12 September 2019

Disturbance near Bahamas slowly getting organized

The broad, disorganized tropical disturbance near the eastern Bahamas that I mentioned yesterday is still centered near the eastern Bahamas but is closer to reaching tropical depression status.  It has maintained thunderstorm activity around it, but seemingly lacks a defined surface circulation. A reconnaissance aircraft will fly through it later today to determine if it has a surface circulation -- it would get upgraded to Tropical Depression Nine or Tropical Storm Humberto if it does.

The National Hurricane Center is giving it a 70% chance of formation by Saturday, somewhere over the Bahamas or south-to-central Florida.  It is technically referred to as Invest 95L now, and could be renamed Potential Tropical Cyclone Nine very soon (if it's not upgraded to a bona fide tropical cyclone). There is actually no difference between the two as far as intensity is concerned.  Both an Invest and a PTC are pre-depression disturbances. The difference is that NHC can issue tropical storm watches and warnings for a PTC but not for an Invest.  Basically, it gives them the ability to start advisories on a disturbance because it could impact land with tropical storm conditions within 48 hours.  This ability and the PTC designation were introduced in 2017.

Model guidance has unfortunately become more cloudy since yesterday.  On Wednesday, there was wide agreement on a westward track into the Gulf of Mexico. That is still a possibility, but more models have started to indicate a northward turn before or around reaching Florida... completely avoiding the Gulf.  Not shown on the map below are the GFS and ECMWF ensembles; the entire GFS ensemble keeps 95L weak and tracking into the northern Gulf, while about 85% of the ECMWF ensemble members strengthen it and turn it north before reaching Florida. This large amount of uncertainty just 2-3 days out is frustrating and makes preparation difficult for people.  It also makes thinking about 4-5 days down the line next to impossible.

Through the weekend, we would not be looking at big impacts in the Bahamas or Florida, even if it does marginally develop. The biggest hazard would be heavy rain, and for a low-end tropical storm there would be some gusty winds. These hazards reaching into Florida are dependent on how far west it tracks.

Much further east, the tropical wave that left the African coast back on the 10th is still a feature of interest. It has a hostile environment ahead of it for the next several days, but is favored by long-range model guidance to start to develop in the vicinity of the Lesser Antilles around Monday.  The latest ECMWF ensemble shows a generally-weak system moving through the Caribbean with lows scattered from the central Caribbean to the far eastern Bahamas next Wednesday.

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