Invest 93L May Become Subtropical/Tropical Storm Alberto This Weekend

Invest 93L:

The feature off the coast of the Carolinas I talked about yesterday has been tagged as Invest 93L this morning. In case you don’t know, areas of interest that may become tropical cyclones are tagged with the name “Invest” and then a number with a following letter that is linked to the oceanic basin the disturbance is in. In the Atlantic Ocean the letter is ‘L’ and the numbers cycle 90-99. 93L is the fourth invest of the 2012 Atlantic Hurricane Season.

As I write this blog, the National Hurricane Center has just raised the chances of 93L becoming a tropical cyclone over the next 48 hours from 20% to 50%. Current satellite imagery shows that 93L is firing convection primarily in the NW quadrant of the circulation:

Visible Satellite Image of 93L

The circulation is experiencing 20 knots of shear from the SW, however the main reason the convection is so lopsided is due to dry air entrainment from the SW as seen on water vapor imagery:

Water Vapor Image of the Eastern US

Also, radar imagery from Wilmington, North Carolina shows 93L has a well developed circulation with banding in the northern semicircle:

Radar Imagery from Wilmington, North Carolina

As you can see, there is even an eye-like feature where the center of circulation is clearly visible, although it is not actually the same as the eyes we see in hurricanes.

So as many of you are wondering, why is this not being called a tropical cyclone already? It has a closed surface circulation and has consistently been generating convection, which would normally satisfy the criteria for being declared a tropical cyclone. Well I will now attempt to simply a complex process in meteorology explaining why 93L is not being called a tropical cyclone at this time.

The reason is, 93L is not generating its energy from the same method as normal tropical cyclones do: the warm waters of the ocean.

Tropical cyclones are called “warm-core” cyclones because they use the heat stored in the tropical oceans to drive their powerful winds through a series of processes. 93L is currently getting only some of its energy from this method. At the moment it is somewhat of a “cold-core” cyclone. 93L originally got its energy from the horizontal temperature differences due to the trough that just moved through the Eastern United States. We call this a “baroclinic” environment in meteorology.

Here was an earlier cross-section of Invest 93L showing that the core of the storm was still “cold-core.”

Cross-section of 93L

In the above diagram, 93L is located where the vertical red line is. Notice that there is not a positive temperature anomaly where 93L is located, meaning that the cyclone is not warm-core yet, or at least at the time of this analysis at 10Z.

However, 93L is currently moving over the Gulfstream, which is a very warm current of water. The Gulfstream is currently warm enough to support the development of a tropical cyclone. As 93L fires off convection, it is taking the latent heat from water vapor that evaporated from the warm Gulfstream waters. This latent heat is released when the water vapor condenses again in the form of clouds in 93L’s circulation. The heat released drives a feedback loop in which the warm air from the latent heat that was released, rises. As the air rises, it lowers the pressure at the surface of the cyclone. As a result of this, surrounding air at the surface wants to replace the air that is rising and the pressure continues to lower if convection can continue to fire and intensify.

This is what currently is happening with 93L. Convection has been firing, and 93L is in the process of developing a warm-core. If a warm-core is established, the National Hurricane Center will classify Invest 93L as either Tropical Depression One or Tropical Storm Alberto. The NHC may use the label “Subtropical Storm” if they feel the core is not completely warm based. Recent buoy and ship reports suggest that 93L probably already has tropical-storm force winds (>35 knots).

Onto the track forecast for 93L.

93L is currently in an area of week steering currents. The background steering currently favors a slow drift to the SW:

Steering Currents

As you can see there is a ridge (high pressure system) to the NW of 93L that is pushing 93L to SW. Computer models are indicating that the ridge will get eroded by a trough moving in from the NW over the next few days. This will allow 93L to complete a cyclonic (Counter-clockwise) loop and move back to the NNE, roughly paralleling the shore. This NNE movement is very tricky since any deviation either W or E will cause 93L to move possibly onshore or on the other hand, farther from the coast. If you live anywhere on the East Coast from South Carolina northward, you need to closely monitor the progress of Invest 93L.

The intensity forecast for 93L is even trickier. I currently believe the computer models do not have a good handle on the delicate process of 93L becoming a warm-core cyclone. A well-used statistical model, the SHIPS, brings 93L up to a 65 mph tropical storm. 93L will have to battle wind shear and dry air throughout its lifetime, so I think this is a reasonable forecast. I would put the chances of 93L becoming a hurricane relatively low at this time (<10%).

Regardless of whether or not 93L develops a warm-core and becomes a tropical cyclone, the main threats will remain the same: heavy rainfall and possibly tropical-storm force winds. Stay tuned to the NHC and your local forecast office for more information on the storm. Remember that the National Hurricane Center is THE authority on tropical cyclones.

Folks hurricane season is here and now is the perfect time to make sure you have your supplies as well as your family plan in case of an emergency.

I will update again when conditions warrant.



One response to “Invest 93L May Become Subtropical/Tropical Storm Alberto This Weekend

  1. Pingback: The Tale of Two B’s | CaneFever·

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