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  • Writer's pictureBryan Norcross


The large tropical disturbance we’ve been watching has finally organized into Tropical Storm Isaias. The system is lopsided, with the strongest winds and thunderstorms to the northeast of the new circulation. Today the system will pass over or near the tall mountains of the Dominican Republic, which should disrupt the organization and strengthening process.

When the system passes over those 10,000-foot mountains on Hispaniola, it’s uncertain what will emerge on the other side. The consensus of the computer forecast models is that a fairly weak circulation will reform north of the islands and move toward the Bahamas.

There’s also a possibility that the disorganized system will track farther west over or near the Dominican Republic, Haiti, and Cuba. Based on the latest computer forecast models, however, that now appears to be the less likely scenario. Although a number of the models still predict that the system will never get very strong.

It is most likely that Isaias will track in the general direction of South Florida. The big question is how strong it will be when it gets near here.

If a somewhat organized circulation emerges into the Atlantic north of Haiti late tonight or early tomorrow, there should be a window of time that it could strengthen as it moves through the Southern and Central Bahamas. The window would seem to be short, however.

Dry air from the Saharan dust layer is moving along with the tropical storm, and is forecast to surround the storm, especially on its west side. If the upper level winds over Florida and the Bahamas develop as forecast, they will try to inject that dry air into the circulation. That should weaken it from whatever peak strength it achieves during its window of opportunity, or at least keep it from getting stronger.

The National Hurricane Center reflects these factors in their forecast, showing a leveling off of Isaias’s intensity when it is in the vicinity of South Florida early Saturday.

It still could cross over into hurricane intensity. There are forces that will work to strengthen the storm and opposing forces weakening it in play at the same time. It’s too close to call. Plus or minus 5 or 10 mph in the wind speed is impossible to predict, but that would make the difference in this case whether it’s called a hurricane or not. Will the peak winds cross 75 mph?

In the end, the structure of the storm will likely be more important than the peak wind speed when it is near the Florida coast.

There are a lot of moving parts here. Each one has a measure of uncertainty and unknowability. How the storm interacts with the mountains is essentially a random event. And the weakening factors as it approaches Florida are based on an accurate forecast of the atmosphere, although they seem reasonable based on what we can see.

If these factors come together, the storm will be very asymmetric when it’s close to us. Most of the rain and wind would be on the north and east side – perhaps well removed from the center of circulation. That means it’s important not to concentrate on the exact track of the center. Remember, that’s what the cone is showing – where the central point in the circulation is forecast to track.

The current thinking is that the worst weather would be over and outside the east side of the cone, even with the center tracking down the middle.

This means, that a track offshore of South Florida, even nearby, would not bring much bad weather onshore. But a track over the middle of the peninsula or to our south could bring an extended period of heavy rain and gusty wind.

But again, this thinking is dependent on the forecast for the dry air and upper winds being correct.

All we can do at the moment is wait to see the state of the system after if interacts with the mountains on Hispaniola. Tomorrow will be our day to prepare whatever seems necessary.

If the bad weather does move in, it could start as early as late tomorrow night. The storm will slow down when it is in our vicinity, but it won’t be very big, so it should be past us by late Saturday. If it tracks to our west, however, we would be on the wet side, and the rain would likely continue into Sunday at least.

Stay informed. Things will develop quickly tomorrow.

After the storm passes South Florida, it could turn into a threat to the Mid-Atlantic and Northeast coast. The same upper-air system that is forecast to push dry air into the circulation when it’s near us, might also propel it north where it could find more conducive conditions for strengthening on the way toward the Mid Atlantic.

Everyone along the East Coast should plan to stay informed through the weekend into early next week.

Elsewhere in the Atlantic, there is another system to watch near Africa, but no threatening development is expected for the next several days.


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