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| #939469 (Received by flhurricane at: 4:45 AM 11.Sep.2018) TCDAT3 Hurricane Helene Discussion Number 16 NWS National Hurricane Center Miami FL AL082018 500 AM AST Tue Sep 11 2018 An Advanced Microwave Technology Sounder (ATMS) 165GHZ overpass on Helene indicated a well-developed closed eyewall and a deep convective outer band with associated cold cloud tops of -76C wrapping around the south through east quadrants of the cyclone. The initial intensity is held at 95 kt for this advisory and is based on a blend of the subjective and objective T-numbers. There still appears to be a small window of opportunity for Helene to strengthen, during the next 12 hours or so, before the ocean and the upper wind environment become less conducive. Afterward, the SSTs decrease significantly and the vertical shear increases, which should induce gradual weakening. By day 3, the sea surface temperatures increase, however, the southwesterly shear persists and the mid-level atmospheric moisture decreases. Interestingly enough, the global models show Helene either maintaining tropical storm strength or even intensifying by the end of the period, possibly due to some mid-latitude dynamic forcing influences. It`s also worth noting that the Florida State Cyclone Phase Evolution analysis and forecast product shows the system retaining a relatively symmetric warm core through the entire forecast. The initial motion is estimated to be west-northwestward, or 300/12 kt. A turn more toward the northwest is forecast early on Wednesday. Beyond mid-period, the large-scale models agree that a mid- to upper tropospheric trough over the northeast Atlantic will dig southward inducing a weakness in the eastern portion of the subtropical ridge. In response to this change in the synoptic steering pattern, Helene should slowly turn northward and then north-northeastward during the next 72 to 96 hours, followed by a turn toward the northeast on day 5. The official forecast is close to the previous one, and lies near the TVCN consensus model and the GFS ensemble mean. FORECAST POSITIONS AND MAX WINDS INIT 11/0900Z 16.0N 33.6W 95 KT 110 MPH 12H 11/1800Z 16.8N 35.0W 100 KT 115 MPH 24H 12/0600Z 18.0N 36.5W 85 KT 100 MPH 36H 12/1800Z 19.5N 37.6W 75 KT 85 MPH 48H 13/0600Z 21.4N 38.3W 65 KT 75 MPH 72H 14/0600Z 25.7N 38.8W 55 KT 65 MPH 96H 15/0600Z 31.4N 36.6W 50 KT 60 MPH 120H 16/0600Z 37.3N 31.2W 50 KT 60 MPH $$ Forecaster Roberts |
