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| #9431 (Received by flhurricane at: 11:18 AM 21.Sep.2004) TCDAT3 TROPICAL STORM LISA DISCUSSION NUMBER 9 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL 11 AM EDT TUE SEP 21 2004 NORTHERLY SHEAR THAT HAD PREVIOUSLY BEEN AFFECTING LISA APPEARS TO HAVE ABATED...AT LEAST FOR THE TIME BEING. THIS HAS ALLOWED OUTFLOW TO REDEVELOP IN THE NORTHERN SEMICIRCLE. THE OVERNIGHT BURST OF DEEP CONVECTION...INDICATED BY CLOUD TOPS COLDER THAN -80C...HAS PERSISTED FOR MORE THAN 9 HOURS NOW. AN UW-CIMSS AMSU PRESSURE ESTIMATE OF 987.4 MB...ALONG WITH A CIMSS 3-HR AVERAGE AODT OF T4.1...OR 66 KT...SUGGEST THAT LISA COULD BE A HURRICANE. HOWEVER ...SUBJECTIVE DVORAK ESTIMATES ARE ONLY 45-55 KT FROM THE THREE SATELLITE AGENCIES. THEREFORE...THE INITIAL INTENSITY IS BASED ON A BLEND OF THESE VARIOUS INTENSITY ESTIMATES FOR THE TIME BEING. THE INITIAL MOTION ESTIMATE IS 285/09. THERE REMAINS NO SIGNIFICANT CHANGE TO THE PREVIOUS FORECAST TRACK OR REASONING. MOST OF THE NHC MODEL GUIDANCE HAS BEEN OF LITTLE HELP...EXCEPT FOR THE GFDL...GFDN AND CANADIAN MODELS. THE GFS...UKMET...AND NOGAPS HAVE ALREADY MISSED THE SHARP NORTHWARD TURN OF HURRICANE KARL LOCATED TO THE NORTHWEST OF LISA. THE 12 LOCATION OF KARL WAS ALREADY 18 HOURS AHEAD OF AND AT LEAST 2 DEGREES OF LONGITUDE EAST OF THOSE MODELS. THIS SUGGESTS THAT RIDGE LOCATED BETWEEN LISA AND KARL IS STRONGER THAN THOSE MODELS HAVE BEEN FORECASTING...SO THE OFFICIAL ELANS MORE HEAVILY TOWARD THE GFDL-GFDN-CANADIAN SOLUTIONS. THE RIDGE TO THE NORTH OF TINY LISA IS EXPECTED TO REMAIN INTACT AND STRONG ENOUGH TO STEER THE CYCLONE WEST-NORTHWEST OR WESTWARD UNTIL THE END OF THE FORECAST PERIOD WHEN A TROUGH IS FORECAST TO DIG SOUTHWARD BETWEEN 50-55W LONGITUDE AND WEAKEN THE RIDGE SLIGHTLY. THIS MAY ALLOW LISA TO BRIEFLY GAIN SOME LATITUDE BEFORE THE TROUGH LIFTS OUT AND THE RIDGE BUILDS BACK IN TO THE NORTH. MODERATE TO WEAK NORTH TO NORTHEASTERLY SHEAR IS EXPECTED TO AFFECT LISA FOR THE NEXT 36 HOURS OR SO. AS SUCH...THE INTENSITY IS HELD CONSTANT DUE TO THE SMALL SIZE OF THE CYCLONE. AFTER THAT...THE SHEAR IS EXPECTED TO WEAKEN TO 10-15 KT FROM THE EAST AND SOUTHEAST ...WHICH MAY ALLOW FOR SOME MODEST STRENGTHENING TO OCCUR...ASSUMING LISA SURVIVES THE AFOREMENTIONED SHEAR PATTERN. NOTE-- SINCE LISA IS A VERY SMALL CYCLONE...IT WILL BE SUBJECT TO RAPID FLUCTUATIONS IN STRENGTH. FORECASTER STEWART FORECAST POSITIONS AND MAX WINDS INITIAL 21/1500Z 14.2N 39.7W 60 KT 12HR VT 22/0000Z 14.6N 41.1W 60 KT 24HR VT 22/1200Z 15.2N 42.7W 60 KT 36HR VT 23/0000Z 15.7N 44.3W 60 KT 48HR VT 23/1200Z 15.9N 45.6W 65 KT 72HR VT 24/1200Z 16.0N 48.0W 65 KT 96HR VT 25/1200Z 16.5N 50.0W 70 KT 120HR VT 26/1200Z 17.5N 52.0W 70 KT |
