Development of Tropical Depression Keeping South Florida on Watch

The evolution of Tropical Depression 13, from disorganized clouds two days ago to a better defined rotation Thursday, has ended up putting it at a latitude that lessens the threat to Puerto Rico and Hispaniola.

Both islands have been left out of the “cone of concern” from the National Hurricane Center, and practically all models take it along a route to the north of the Antilles. That would mean less impact because the south side of the system is weaker than the north.

At the moment, there is a Tropical Storm Watch for the Leeward Islands. Rainfall accumulations of up to 6 inches are forecast in Puerto Rico, but the potential for tropical storm winds is low. If it comes too close, obviously the effects will be greater.

We also have to take into account the obstacles that the depression faces. There is dry Saharan air to the north that could infiltrate and weaken the disturbance. There will also be shearing winds in the middle and upper levels of the atmosphere when it is near the northeast Caribbean.

That is why the European model insists that it would only be a tropical wave when passing to the north. The American GFS model takes it as a depression or weak storm near or north of Puerto Rico. The specialized models project a stronger system, but suffer from an overzealous intensity bias for tropical organisms in this area.

My attention begins to pivot to the Bahamas and Florida. The hostile conditions that should inhibit the intensity of the depression when it’s near Puerto Rico would not be present in the waters between the Greater Antilles and the Bahamas.

Many track forecasts currently maintain the center of a potential storm not over the Greater Antilles, but over that super-hot stretch of ocean to the north, eliminating topographic effects and accentuating the energy that a tropical cyclone can acquire from the ocean. Wind shear would disappear.

In other words, if there is a viable storm in the Bahamas or north of Cuba, the intensity could suddenly skyrocket. It is difficult to predict at present if the possible future trajectory would take it near the Florida Keys or over the peninsula.

It should be understood that not all models project that there will be a viable storm this weekend – the European’s operational run keeps it as a wave over Cuba but some members of its ensemble do elevate it to a tropical cyclone in waters north of Cuba. All other operational models from last night carry a tropical cyclone somewhere between the Keys and southeast Florida by Monday night.

These will be the key moments: does the depression survive dry air and wind shear when it is near the northeast Caribbean? If it doesn’t, does it cross the Greater Antilles as just a tropical wave?

If it survives, what track does it take? One further south passes over the Straits of Florida. Further north it leads to a direct hit from what would likely be a hurricane in southeast Florida.