Atlantic/2018/02L/Archive/2

Tropical Depression Two Discussion Number 2

Wiki-Hurricanes Forecast Center

11:00 AM AST Thu Jul 5 2018

There have not been many changes to the structure of Two overall during the overnight hours, with more or less the same appearance of a single vigorous bulb of convective activity with some minor rainbands pulled from the south as it tries to draw moisture from the nearby intertropical convergence zone. A ScatSat pass from 1136Z eliminated any outstanding doubts that a closed circulation existed, showing a very well-defined surface circulation. Not too much could be gleaned from the 0922Z SSMIS microwave scan this morning due to Two's small size, though at the time the tropical depression sported a strong northern band. Another microwave scan at 1204Z showed a low-resolution curved band wrapping around the south. SAB and TAFB both gave T2.0/30kt final-T estimates for Two. Given no significant changes since the last advisory, there is no convincing reason to raise the assessed intensity from 30 kt. All intensity parameters have been maintained from the previous advisory, which may be conservative as small storms like Two have a penchant for exceeding their appearances.

While it is slightly difficult to determine where the surface circulation is located relative to the powerful mid-level circulation, microwave and infrared satellite data would suggest that Two's main convective region has actually lost some latitude since last night. Regardless, the speedy easterly motion is being propelled by an extended bifocal ridge over the subtropical Atlantic, which has established a 20 kt steering flow across the main development region within which Two is embedded. This track towards the west should continue, with some latitudinal gains coming from a slight upper-level weakness in this ridging. As Two has been coming in south of forecast, the forecast track in on the southerly end of statistical gudiance.

Marginally favorable conditions for Two continue this morning as Two maintains a delicate balance between escaping the Saharan Air Layer and posessing a small size susceptible to disruption by subsidence. Analyses indicate neutrally favorable sea surface temperatures where Two is located, and while temperatures may cool slightly over a small swath where the tropical depression is headed, the system is moving fast enough so as to minimize the effects of this slight cooling. Two also also carted along the same moisture pouch it left Africa with which is helping to insulate the system against the dry air to its north. As Two gains latitude, support from the monsoon trough will decrease and permeability of the moisture pouch will increase, making it more susceptible to dry air. A tropical upper-tropospheric trough draped into the eastern Caribbean will force strong unfavorable upper-level southwesterlies across the system after 48 hours which will likely allow for the rapid degeneration of Two into a trough. COAMPS parameters are friendlier than SHIPS parameters, but an anomolously strong TUTT in the region all but guarantees a swift end to organized convection. Gradual strengthening is depicted out to 36 hours before weakening commences thereafter, though quick trades will elevate the remnant trough associated with Two some even after it dissipates. These winds may provide some gusty conditions, coupled with scattered rainfall, in portions of the Lesser Antilles in around four days.

FORECAST POSITIONS AND MAX WINDS

INIT 05/1500Z 10.4N 41.4W 30KT

12H 06/0000Z 10.5N 43.6W 35KT

24H 06/1200Z 11.1N 46.2W 35KT

36H 07/0000Z 11.7N 48.8W 35KT

48H 07/1200Z 12.3N 51.1W 30KT

72H 08/1200Z ... DISSIPATED