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Tropical Depression Three Discussion Number 1

Wiki-Hurricanes Forecast Center

11:00 AM AST Fri Jul 5 2018

A non-tropical low centered between Bermuda and Savannah, Georgia has been producing organized convection since the overnight hours. Visible images from yesterday evening confirmed a closed low-level circulation which has expanded on the basis of a 20kt west-northwesterly wind observed by a ship south of the suspected center of the area of low pressure. High resolution rapid scan is confirming this center is shrouded by convection this morning with microwave seeing definite circulation and internal banding at the mid-levels. TAFB estimated T1.5/25kt at 1200Z, but since the system has improved in coverage of the center I have set the intensity to 30 kt for this first advisory on the newly formed Tropical Depression Three.

The steering forecast for Three is complicatedf to say the least. Three is currently moving northwest around a subtropical ridge in the general direction of the Outer Banks, with a shortwave trough over Quebec providing some northerly ascent. However, global guidance suggests that the frontal boundary associated with this trough takes a rather shallow angle on the developing system, forcing it to stall or back southward as opposed to the ordinary action of kicking it out towards the northeast. Building heights behind the passing front could impart some westerly motion on Three as it stalls for a few days, waiting for another trough around the day 5 time period to kick the system out northeast. The current forecast track is close to the mean of statistical guidance and follows this stall, westward drift and eventual northeast trajectory, though the northeastern push will mostly take place towards the end of the forecast window and after the forecast window.

The upper-air environment for Three is rather conducive for development, with cold -55C upper-air temperatures and warm Gulf Stream waters in excess of 28.6C throughout the forecast period, owing to anomolously warm subtropical waters off the East Coast this year some 2-3C above normal. Before the cold front clears past the developing tropical depression, the system is expected to remain rather dormant. Afterwards, intensification should commence fairly steadily through the end of the forecast window.

FORECAST POSITIONS AND MAX WINDS

INIT 06/1500Z 30 KT 35 MPH

12H 07/0000Z 30 KT 35 MPH

24H 07/1200Z 30 KT 35 MPH

36H 08/0000Z 30 KT 35 MPH

48H 08/1200Z 35 KT 40 MPH

72H 09/1200Z 40 KT 45 MPH

96H 10/1200Z 45 KT 50 MPH

120H 11/1200Z 60 KT 70 MPH