The core of an old occluded extratropical cyclone was designated at Invest 91L on Monday.  At 8:00 a.m. EDT on Monday the center of Invest 91L was located at latitude 32.7°N and longitude 47.1°W which put it about 1180 miles (1900 km) east of Bermuda.  Invest 91L was moving toward the south-southeast at 7 m.p.h. (11 km/h).  The maximum sustained wind speed was 60 m.p.h. (95 km/h) and there were wind gusts to 75 m.p.h. (120 km/h).  The minimum surface pressure was 986 mb.

A large surface high pressure system near Europe is blocking the northeastward movement of an extratropical cyclone over the North Atlantic Ocean.  The extratropical cyclone proceeded through the normal stages of development and it matured into an occluded low during the past few days.  The occluded low has been nearly stationary over the Atlantic Ocean during that period.  Drier air wrapped around the western and southern sides of the low and the occluded front was pushed east of the core of the circulation.  More showers and thunderstorms formed closer to the center of circulation and a well defined, tightly wound circular center is apparent on the visible satellite images this morning.  The appearance of the tight circular center prompted the designation of Invest 91L.

Invest 91L is in an environment that would not be favorable for the classical development of a tropical cyclone.  It is over water where the Sea Surface Temperature (SST) is around 20°C.  However, it is possible for a transition to a tropical or subtropical cyclone to occur over colder SSTs if the air aloft is also cold enough to allow thunderstorms to develop.  It appears that there may be enough instability to allow for thunderstorms to form near the core of Invest 91L.  There is currently a cold cut off low associated with the occluded surface low over the top of Invest 91L.  Cold temperatures in the upper levels mean that Invest 91L is still an extratropical cyclone.  However, the upper low is also rotating in the same direction as the surface low.  So, there is not very much wind shear over the top of Invest 91L.

Invest 91L is forecast to move slowly southward during the next several days.  As it moves south, Invest 91L will gradually move over warmer water.  If more thunderstorms develop around the center of circulation and a warm core starts to develop, Invest 91L could be classified as a subtropical or tropical cyclone.  Since the maximum sustained wind speed is 60 m.p.h., Invest 91L could be classified as a subtropical or tropical storm if that happens.

The large surface high pressure system is forecast to continue to block the movement of Invest 91L.  Invest 91L is forecast to moved slowly toward the south or south-southeast during the next day or two.  The system could meander over the central North Atlantic Ocean during much of the week.