94L and 95L

(NOTE: I would like to apologize for the spacing in this post. The blogger system doesn't want to cooperate and space properly, so this will have to be how it is presented.)

Fay is still the headline story, but the main threats and track are fairly understood at this point. Fay will move westward, possibly over water again today, hold weak tropical storm strength, and then begin dissipation over Mississippi or Louisiana. The storm will produce heavy rainfall through Thursday...at least. This heavy rainfall will lead to flash flooding and river flooding in the areas that receive the heaviest rainfall. However, other tropical stories are currently playing out in the Atlantic, and this update is going to touch on these.

Currently there are two potential tropical systems, 94L (Red circle on water vapor below) and 95L (Blue circle). Each system currently contains robust convection, much more than yesterday, and are in areas of little shear. Both have a weak cyclonic rotation, but no closed surface low. This discussion will go into each system's potential and try to come to some sort of conclusion or prediction.

Water Vapor Imagery (Red: 94L, Blue: 95L)

Current Shear (Low in Tropical Development Region)

850 mb Vorticity (Highest Orange and Yellows)

95L - The Little Guy

Needless to say by the title, I am not quite as worried about this system as 94L. Yes, broad circulation is currently seen on the visible satellite image and convection has increased today, but its location doesn't make me as nervous as 94L's does. It is currently near 20N, 49W which is north of the islands. The system is currently in an area with 5 knots of shear, which is favorable for development, but continued westward motion will march it directly into 30 knot shear, which would destroy the system. The SHIPS model forecasts shear to stay below 10 knots until 72 hrs, where it only briefly brings it to 17 knots and then back to 5 knots. This seems a bit under estimated, considering the high shear environment to the west, which likely explains why the SHIPS strengthens 95L to a 85 knot storm in 120 hours while most models dissipate the system. The trough which is producing the high shear should pull out some, but not completely over the next 120 hours.

Beyond the shear, 95L has to deal with a very dry environment. Dry are is surrounding 95L to the east, north, and west. Even if the storm were to start getting its act together, the system would likely entrain this dry air which would help to weaken it once again. Currently a surface circulation is not visible, and not present on QSCAT images either, so the prospects of rapid strengthening or development look unlikely. 850 mb vorticity is also very low in the vicinity of 95L.

QSCAT for 95L: No Circulation Noted

Considering all of the negative issues with 95L, I would give it a very low probability of development, which is about what the NHC is currently saying (Low Chance). Even if it did develop, the track of 94L tells me that 95L would interact with 94L resulting in an even greater chance of the system not developing. Below are the current models for 95L, with the Canadian (Bold Green Line) being the only 'reliable' tropical model currently forecasting 95L to develop.

Model Tracks (Bold Green: Canadian Model)

94L: The Threat

94L is currently located near 12N, 57W which is very close to the continent of South America. Normally when a system is located in this region it would be considered a non-threat because it would interact with South America, entraining drier air from the interior of the continent, killing the circulation. However, looking at the mid-level steering maps, the system looks like it will be steered to the north and west over the next few days. This would bring it away from South America giving it a much better chance of development.

Steering Currents

Additionally, the highest water temperatures and tropical cyclone heat potential waters in the deep Atlantic are located just off of the South American coast, which likely indicates why such explosive thunderstorm development has occurred in the last 12-24 hours. Low shear is present over the system, and low shear continues (and is forecasted to continue) throughout the Caribbean and into the Gulf of Mexico. The SHIPS model predicts shear to stay at 10 knots or lower for the next 120 hours, which this time seems reasonable, and develops the system into a 83 knot storm, again reasonable for the shear forecast. 850 vorticity is the highest, in the entire tropical Atlantic (besides Fay) in the region around 94L and strong upper-level divergence and lower-level convergence is already present with the system. However, one major factor has not yet come together for 94L, and that is a low level circulation.

The QSCAT images currently show a low level wind shift boundary, but no closed circulation with 94L. This wind shift boundary is a starting point, and if convection can continue to occur, a surface low could develop along this boundary with time. However, with 94L so far south, the Coriolis force is not quite as strong as if it were at 15N (or further north). Thus, as the system follows the steering currents to the northwest towards the islands, the Coriolis force will increase, helping to spin up a system in a few days. Of course, if convection were to subside before the system can move north, this would reduce the chances of a surface circulation, but the low level wind shift boundary should continue to enhance thunderstorm development.

QSCAT for 94L: Notice the boundary parallel to the South American coast.

Currently 30 knot surface winds have been detected by QSCAT, which would be strong enough to designate the system a tropical depression if a surface low was present. Thus, whenever this system gets a low level closed circulation, we will see a tropical depression designated. The system will then track northwest into the Caribbean towards Hispaniola, as forecasted by the European (Red Line) and Canadian (Green Line) in the image below. Some of the less reliable models forecast 94L to move more westward, but this is most likely only if 94L stays weak. A trough will weaken the ridge to the north of Hispaniola over the next 72-96 hours, which should allow 94L to drift north before the ridge re-builds. I think 94L should get near Hispaniola before the ridge builds back in and forces it back westward. The system will act much like Fay did over Florida when it stalled out and moved back towards the west.

94L Model Tracks (Red Line: European, Green Line: Canadian)

78 Hour GFS Forecast

I give 94L a high chance of development, which is a bit stronger than the NHC (Medium Chance). Everything is coming together for a system in the Caribbean, a strong surface boundary is already present, and many of the models are predicting a system to develop from this wave. I think we will likely see a named storm (Gustav) from this system in 2-4 days. It will be a threat to the islands of the Caribbean before moving further west. If the system does reach Hispaniola, it could get weakened by the mountainous island, but should keep its circulation intact to traverse towards Florida or the Gulf as a strong ridge of high pressure will build off of the southeast coast of the United States.

Beyond 94L and 95L:

The 'wave train' that often occurs this time of year is starting to come together. The Cape Verde season will be very active over the next two to three weeks, with two strong waves already present over Africa (shown in the satellite below). Many of the models, including the Canadian, European, and GFS are indicating these waves could develop into tropical systems, so it will be important to watch them carefully over the next few weeks.

Tropical Waves in Africa