Tropical Storm Bret is about to bring tropical storm conditions to the Windward Islands as it begins to become more disorganized. Bret is likely to dissipate over the central Caribbean as wind shear is expected to increase. In addition to Bret, a tropical wave developed into Tropical Depression Four early this morning. TD Four is likely to intensify into a weak-moderate tropical storm, but should not be a threat to any land areas.

As of 11:00 a.m. EDT Thursday, Tropical Storm Bret was centered near 13.8°N 57.7°W, and was moving westward at about 14 mph. Maximum sustained winds were 60 knots (70 mph), with an estimated minimum pressure of 999 mb. Around 1:00 a.m. EDT this morning, reconnaissance data suggested that Bret had developed an eyewall open to the south, and the SFMR instrument measured winds as high as 69 knots in the deep convection, although flight-level winds were lower. Bret was likely very close to hurricane intensity at the time, though it is possible the winds were transient. The strong winds necessitated the issuance of a Hurricane Watch for St. Lucia. Unfortunately, the reconnaissance aircraft left Bret after this time, making Bret’s peak intensity somewhat uncertain. Since that time, Bret’s convective organization has become rather ragged, the central pressure has risen significantly, and the low-level circulation appears to be becoming exposed in the southwestern quadrant. Therefore, the current intensity estimate is likely generous, and Bret appears to be starting a weakening trend already. Bret could maintain its strength for a bit longer if it is able to re-develop deep convection near the center. Reconnaissance aircraft data suggests Bret’s center has been wobbling southwestward this morning, which commonly occurs with weak, sheared tropical cyclones in this region. According to the GFS-based SHIPS guidance, vertical shear is expected to increase further to greater than 25 knots in 24 hours or so, while Bret’s forward speed will increase as well. This should cause Bret to degenerate into a trough of low pressure on Saturday over the central Caribbean, but at the rate Bret is losing organization today, it could be sooner.

A Hurricane Watch is currently in effect for St. Lucia. A Tropical Storm Warning is currently in effect for Dominica, St. Lucia, and Martinique. A Tropical Storm Watch is currently in effect for Barbados, St. Vincent and the Grenadines. The primary threat with Bret in the Windward Islands will be heavy rainfall, but there will be a risk of strong winds on the northern side. Bret is likely to produce 3-6 inches of rain in the Lesser Antilles with isolated amounts of greater than 10 inches possible.

A tropical wave that has been monitored over the tropical Atlantic for the past several days organized into Tropical Depression Four early Thursday morning. As of 11:00 a.m. EDT, Tropical Depression Four was centered near 11.3°N 42.9°W, and was moving west-northwestward at approximately 15 mph. Maximum sustained winds were 30 knots (35 mph), with an estimated minimum pressure of 1007 mb. The low-level center of TD Four, which still appears to be slightly elongated, is currently near the eastern edge of the convective mass. The formation of TD Four is the first such occurrence of two tropical cyclones forming in the tropical Atlantic east of the Lesser Antilles during June. TD Four should continue on its west-northwestward track over the next several days, and is not likely going to affect any land areas. The environment for TD Four appears favorable for some modest intensification during the next 36 hours or so, with wind shear less than 10 knots, and sea surface temperatures of approximately 27-28°C (80.6-82.4°F). After that time, the vertical shear is expected to increase, which should start a weakening trend. A small, weak cyclone such as TD Four is not likely going to be able to survive the shear, so TD Four is likely going to be a moderate tropical storm at strongest. Regardless, the genesis of two tropical cyclones in the Atlantic Main Development Region (MDR) during the month of June is a climatological anomaly.