[Tuesday Evening] Franklin Impacting Mexico; Watching Invest 99L for Development in Central Atlantic

   Posted by Levi at 12:52am on August 9, 2017

Tropical Storm Franklin has emerged in the Bay of Campeche this evening with winds of around 40 mph, after crossing the Yucatan Peninsula. Core convection has declined, as is typical for a storm moving over land, but during the hours before sunset, convection appears to be increasing again in the southern semicircle. An ongoing recon mission into Franklin recently measured a central pressure near 996mb. Tropical storms usually require half a day or longer to recover after crossing the Yucatan, and Franklin is likely no different. However, some storms recover faster than others, and this makes the expected amount of restrengthening somewhat uncertain. The environment ahead of Franklin is generally favorable for intensification, with light northerly shear the only potential limiting factor during the next 2 days. The latest official NHC forecast expects Franklin to be near hurricane intensity at its second landfall in Mexico on Thursday morning, and a Hurricane Watch is in effect along the southwestern gulf coast of Mexico. For further details, see the latest public advisory from the National Hurricane Center.

Invest 99L, a well-defined tropical wave east of the Caribbean, continues to be monitored for development. The wave remains elongated and impacted by vertical wind shear, and any development is likely not imminent. However, the wave’s envelope is moist enough to support moderate convection, and 99L’s robust flow signature means that it will survive for the next few days as it travels WNW or NW into the region north of Puerto Rico. An upper low is currently located to the north of 99L (see the GFS analysis below), and model forecasts indicate that this low will interact closely with 99L over the next couple of days, with the two features potentially becoming vertically stacked. The ECMWF and GFS models then disagree significantly on the evolution after that point. The ECMWF generates deep convection over the wave axis, eroding the upper low and developing upper-level outflow, ultimately resulting in the formation of a tropical storm. The GFS, on the other hand, generates less convection, and maintains the upper low for longer, preventing development of 99L. It is not yet clear which model is correct about this interaction. However, given the wave’s well-defined signature and the presence of moderate convection already, it would not be surprising to see some development of the system over the next several days. The NHC currently gives 99L a 40% chance of becoming a tropical cyclone during the next 5 days. The disturbance is currently expected to track to the north of the Caribbean and NE of the Bahamas, though there will be a little extra uncertainty in this forecast while a closed low does not exist along the wave axis.

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