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| #939039 (Received by flhurricane at: 4:56 AM 08.Sep.2018) TCDAT3 Tropical Storm Helene Discussion Number 4 NWS National Hurricane Center Miami FL AL082018 500 AM AST Sat Sep 08 2018 Helene`s structure appears to be quickly improving this morning. The initial intensity has been increased to 40 kt based on a blend of the latest subjective Dvorak intensity estimates and the UW-CIMSS SATCON. Several microwave passes overnight indicate that the cyclone has become more vertically aligned, likely due to a reformation of the center beneath the strongest convection. In fact, a 0733 UTC SSMIS image shows the early development of a low- to mid-level eye, which is often a precursor to intensification. Nearly all of the intensity guidance seems to have picked up on the improved structure of Helene, and the models show far more intensification than before, especially through 48 h. Consequently, a significant change has been made to the intensity forecast, which now calls for Helene to become a hurricane on Sunday while it is near the Cabo Verde islands. Once the cyclone passes the islands, the environment is expected to remain favorable for intensification for at least 72 h. By the end of the forecast period, all of the guidance shows some weakening due primarily to increased shear associated with an approaching mid- to upper-level trough over the central Atlantic. Given the recent microwave imagery signature, low shear, and warm SSTs, its possible that the current forecast is still too conservative, especially for the first 36 h of the forecast, and further adjustments could be required if it becomes clear that Helene is intensifying at a faster rate than expected. Smoothing through the possible reformation of the center, Helene appears to be moving west with an initial motion around 275/10 kt. Little change was required to the track forecast, which has been nudged only slightly south for the first 48 h of the forecast, in line with the latest track model consensus. A westward motion should continue for the next couple of days, as Helene is steered by a mid-level ridge to the north. By day 5, the aforementioned trough over the central Atlantic should force Helene to turn toward the northwest. FORECAST POSITIONS AND MAX WINDS INIT 08/0900Z 13.7N 19.6W 40 KT 45 MPH 12H 08/1800Z 13.9N 21.2W 50 KT 60 MPH 24H 09/0600Z 14.1N 23.5W 60 KT 70 MPH 36H 09/1800Z 14.7N 26.3W 70 KT 80 MPH 48H 10/0600Z 15.4N 29.3W 75 KT 85 MPH 72H 11/0600Z 17.2N 35.1W 80 KT 90 MPH 96H 12/0600Z 19.0N 39.5W 70 KT 80 MPH 120H 13/0600Z 22.0N 42.5W 65 KT 75 MPH $$ Forecaster Zelinsky |
