As we near the peak of hurricane season, the tropical Atlantic and eastern Pacific are both quite active today. Tropical Storm Florence over the eastern tropical Atlantic has weakened slightly this morning as its low-level circulation has become exposed.  Invest 91L, currently located near the Bahamas, is likely to develop into a tropical cyclone over the eastern Gulf of Mexico in a couple days. In the eastern Pacific, Hurricane Norman has re-attained Category 4 hurricane status, while Tropical Storm Olivia remains weak and poorly organized.

As of 11:00 a.m. EDT Sunday, Tropical Storm Florence was centered near 17.0°N 33.2°W, and was moving west-northwestward at about 18 mph. Maximum sustained winds were 45 knots (50 mph), with an estimated minimum pressure of 1000 mb. After a period of strengthening late last night and early this morning in which Florence reached maximum sustained winds of 50 knots (60 mph), Florence has become less organized this morning with the well-defined circulation center now exposed to the south and west of the deep convection. Moderate wind shear, which was not well forecast by the SHIPS guidance, appears to be the cause of the weakening. Florence is likely to maintain intensity, or perhaps weaken some more, over the next few days since it is headed towards even stronger wind shear of above 20 knots (25 mph). In fact, it is not out of the question that Florence becomes a remnant low at some point over the next few days, though this appears unlikely at this time since Florence’s low level circulation is quite vigorous. Marginal sea surface temperatures of about 26°C (78.8°F) are also likely to limit intensification for the next couple days. In about five days or so, wind shear is expected to drop to around 10 knots while sea surface temperatures warm to above 28°C (82.4°F), and after that time, Florence is likely to re-intensify or regenerate. This is shown by the GFS and ECMWF models, which strengthen Florence into a powerful hurricane in the western subtropical Atlantic. Although it appears unlikely at this time that Florence will affect the United States, some recent ECMWF runs have showed the subtropical ridge restrengthening in about 10 days, which could push Florence more to the west. The GFS model continues to show Florence recurving east of 60W, but it appears to erroneously show Florence stronger in the short term. Bermuda will also need to closely monitor the progress of Florence as well. Florence is likely to be around for ten days or longer.

91L likely to develop over the eastern Gulf of Mexico in a couple days

As of 12:00 UTC Sunday, Invest 91L was located near 21.8°N 75.5°W, and was moving slowly west-northwestward. Maximum sustained winds were 20 knots (25 mph), with an estimated minimum pressure of 1012 mb. 91L consists of a tropical wave interacting with an upper-level trough. The showers and thunderstorms associated with 91L are gradually becoming better organized, though tropical cyclogenesis is not imminent. Little development is expected today due to strong upper-level winds, but wind shear is likely to start to relax tomorrow as 91L approaches southern Florida. By Tuesday, 91L is likely to begin organizing as it emerges into the eastern Gulf of Mexico. Although environmental conditions are expected to be favorable for development in the Gulf of Mexico, it is unlikely that 91L will become anything more than a tropical storm since it only will have about 36 hours over water before moving inland over Mississippi or southeastern Louisiana. The 00z Sunday GFS, UKMET and ECMWF models all showed development of 91L, but all kept it fairly weak. The ECMWF shows a more westward component of motion after landfall, with 91L or its remnants moving westward towards Texas. GFS does not show this, bringing 91L’s remnants towards Arkansas. Residents along the Gulf coast will need to closely monitor 91L’s progress over the next few days. The National Hurricane Center (NHC) gives 91L a 50 percent chance of development within 48 hours, and an 80 percent chance within five days.

Norman regains Category 4 strength, TS Olivia currently disorganized but expected to strengthen

In the eastern Pacific basin, Hurricane Norman has regained Category 4 strength. As of 8:00 a.m. PDT (11:00 a.m. EDT) Sunday, Hurricane Norman was centered near 17.6°N 129.1°W, and was moving westward at about 18 mph. Maximum sustained winds were 115 knots (130 mph), with an estimated minimum pressure of 948 mb. Norman has unexpectedly restrengthened after being a Category 2 hurricane early this morning, with a well-defined eye appearing surrounded by a ring of very deep convection. Norman is likely to begin to weaken again soon as it heads for cooler waters. The model guidance is in good agreement that Norman will turn to the northwest before approaching the Hawaiian islands, and impacts to the islands appear unlikely at this time.

To the east of Norman, Tropical Storm Olivia, the fifteenth named storm of the very active Pacific hurricane season, was named early Sunday morning. As of 9:00 a.m. MDT (11:00 a.m. PDT), Tropical Storm Olivia was centered near 17.0°N 112.4°W, and was moving northwestward at about 7 mph. Maximum sustained winds were 40 knots (45 mph), with an estimated minimum pressure of 1002 mb. Olivia is currently a sheared tropical cyclone with nearly all of the deep convection to the south of the center. Environmental conditions are expected to become more conducive for strengthening, and Olivia could become a hurricane by Tuesday as it moves west-northwestward. Olivia is not likely to pose a threat to any land areas.

I will be back with another post tomorrow.