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Posted at 11:15 AM ET, 08/20/2012

Very active tropical Atlantic as we enter peak of hurricane season; Isaac next?

As one-time hurricane Gordon and one-time tropical storm Helene move out of the picture, three other disturbances are brewing in the tropical Atlantic.

Let’s begin examining the disturbance known as AL94. Of the active systems, it has the greatest potential to impact the U.S. but its future course and development remain highly uncertain.

AL94 - Isaac???

This disturbance has been trekking westward across the deep tropics since it left the African coast on August 16. It still has not reached the tropical depression stage, and this morning’s satellite presentation is less than impressive. However, it could intensify into a tropical storm in the next couple days, earning the name Isaac.

Visible satellite image from 9:15 a.m. EDT showing the disturbance 1000 miles east of the Lesser Antilles island chain. (NASA/MSFC)
While there is an established circulation and 1010mb low pressure center, there is minimal deep convection (strong thunderstorms) associated with it, which is a key ingredient in its development. It is located 1000 miles due east of the Lesser Antilles and moving west at 20mph. At this rate, it will reach the Lesser Antilles on Wednesday.

Surface winds, pressure, and precipitation from the GFS model valid next Sunday night (NOAA)
The most recent run by the GFS global model has an ominous solution: it places this system as a hurricane right over southern Florida next week.

The path it takes to get there takes it over Hispaniola, then between Cuba and the Bahamas where it picks up in intensity. Of course, models that far out aren’t very reliable, and shift around a bit with each run, but it’s wise to be aware of the possibilities.

Rather than focusing on a single run from a single model, it is common practice to assemble output from various models, and ensembles of those models, monitor their trends and biases, then make a forecast from that information. In the 5-7 day period and beyond, weather details become fairly unpredictable, and with hurricanes, their track affects the intensity, and their intensity affects the track, making it especially complicated.

The plot below shows the track forecast output from several models and ensemble members (the ensemble in this case is made by running the same model with somewhat different initial conditions) through the next five days.
5-day track forecasts from a large number of models and ensemble members, providing an idea of the uncertainty and spread of options given the current state of the atmosphere.
The intensity forecasts in five days ranges from a tropical depression to a category 3 hurricane, so clearly, we need to nail down the track better before we can even begin to think about getting the intensity right. Given the uncertainty, and knowing the potential, anyone in the Lesser Antilles, Puerto Rico, Hispaniola, Cuba, Jamaica, the Bahamas, etc should be watching this system VERY closely.


Over the weekend, Gordon, which began as an easterly wave off the coast of Africa on August 9th, was first declared a tropical depression on August 15, then a tropical storm on August 16, and then a hurricane on the 18th. Gordon was the third hurricane of the Atlantic season, after Chris and Ernesto.

It never got near land, or even the Caribbean Sea... it recurved to the north at 55W longitude and then headed east toward the Azores Islands, briefly reaching category 2 intensity along the way. It made its closest approach to those islands on Sunday evening as a category 1 hurricane. The visible satellite image shown here is from yesterday at 5:15pm local time in the Azores as the storm was zipping by to the south. It continues to weaken as it heads east toward Portugal over the next few days.
Visible satellite image of Hurricane Gordon on August 19th at 1:15 p.m. EDT (NOAA)

Helene and AL95

On Friday evening, the remnants of tropical depression 7 were revived after spending six days as an open wave. Aircraft reconnaissance flights into the disturbance in the southwestern Gulf of Mexico found 45mph winds and a 29.65“ (1004mb) central pressure, so it was upgraded to tropical storm helene.

Visible satellite image of the disturbance in the western Gulf of Mexico from 9:15 a.m. EDT today (NOAA)
It was quite close to land (east of Tampico, Mexico) and would only spend another twelve hours over the water until it made landfall. It dissipated and lingered over coastal Mexico, and now, a new disturbance is brewing near the same location... “borrowing” some of ex-Helene’s energy and taking advantage of a nearby trailing cold front. While worth watching as it drifts northward, it will most likely not be a significant player. Helene was the season’s eighth named storm.


The third disturbance in the Atlantic is an easterly wave that exited the African coast on Saturday. It’s centered a couple hundred miles south of the Cape Verde islands and is still in the formative stage. This is no threat to land any time in the near future, and may not be too quick to develop either.

Seasonal status

Climatologically, we only get to the 9th name on the list on October 4, so if Isaac forms this week, we are way ahead of an average season. As far as hurricanes go, we’ve had 3 so far, and during an average season, the third occurs on September 9.

On the graph below, the climatological seasonal cycle of tropical storms and hurricanes (red) and hurricanes-only (yellow) is shown, with today’s date marked by the vertical green line. You can clearly see that we are just getting into the heart of hurricane season now, and the abundance of activity in the basin normally occurs between about August 20 and October 20.

Annual cycle of tropical cyclone frequency in the Atlantic basin. tropical storm and hurricanes combined are shown in the red curve, while hurricanes alone are shown in the yellow curve (NOAA)

The author, Brian McNoldy, is a senior tropical weather researcher at the University of Miami/Rosenstiel School of Marine and Atmospheric Science. He is a new tropical weather blogger for the Capital Weather Gang.

By Brian McNoldy  |  11:15 AM ET, 08/20/2012

Categories:  Latest, Tropical Weather

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