Hurricane Gert transitioned into a post-tropical cyclone Thursday afternoon after its circulation became very elongated and became associated with frontal boundaries. As soon as Gert transitioned into an extratropical cyclone, however, Potential Tropical Cyclone Nine developed into Tropical Storm Harvey east of the Windward Islands. Harvey is expected to approach the Windwards as a tropical storm, and tropical storm warnings have been issued for parts of the area. East of Harvey is Invest 92L, which has a high chance to develop into a tropical cyclone before conditions are expected to become unfavorable. However, conditions for 92L could become a little more favorable south of the Bahamas if it manages to survive the harsh environment and avoids land interaction with Hispaniola and Cuba.

Farewell, Gert

As of 5:00 p.m. EDT Thursday, Post-Tropical Cyclone Gert was centered near 44.8°N 46.0°W and was moving east-northeast at 39 mph. Maximum sustained winds were 55 knots (65 mph), with an estimated minimum pressure of 985 mb. What was previously Hurricane Gert is now just an elongated extratropical cyclone with some deep convection associated with frontal boundaries. Gert will not regenerate, and is expected to slowly spin down over the next few days over the cold waters of the Northern Atlantic.

Tropical Storm Harvey is born east of the Lesser Antilles

A low pressure system (Invest 91L) was designated Potential Tropical Cyclone Nine by the National Hurricane Center (NHC) on Thursday morning. An Air Force Reserve Hurricane Hunter aircraft investigated the disturbance this afternoon, and found a sufficiently well-defined center to designate it Tropical Storm Harvey. Harvey is the eighth named storm of the 2017 Atlantic hurricane season, and is the earliest “H” storm to form in the Atlantic since Helene in 2012. The average eighth named storm in the Atlantic does not form until September 24.

As of 8:00 p.m. EDT, Tropical Storm Harvey was centered near 12.9°N 56.5°W, and was moving westward at about 18 mph. Maximum sustained winds were 35 knots (40 mph), with an estimated minimum pressure of 1005 mb. Harvey is not very well organized, with the center over the eastern edge of the main convective mass due to some moderate easterly wind shear. Harvey is likely to approach the Windward Islands as a Tropical Storm. Tropical Storm Warnings have been issued for Barbados, Martinique, St. Lucia, and St. Vincent and the Grenadines. A Tropical Storm Watch has been issued for Dominica. Residents of these islands should closely monitor the progress of Harvey. Harvey is then expected to move into the Caribbean Sea, and could pose a possible long-range threat to Nicaragua, Honduras or Belize.

In the short term, significant strengthening of Harvey appears to be unlikely. Although sea surface temperatures are warm, near 29°C (84.2°F), moderate easterly wind shear of about 15 knots and mid-level relative humidity values of less than 50 percent. In addition, the cyclone’s fast forward speed could limit organization. In fact, it is possible that Harvey could degenerate into an open tropical wave over the Eastern Caribbean Sea. This outcome is suggested by the ECMWF and GFS global models. The ECMWF model does suggest, however, that Harvey could redevelop in the Bay of Campeche if that scenario occurs. By Sunday or so, wind shear around Harvey is expected to decrease while the cyclone moves over warmer waters and into an increasingly moist environment. Regeneration or strengthening is possible at that point. The official NHC forecast shows Harvey being very close to the Honduras coast by Monday at near hurricane strength, although this is highly uncertain. The 18z HWRF model run is very similar to the official NHC forecast. The intensity guidance is split between showing a weak cyclone and a low-end hurricane. Harvey is not likely to pose a threat to the United States.

92L likely to develop, but will it survive the unfavorable conditions?

Another area of low pressure, located to the east-northeast of Harvey (Invest 92L) has a high chance of developing into a tropical depression during the next 48 hours. NHC gives this disturbance a 70 percent chance of development within both the next 48 hours and five days. As of 00:00 UTC Friday, Invest 92L was centered near 16.0°N 46.4°W, and was moving west-northwestward at about 20 mph. Maximum sustained winds were 25 knots (30 mph), with an estimated minimum pressure of 1009 mb. Convection has recently begun to burst over 92L, but it is not clear of this convection will persist. The ASCAT instrument unfortunately missed 92L, and it is currently inconclusive if a well-defined circulation exists. Global models appear to support some slight development of 92L through the weekend. 92L is expected to encounter strong wind shear this weekend associated with a Tropical Upper Tropospheric Trough (TUTT), and this is likely to weaken or perhaps even cause 92L to dissipate. If 92L manages to survive the TUTT, then it could encounter more favorable conditions south of the Bahamas, with lower wind shear and increasing moisture. It is possible, though, that land interaction with the Greater Antilles could suppress development. The UKMET model is the most aggressive and predicts that 92L will be over the Florida Straits as a tropical storm in about seven days. The GFS and ECMWF models do not show much development with 92L at this time, however, many GEFS ensemble members predict that 92L could be a hurricane over the Eastern Gulf of Mexico. It is too soon to determine what impacts 92L will have on the United States, if any. The Leeward Islands, however, should monitor the progress of 92L.

Wave behind 92L becoming less likely to develop

A tropical wave located over the Eastern Atlantic is producing only limited shower and thunderstorm activity. The NHC gives this wave a near zero percent chance of development within 48 hours, and a 30 percent chance of development within five days. The ECMWF model is only supporting some slight development with this wave, and development is becoming less likely. This wave does no imminent threat to land.

The next two names on the Atlantic naming list are Irma and Jose. I will be back tomorrow for another post.