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| #698529 (Received by flhurricane at: 11:04 AM 01.Jul.2014) TCDAT1 TROPICAL STORM ARTHUR DISCUSSION NUMBER 3 NWS NATIONAL HURRICANE CENTER MIAMI FL AL012014 1100 AM EDT TUE JUL 01 2014 Radar and satellite imagery indicate that the convective organization of the cyclone has improved since the previous advisory, and the cyclone is being upgraded based on a sustained wind report of 33 kt from Settlement Point (SPGF1) on Grand Bahama Island earlier this morning that was outside of the deep convection. After remaining nearly stationary earlier this morning, Arthur appears to to be drifting northwestward now with an uncertain motion of 315/02 kt. Otherwise, there is no significant change to the previous forecast track. The latest model guidance has continued the trend of a pronounced mid-tropospheric trough digging southeastward from the upper-midwest into the northeastern and mid-Atlantic region of the United States by 72 hours. The 500 mb flow pattern is almost identical in the GFS and ECMWF models, which increases the confidence in this evolving pattern. As a result, a steady increase in southwesterly steering flow over the southeastern United States is expected to gradually turn the tropical cyclone northward over the next 24-36 hours, and then accelerate the system faster toward the northeast on Thursday and Friday. By Days 4 and 5, Arthur is forecast to move over the far north Atlantic as an extratropical cyclone. The NHC track forecast is just an update of the previous advisory track, and lies down the middle of the tightly packed guidance envelope close to the consensus model TVCA. Northwesterly vertical wind shear is forecast by the models to gradually subside over the next 48 hours, which should allow the cyclone to develop its own upper-level outflow pattern. In fact, latest visible and water vapor imagery indicates that cirrus outflow has been expanding on the north side of the system during the past few hours, suggesting that the shear conditions could already be subsiding. The low shear conditions and warm sea-surface temperatures should allow for at least steady strengthening, and the cyclone is expected to become a hurricane by 72 hours. The official intensity forecast is similar to the latest intensity model consensus IVCN through 36 hours, and then slightly higher after that. FORECAST POSITIONS AND MAX WINDS INIT 01/1500Z 27.6N 79.3W 35 KT 40 MPH 12H 02/0000Z 27.8N 79.4W 35 KT 40 MPH 24H 02/1200Z 28.7N 79.6W 40 KT 45 MPH 36H 03/0000Z 29.8N 79.5W 50 KT 60 MPH 48H 03/1200Z 31.2N 78.9W 60 KT 70 MPH 72H 04/1200Z 35.4N 75.2W 70 KT 80 MPH 96H 05/1200Z 40.8N 67.3W 65 KT 75 MPH 120H 06/1200Z 45.5N 59.5W 45 KT 50 MPH...POST-TROP/EXTRATROP $$ Forecaster Stewart |
