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Tropical Storm Ophelia Forecast Discussion Number 3
2017-10-09 22:37:49| National Hurricane Center (Atlantic)
Issued at 500 PM AST Mon Oct 09 2017 000 WTNT42 KNHC 092037 TCDAT2 Tropical Storm Ophelia Discussion Number 3 NWS National Hurricane Center Miami FL AL172017 500 PM AST Mon Oct 09 2017 Ophelia's convective pattern has continued to steadily improve with the formation of strong thunderstorms with cloud tops near -80C near the center, along with the development of tighter curved banding features in the southeastern semicircle. There has also been a noticeable increase in lightning activity within 30 nmi of the center. The TAFB Dvorak intensity estimate at 1800Z was T2.5/35 kt. However, NHC objective Dvorak estimates are ranging from T3.2/49 kt using a curved band pattern to T3.5/55 kt for a shear pattern. Based on the aforementioned estimates and the continued improvement in Ophelia's convective organization since the 1800Z TAFB estimate, the initial intensity is increased to 40 kt. The initial motion estimate is 055/03 kt. There is no significant change to the previous forecast track or reasoning. Ophelia is expected to remain embedded within a weak flow regime for the next 72 hours due to the cyclone being cut off from the mid-latitude westerlies, resulting in a slow and gradual turn toward the east and southeast during that time frame. Thereafter, a broad mid- to upper-level trough to the northwest, in conjunction with a building subtropical ridge to the south, is forecast to gradually accelerate Ophelia toward the northeast at a slightly faster forward speed of near 10 kt. The new NHC forecast track is close to the northern edge of the guidance envelope out of respect for the ECMWF model, but the forward speed is slower than the ECMWF and similar to a blend of the HCCA and TVCX consensus models. Ophelia is expected to remain over marginally warm SSTs of 26.5-27C, but temperatures aloft that are 2-3 deg C colder than normal should continue to produce steep lapse rates and strong instability, which will allow for deep convection to develop for the next 96 h or so. Both the GFS and ECMWF models continue to forecast the vertical wind shear to decrease sharply after 24 h, with the ECMWF forecasting shear values to decrease to less than 10 kt from 36-96 hours. Based on the availability of strong instability and lower shear, slow but steady strengthening is expected for the next 3-4 days, with only the occasional entrainment of very dry mid-level air preventing rapid intensification from occurring, at least in the short term. After 96 h, Ophelia will be encountering increasing southwesterly wind shear of 15-20 kt and also be moving over 25C sea-surface temperatures, a combination that should cap the intensification process and possibly even induce weakening. The SHIPS and LGEM statistical models, along with the HCCA and IVCN consensus models, have come into much better agreement on their intensity forecasts. Therefore, there is a higher degree of confidence in the official intensity forecast, which basically remains unchanged from the previous advisory. Note, it is possible that Ophelia could reach a higher peak intensity than indicated between 72-96 hours when the wind shear will be at its lowest value. FORECAST POSITIONS AND MAX WINDS INIT 09/2100Z 31.5N 39.6W 40 KT 45 MPH 12H 10/0600Z 31.6N 39.1W 45 KT 50 MPH 24H 10/1800Z 31.4N 38.5W 50 KT 60 MPH 36H 11/0600Z 30.8N 37.9W 55 KT 65 MPH 48H 11/1800Z 30.1N 37.6W 60 KT 70 MPH 72H 12/1800Z 29.7N 36.1W 65 KT 75 MPH 96H 13/1800Z 31.0N 33.7W 65 KT 75 MPH 120H 14/1800Z 32.4N 30.0W 65 KT 75 MPH $$ Forecaster Stewart
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