Subtropical Depression Four developed southeast of Cape Cod, Massachusetts on Monday afternoon. At 5:00 p.m. EDT on Monday the center of Subtropical Depression Four was located at latitude 38.2°N and longitude 65.7°W which put it about 310 miles (495 km) southeast of Cape Cod, Massachusetts. The depression was moving toward the east-northeast at 9 m.p.h. (15 km/h). The maximum sustained wind speed was 35 m.p.h. (55 km/h) and there were wind gusts to 45 m.p.h. (75 km/h). The minimum surface pressure was 1009 mb.
The structure of a previously non-tropical low pressure system southeast of Cape Cod evolved into a more symmetrical circular shape on Monday. Bands of showers and thunderstorms revolved more closely around the center of the low. The low pressure system exhibited a more tropical like structure on visible satellite imagery. However, the National Hurricane Center (NHC) observed that the surface low was under an upper low which meant the system had a cold core in the upper troposphere. So, NHC designated the system as Subtropical Depression Four when it initiated advisories on Monday afternoon.
The circulation around Subtropical Depression Four had a hybrid structure with a tropical like appearance in the lower levels and a non-tropical cold core in the upper levels. The depression will move through an environment favorable for intensification during the next 12 to 18 hours. The core of the depression will move near the northern part of the Gulf Stream where the Sea Surface Temperature is near 26°C. It will move near the south side of the core of the upper low where the winds are weaker. Those winds will cause some vertical wind shear, but the shear will not be too strong during the next 12 hours. The depression could strengthen into a subtropical storm. There is a chance that the circulation could develop a warm core in the upper levels if the depression stays near the Gulf Stream. The warm water in the Gulf Stream and the cold air in the upper troposphere will enhance instability which will contribute to the development of thunderstorms. If thunderstorms release enough latent energy in the upper troposphere, then a warm core could form and the system could make an additional transition into a tropical storm.
Subtropical Depression Four is likely to move in tandem with the upper low during the next day or two. The upper low will move toward the east-northeast on Tuesday and then it will move more toward the northeast on Wednesday. On its anticipated track Subtropical Depression Four will move away from the northeast U.S. The depression will remain south of Nova Scotia, but it could pass near southeastern Newfoundland on Thursday.