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Tropical Depression NINE Forecast Discussion Number 1
2013-09-08 22:42:59| National Hurricane Center (Atlantic)
Issued at 500 PM AST SUN SEP 08 2013 000 WTNT44 KNHC 082042 TCDAT4 TROPICAL DEPRESSION NINE DISCUSSION NUMBER 1 NWS NATIONAL HURRICANE CENTER MIAMI FL AL092013 500 PM AST SUN SEP 08 2013 SATELLITE IMAGES INDICATE THAT THE LOW PRESSURE SYSTEM BETWEEN THE CAPE VERDE ISLANDS AND THE WEST COAST OF AFRICA HAS DEVELOPED ENOUGH CONVECTIVE ORGANIZATION TO BE DESIGNATED AS A TROPICAL DEPRESSION. THE INITIAL INTENSITY IS SET AT 25 KT BASED ON DVORAK ESTIMATES OF T2.0 FROM TAFB AND T1.5 FROM SAB. THE LOW-LEVEL CENTER APPEARS TO BE ON THE EASTERN EDGE OF THE DEEP CONVECTION DUE TO A LITTLE BIT OF NORTHEASTERLY SHEAR. THE INITIAL MOTION ESTIMATE IS 270/9 KT. THE DEPRESSION IS LOCATED TO THE SOUTH OF A LARGE MID-TROPOSPHERIC ANTICYCLONE CENTERED NEAR THE AZORES...AND THE CURRENT WESTWARD MOTION IS EXPECTED TO CONTINUE FOR ANOTHER 36 HOURS OR SO. AFTER THAT TIME...THE ANTICYCLONE IS FORECAST TO WEAKEN WHILE A CLOSED MID/UPPER-LEVEL LOW DEVELOPS NEAR THE CANARY ISLANDS AND A RIDGE BUILDS WESTWARD FROM AFRICA. THIS PATTERN WILL ALLOW THE DEPRESSION TO TURN RATHER SHARPLY TO THE NORTH-NORTHWEST AFTER 48 HOURS AND CONTINUE THAT TRAJECTORY THROUGH THE END OF THE FORECAST PERIOD. SURPRISINGLY... THE LARGEST SPREAD AMONG THE TRACK GUIDANCE OCCURS WITHIN THE FIRST 48 HOURS OR SO...BUT THE MODELS SEEMS TO CONVERGE BY 72 HOURS. THE NHC OFFICIAL TRACK FORECAST IS CLOSE TO THE MODEL CONSENSUS FOR MUCH OF THE FORECAST. A MOIST AIR MASS...WARM WATERS...AND RELATIVELY LIGHT SHEAR SHOULD ALLOW THE DEPRESSION TO STRENGTHEN AT LEAST GRADUALLY DURING THE NEXT FEW DAYS. HOWEVER...SOME OF THE MODELS...MAINLY THE STATISTICAL-DYNAMICAL GUIDANCE...ARE FAIRLY AGGRESSIVE WITH THE INTENSIFICATION RATE AFTER 24 HOURS. SHIPS AND LGEM HAVE TENDED TO HAVE A HIGH BIAS THIS HURRICANE SEASON...SO THE OFFICIAL FORECAST IS KEPT A BIT LOWER. THE FORECAST DOES...HOWEVER...ANTICIPATE THE CYCLONE BECOMING A HURRICANE BY 72 HOURS. GIVEN THE FORECAST...AND THE UNCERTAINTY WITH HOW BIG THE WIND FIELD WILL GET ONCE THE DEPRESSION BECOMES A TROPICAL STORM...A TROPICAL STORM WARNING HAS BEEN ISSUED FOR THE SOUTHERN CAPE VERDE ISLANDS. FORECAST POSITIONS AND MAX WINDS INIT 08/2100Z 13.1N 20.7W 25 KT 30 MPH 12H 09/0600Z 13.2N 22.1W 30 KT 35 MPH 24H 09/1800Z 13.5N 24.3W 35 KT 40 MPH 36H 10/0600Z 13.9N 26.5W 40 KT 45 MPH 48H 10/1800Z 14.5N 27.9W 55 KT 65 MPH 72H 11/1800Z 17.5N 29.5W 70 KT 80 MPH 96H 12/1800Z 21.0N 31.0W 70 KT 80 MPH 120H 13/1800Z 23.5N 32.5W 65 KT 75 MPH $$ FORECASTER BERG
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