• Bryan Norcross

A STORMY PERIOD WITH POSSIBLE FLOODING RAIN IS FORECAST FOR SOUTH FLORIDA

The strong consensus of the computer forecast models is that a storm is going to form in the Caribbean on Friday or Saturday out of the remnants of Ex-Hurricane Eta. The system will get grabbed by a sharp dip in the jet stream over the Gulf and be lifted, along with gobs of tropical moisture, toward South Florida.

The moisture will precede the storm, with a notable increase starting late today. First the front that brought the cool, breezy weather early in the week will be pulled back north. That will be a focus for periods of heavy rain. Then, slowly, the new storm will migrate north toward Cuba and into the general vicinity of South Florida over the weekend.

It’s already quite breezy at the coast unrelated to the circulation around Eta. The wind is caused by the pressure difference between Eta to the south and high-pressure to the north. The air gets squeezed in between the two systems like toothpaste from a tube.


If Eta’s center tracks fairly close to South Florida late in the weekend or early next week, then we’ll get some extra wind from the actual circulation of the storm. That’s iffy because the exact track is uncertain, and it’s most likely to happen in the Keys.

It is important not to look at the National Hurricane Center cone or other forecast tracks of where the center of the storm may go. Because the storm will be involved with the upper-level jet-stream dip, the worst of the weather is likely to be well displaced from the center, which is what the cone is tracking. Eventually the storm may loop and wiggle across the Gulf in impossible to predict ways.

Even when Eta’s center is well south of the state, bands of heavy rain in gusty squalls can be expected in South Florida.

The timing of all of this is an open question. As is the exact track. Most forecasts, including the official National Hurricane Center forecast, keep the storm in the general vicinity of Cuba and South Florida into early next week.

There is a mixture of positive and negative atmospheric factors influencing the system. It is expected to be big, a legacy of the broad mid-level circulation associated with Hurricane Eta. Broad systems take longer to spin up because more air has to get moving. On the other hand, the atmospheric environment and the seawater temperatures would seem to support strengthening.

The net is that moderate strengthening is forecast, so Eta is expected to be a robust tropical storm when it is in our general vicinity with a broad area of noticeably high winds. The National Weather Service is forecasting 7 to 10 inches of rain to fall in the next several days over the Keys with similar amounts over metro South Florida.

The nature of the system that will develop in the Caribbean is also an open question. Because the system will likely be wrapped up in the upper-level jet-stream dip when it’s near Cuba and South Florida, and it will be deriving some of its energy from the jet stream in addition to the warm ocean water, it may technically meet the definition of a subtropical storm.

So don’t be surprised if it’s called Subtropical Storm Eta if and when it regenerates. The status isn’t important to the weather effects of wind and heavy rain, however.

We probably won’t have any more confidence in the forecasts until late tomorrow when the system is forecast to be back over the Caribbean waters. But it’s time to think about securing boats and outdoor items that could be caught in gusty winds, especially in the Keys and on the extreme southeast coast.

As a rule, we don’t put up shutters for tropical storms. We do, however, bring in items that can easily blow around. Use your good judgement.

While we wait for Eta’s remnants to move over the Caribbean, it continues to dump multiple feet of rain over the mountains of Central America. It’s hard to imagine that a catastrophe is not underway in those countries.

For South Florida, plan on an extended period of wind with periods of heavy rain. This is not a situation where the center of the storm will make a huge difference. It’s a mass of moisture coming north, with a windy circulation embedded in it. Don’t be misled by looking at the cone.

© 2019 by Bryan Norcross Corporation

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