUMBER 4 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082009 1100 AM AST SAT SEP 26 2009 SATELLITE IMAGERY THIS MORNING INDICATES THAT TROPICAL DEPRESSION EIGHT HAS BECOME A LITTLE LESS ORGANIZED...WHICH IS LIKELY TO DUE TO 20 KT OF WEST-SOUTHWESTERLY VERTICAL WIND SHEAR INDICATED BY ANALYSES FROM CIMSS AT THE UNIVERSITY OF WISCONSIN. THE CIRCULATION HAS BECOME ELONGATED ALONG A NORTHEAST-SOUTHWEST AXIS...WITH TWO OR THREE EMBEDDED VORTICITY CENTERS NOTED. IN ADDITION...THE CONVECTIVE BANDING IS FARTHER FROM THE CENTER THAN 24 HR AGO. SATELLITE INTENSITY ESTIMATES FROM TAFB AND SAB REMAIN 30 AND 25 KT...SO THE INITIAL INTENSITY REMAINS 30 KT. THE INITIAL MOTION IS A SOMEWHAT UNCERTAIN 295/12. THE LARGE-SCALE MODELS FORECAST A LARGE-DEEP LAYER TROUGH AND ASSOCIATED SURFACE COLD FRONT CURRENTLY OVER THE WESTERN ATLANTIC TO MOVE EASTWARD DURING THE NEXT 48-96 HR. THIS SHOULD CAUSE THE SUBTROPICAL RIDGE OVER THE EASTERN ATLANTIC TO WEAKEN...AND SHOULD ALLOW THE TROPICAL DEPRESSION TO TURN NORTHWARD AND THEN RECURVE INTO THE WESTERLIES ON THE SOUTH SIDE OF THE TROUGH. MOST OF THE TRACK GUIDANCE CALLS FOR THIS SCENARIO...AND THE NEW TRACK FORECAST IS FOR RECURVATURE AFTER 72 HR. AN ALTERNATE SCENARIO IS SHOWN BY THE ECMWF...WHICH FORECASTS THE DEPRESSION TO WEAKEN WITH THE REMNANT TROUGH MOVING WESTWARD IN THE LOW-LEVEL FLOW. THE INTENSITY FORECAST IS A LITTLE PROBLEMATIC. THE DEPRESSION IS NOT ONLY ENCOUNTERING MODERATE SHEAR...BUT IS ALSO OVER A TONGUE OF COLDER SEA SURFACE TEMPERATURES AND HAS AN AREA OF DRY AIR JUST TO ITS WEST. WHILE THE SHEAR IS FORECAST TO CHANGE LITTLE DURING THE NEXT 48-60 HR...THE SSTS INCREASE ALONG THE FORECAST TRACK. THE MOST LIKELY SCENARIO IS THAT THE COMBINATION OF SHEAR AND DRY AIR WILL CAUSE THE DEPRESSION TO GRADUALLY DECAY OVER THE NEXT 48-72 HR. ON THE OTHER HAND...WITH THE INCREASING SSTS...THE CYCLONE COULD STRENGTHEN IF AND WHEN THE SHEAR DECREASES...AS SUGGESTED AFTER 72 HR BY THE SHIPS...LGEM...AND GFDL MODELS. THE NEW INTENSITY FORECAST GOES WITH THE WEAKENING SCENARIO...CALLING FOR THE DEPRESSION TO DECAY INTO A REMNANT LOW IN ABOUT 72 HR. HOWEVER... THERE IS LOWER CONFIDENCE IN THIS SCENARIO THAN THERE WAS YESTERDAY. THE DYNAMICAL MODELS SHOW THE DEPRESSION OR ITS REMNANTS SURVIVING LONGER THAN THEY DID 24 HR AGO...SO A 96 HR FORECAST POINT HAS BEEN ADDED. THE SYSTEM IS EXPECTED TO DISSIPATE OR BE ABSORBED INTO THE BAROCLINIC TROUGH BY 120 HR. FORECAST POSITIONS AND MAX WINDS INITIAL 26/1500Z 18.0N 34.8W 30 KT 12HR VT 27/0000Z 19.1N 36.4W 30 KT 24HR VT 27/1200Z 20.5N 37.8W 30 KT 36HR VT 28/0000Z 21.9N 38.9W 25 KT 48HR VT 28/1200Z 23.0N 40.0W 25 KT 72HR VT 29/1200Z 24.5N 40.5W 20 KT...REMNANT LOW 96HR VT 30/1200Z 26.0N 37.0W 20 KT...REMNANT LOW 120HR VT 01/1200Z...DISSIPATED $$ FORECASTER BEVEN |
