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Tropical Depression Seventeen Forecast Discussion Number 1
2020-09-07 04:37:38| National Hurricane Center (Atlantic)
Issued at 1100 PM AST Sun Sep 06 2020 306 WTNT42 KNHC 070237 TCDAT2 Tropical Depression Seventeen Discussion Number 1 NWS National Hurricane Center Miami FL AL172020 1100 PM AST Sun Sep 06 2020 The tropical low that the National Hurricane Center has been tracking the past few days west of the Cabo Verde Islands has developed enough convective banding for the system to be classified as a tropical depression. A 06/2345Z ASCAT-B scatterometer pass indicated surface wind speeds of 30-31 kt north of the center, which are supported by a T2.0/30 kt Dvorak satellite intensity estimate from TAFB at 07/0000 UTC. The initial motion estimate is an uncertain 270/04kt. The center has been jumping around somewhat due to small, brief convective bursts that then dissipate, leaving small swirls rotating around the mean circulation center. However, the latest ASCAT data showed a much better defined overall circulation, especially in the inner core wind field region, so the forward motion should stabilize fairly soon. The depression is expected to remain caught in weak steering flow for the next 5 days, so only a slow westward motion is expected tonight and Monday, followed by a west-northwestward motion by later Monday and Tuesday that will continue through the remainder of the 120-h forecast period. Possible track forecast complications could develop by days 3-5 if binary interaction occurs between the depression and the large low (Invest 93L) currently located southeast of the Cabo Verde Islands, resulting in TD-17 not gaining as much latitude as currently indicated. The NHC forecast track lies down the middle of the simple- and corrected-consensus model guidance envelope. The depression is only forecast to slowly strengthen for the next 4 days or so due to expected intrusions of dry mid-level air. By day 5, increasing southerly vertical wind shear on the east side of a large upper-level low that is forecast to cut off north of Puerto Rico is expected to induce gradual weakening. However, both the timing of the development of the low and its west-southwestward motion will determine if sufficient shear will prevent further strengthening after 96 h; a slower formation of the low and/or a slower retrograding motion would result in at least less weakening. The NHC intensity forecast closely follows an average of the IVCN and NOAA-HCCA intensity consensus models. FORECAST POSITIONS AND MAX WINDS INIT 07/0300Z 17.0N 41.5W 30 KT 35 MPH 12H 07/1200Z 17.0N 42.1W 30 KT 35 MPH 24H 08/0000Z 17.2N 42.7W 35 KT 40 MPH 36H 08/1200Z 17.5N 43.4W 40 KT 45 MPH 48H 09/0000Z 18.0N 44.6W 45 KT 50 MPH 60H 09/1200Z 18.6N 46.3W 50 KT 60 MPH 72H 10/0000Z 19.2N 48.2W 55 KT 65 MPH 96H 11/0000Z 20.4N 51.9W 50 KT 60 MPH 120H 12/0000Z 21.2N 55.5W 45 KT 50 MPH $$ Forecaster Stewart
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