• Bryan Norcross

IT’S TIME TO PAY ATTENTION TO THE TROPICS BUT A COMPLEX WEATHER PATTERN MAKES FORECASTS UNCERTAIN

There are two disturbances that have our attention. Disturbance #1 has moved into the Caribbean, and a southern extension of the system extends south over Venezuela. It is producing heavy rain and gusty winds on the southern Caribbean islands and over the continent. Disturbance #2 is of more concern to Floridians because of its location in the ocean and the variety of computer forecast models that show it coming in the general direction of the state.

We should not be looking at long-range computer models at this point, however. There are too many variables in play between the eastern Atlantic and our side of the ocean.

Disturbance #2 is part of an elongated area of disturbed weather that is made up of at least two separate prominent circulations. The one the National Hurricane Center is focusing on appears to be the best developed, but having multiple circulations near each other makes the starting point of the computer model forecasts questionable. Until there is a clear, defined center of rotation, the models will jump around.

There is also very dry air just north of the broad disturbance. Unless a strong circulation can break free and create a moist bubble that holds off the Saharan air as it moves toward the west, which is possible but not likely, the dry air is going to be another complicating factor. Most of the computer model forecasts do not strengthen the system very quickly because of these and other issues.

The rule that forecasts for weak or developing systems are prone to high errors applies doubly in this situation.


In broad strokes, it seems likely that the disturbance will lift in a general north-of-west direction because the blocking high pressure system to the north is temporarily weakened. By the end of the week, however, the high should restrengthen, and block further northward progress. At that time, the system would likely be pushed generally westerly.

Where exactly it would track is anybody’s guess, and a lot of guesses are being made. But they are only guesses at this point because of the obstacles in the system’s path.

On the current schedule, the system would be in the vicinity of the Caribbean islands around Friday. Before that time, we’ll have a much better idea what we’re working with and be able to make better judgements about its potential future path. If it were to affect Florida, it would be roughly early next week.

Disturbance #1 will also move toward the west. When it reaches the western Caribbean later in the week, it has a decent chance of organizing into a tropical depression or tropical storm. A big dip in the jet stream is forecast to develop over the Gulf of Mexico about that time, which may try to pull the system north over the weekend.

Its path is very dependent on how strong it gets, however. A stronger system will likely turn harder north if the circulation develops far enough north to get grabbed by the jet stream, and a weaker one is more likely to track farther west. Since we can’t accurately predict how strong it will be or where exactly it will develop late in the week, the track beyond that time is unknowable.

This is a good time to throw out the reminder about spaghetti plots, since they are all over the internet and the television. If you look at each line of spaghetti individually, it’s very misleading. To fully understand what it is telling you, you should mentally draw a cone of uncertainty around it. Each computer model forecast has an average error at each time period in the future, and those errors are normally larger than the errors in the National Hurricane Center’s forecasts, which dictate the width of the cone of uncertainty that we all look at.

So, when you look at a spaghetti plot of multiple models, imagine each of them has an error cone drawn around it. When people distribute, irresponsibly I think, 10-day forecasts as spaghetti plots on the internet, the cone representing the average error would be 1,000 miles wide or more for every line, if it were drawn. That’s why hurricane forecasts aren’t made that far in the future.

The bottom line is, let’s not forecast the unforecastable. Let’s wait and just stay alert because things are likely to change day to day. Not just with these two systems, but for the next eight weeks or so through the peak of the hurricane season.

© 2019 by Bryan Norcross Corporation

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