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| #198944 (Received by flhurricane at: 5:08 AM 04.Jul.2008) TCDAT2 TROPICAL STORM BERTHA DISCUSSION NUMBER 5 NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL022008 500 AM EDT FRI JUL 04 2008 A STRONG BURST OF DEEP CONVECTION BEGAN AT ABOUT 05Z...RESULTING IN SUBJECTIVE DVORAK INTENSITY ESTIMATES AT 06Z OF 45-55 KT. THESE ESTIMATES...HOWEVER...WERE CONTINGENT UPON THE LOW-LEVEL CIRCULATION CENTER BEING LOCATED MUCH FARTHER NORTH THAN PREVIOUSLY THOUGHT...AND RIGHT IN THE MIDDLE OF THE CONVECTION. PASSIVE MICROWAVE IMAGES FROM TRMM AT 0146Z AND AMSR-E AT 0326Z SUGGEST THE LOW-LEVEL CENTER HAS BEEN MOVING NORTH OF THE PREVIOUS MOTION ESTIMATE...BUT BENEATH THE SOUTHERN PORTION OF THE COLD CLOUD TOP CANOPY. SINCE 06Z...THE CLOUD TOPS HAVE WARMED A BIT...SO I DO NOT THINK THE DVORAK ESTIMATES WOULD BE AS HIGH NOW. BEST I CAN TELL FROM FIRST LIGHT VISIBLE IMAGES...THE LOW-LEVEL CENTER SEEMS TO BE MOVING ALONG THE HEADING SUGGESTED BY THE MICROWAVE IMAGERY...OR 290/12... BENEATH THE SOUTHERN PORTION OF THE DEEP CONVECTION. THE INITIAL INTENSITY WILL BE HELD AT 40 KT FOR NOW...TO SEE IF THE DEEP CONVECTION PERSISTS AS BERTHA HEADS TOWARD SLIGHTLY COOLER WATERS TODAY. THE FORECAST REASONING HAS NOT CHANGED TOO MUCH. THE FIRST TWO DAYS OF THE TRACK FORECAST HAVE BEEN ADJUSTED SLIGHTLY NORTHWARD... MAINLY TO ACCOUNT FOR THE CHANGE IN THE INITIAL MOTION. THEREAFTER...ONCE BERTHA FINDS ITSELF DUE SOUTH OF THE MID-LEVEL RIDGE TO ITS NORTH...THE TRACK HAS BEEN SPED UP AND EDGED A LITTLE TO THE LEFT OR WEST...IN ACCORDANCE WITH A WESTWARD SHIFT IN MOST OF THE MODELS. ALL OF THE MODELS STILL FORECAST A WEAKNESS IN THE RIDGE TO ALLOW BERTHA TO TURN TO THE RIGHT LATE IN THE FORECAST PERIOD...BUT RECENT RUNS HAVE BEEN TRENDING TOWARD DELAYING THIS TURN. THE NEW OFFICIAL TRACK FORECAST IS KEPT CLOSE TO THE MODEL CONSENSUS...BUT THE MODEL SPREAD IS SUBSTANTIAL AT DAYS 4-5. THE MODELS THAT FORECAST A STRONGER BERTHA...SUCH AS THE GFDL...FORECAST MORE OF A NORTHWESTWARD MOTION BY DAY 5...WHILE MODELS SUCH AS THE ECMWF AND UKMET INSIST ON A WEAKER CYCLONE CONTINUING WEST-NORTHWESTWARD. THE OFFICIAL INTENSITY FORECAST...LIKE THE TRACK FORECAST...IS CLOSE TO THE CONSENSUS...TAKING A STRONG TROPICAL STORM DOWN THE MIDDLE OF THE GUIDANCE ENVELOPE...IN CLOSEST AGREEMENT WITH THE LGEM GUIDANCE. STRENGTHENING SHOULD BE LIMITED BY COOL WATERS DURING THE NEXT TWO DAYS...AND BY INCREASING SHEAR ON DAYS 4 AND 5. IT IS NOTABLE THAT THE GFDL AND HWRF INTENSITY FORECASTS ARE VERY MUCH IN DISAGREEMENT...INDICATIVE OF GREAT UNCERTAINTY IN THE INTENSITY FORECAST AS WELL. FORECAST POSITIONS AND MAX WINDS INITIAL 04/0900Z 14.2N 28.3W 40 KT 12HR VT 04/1800Z 15.0N 30.6W 45 KT 24HR VT 05/0600Z 15.9N 34.0W 45 KT 36HR VT 05/1800Z 16.6N 37.7W 50 KT 48HR VT 06/0600Z 17.3N 41.4W 55 KT 72HR VT 07/0600Z 19.0N 48.0W 60 KT 96HR VT 08/0600Z 21.5N 53.0W 60 KT 120HR VT 09/0600Z 25.0N 56.5W 60 KT $$ FORECASTER KNABB |
