WTNT41 KNHC 220249

Tropical Depression Eleven Discussion Number 1
NWS National Hurricane Center Miami FL AL112018
1100 PM AST Fri Sep 21 2018

The small, but well-defined, low pressure system located about 500 miles east of the Lesser Antilles has been producing steady convection for the past several hours. Although this convection is displaced to the south and east of the exposed low-level center due to strong westerly wind shear, it has persisted long enough to meet the NHC definition of a tropical cyclone. On this basis, advisories are being initiated on Tropical Depression Eleven.

The initial intensity is set at 30 kt based on ASCAT data from earlier today which showed a couple of 25-30 kt wind vectors in the northeast quadrant of the circulation. The intensity guidance is in extremely good agreement that the depression is unlikely to strengthen. All of the global models forecast an increase in upper-level winds over the small cyclone during the next 24 h. In fact, SHIPS diagnostics indicate that the 850-200 mb shear could exceed 30 kt by tomorrow morning, and will be near 40 kt within 48 h. As a result of this shear and some dry air also in the vicinity, the dynamical models unanimously forecast dissipation within 72 h, and most show that the depression will open into a trough of low pressure sooner than that. The NHC forecast conservatively maintains the tropical cyclone for 48 h, but it could weaken and dissipate sooner than currently indicated.

The depression has recently been moving almost due west, but a longer-term motion yields an initial motion of 290/5 kt. A break in the subtropical ridge to the north of the depression should result in fairly weak steering flow for the next day or two, and only a slow west-northwestward to westward motion is anticipated. All of the typically-reliable track models are in fairly good agreement on this scenario. The NHC forecast is very close to HCCA at all forecast hours, and lies near the south edge of the tight guidance envelope.


INIT 22/0300Z 13.1N 53.4W 30 KT 35 MPH 12H 22/1200Z 13.5N 54.2W 30 KT 35 MPH 24H 23/0000Z 13.8N 55.0W 30 KT 35 MPH 36H 23/1200Z 14.2N 56.3W 30 KT 35 MPH 48H 24/0000Z 14.5N 58.0W 25 KT 30 MPH 72H 25/0000Z...DISSIPATED

$$ Forecaster Zelinsky