Atlantic/2018/03L/Archive/2

Tropical Depression Three Discussion Number 2

Wiki-Hurricanes Forecast Center

5:00 PM AST Fri Jul 5 2018

Convection has increased over the center of circulation of Tropical Depression Three this afternoon, taking residence over the low-level vorticity as opposed to the mid-level one like this morning, which is displaced slightly to the east. Some small banding features are also beginning to develop, particularly in the comparatively moist western semicircle. ScatSat and ASCAT scatterometer passes at 14Z and 15Z confirmed the well-defined surface circulation identified in high-resolution mesoscale imagery. Peak winds were found to be lower than anticipated this morning, with the scatterometer data only finding isolated patches of 20kt winds far from the center. However, given a fair number of ship observations near the center finding 20kt winds, the intensity for this advisory has only been reduced marginally to 25kt.

Three is tracking to the northwest in a convenient steering flow generated by a tight upper-level low moving ashore the Georgia coast this afternoon and a large subtropical ridge centered northeast of Bermuda. A cold front now beginning to move off the Mid-Atlantic coast is expected to hit Three with a glancing blow this weekend which will halt northwestward advances from the tropical cyclone. Once the front passes, steering currents will be quite weak over the region as several steering disturbances concurrently tug on Three. However, localized ridging in the wake of the trough over the Eastern United States is expected to weakly bring Three westward some before a second trough, this time with a better angle on Three, is expected to cart the wandering system to the northeast Tuesday to Wednesday next week.

Three is presently in a somewhat dry environment but has the aid of some moisture tapped from a second disturbance to the southeast. Three has managed to develop convection within these dry conditions due to light upper-level winds caused by the alignment of cyclonic and anticyclonic flow over the system and steep lapse rates resulting from unseasonably warm sea surface temperatures above 28°C. However, dry conditions exacerbated by the leeward environment of a passing front with associated shear are expected to cap intensification this weekend, before Three finds a weak steering region over the Gulf Stream during which Three can strengthen in relative peace. The intensification of Three will depend largely on how long it can spend away from the approaching second trough. The HWRF brings Three quite far southwest and allows it to intensify into a major hurricane, marking the high end of model guidance. Most guidance remains weaker. UKMET-G has a significant intensification episode, but the model also brings the system much closer to shore, perhaps from overemphasizing the eastern US ridge. For now, the intensification philosophy from the last advisory is carried over to this advisory with few adjustments.

FORECAST POSITIONS AND MAX WINDS

INIT 06/2100Z 25 KT 30 MPH

12H 06/0600Z 25 KT 30 MPH

24H 07/1800Z 30 KT 35 MPH

36H 07/0600Z 30 KT 35 MPH

48H 08/1800Z 35 KT 40 MPH

72H 09/1800Z 40 KT 45 MPH

96H 10/1800Z 50 KT 60 MPH

120H 11/1800Z 65 KT 75 MPH