Lee Update – why should it turn away from Florida?

This morning’s (Thu. 7 Sept 2023) 5am forecast was pretty much the same as the previous forecasts. We’ll take a look at why in a bit, but first lets look at the forecast. As usual your go-to source are NHC’s Key Messages regarding Hurricane Lee (en Español: Mensajes Claves), and here is my TAOS/TC impact model estimate based on that forecast:

click to embiggen.

On this track the main concern are waves – they will cause dangerous rip currents and erosion on the coast of the northern Caribbean (including Puerto Rico and the USVI) as well as the Bahamas this weekend. It’s possible, as Lee intensifies, the outer winds might touch the Leeward Islands, but doesn’t seem the case at the moment. If you live there still monitor for unexpected changes.

The big non-news is that Lee is still expected to rapidly intensify in the next few days. It is moving in to great conditions for a hurricane – warm water, low vertical shear, good upper level outflow. The intensity forecast is rapid, NHC holding just short of a Category Five but the major models not holding back …

Lee continues to “barrel” (why do the TV talking heads all say that?) towards Florida as it churns (another favorite phrase) the Atlantic. The NHC forecast goes out five days, so it’s just starting to “see” the expected turn; you can tell because the storm is forecast to slow down, which usually a sign of an impending change of direction. The official forecast only goes out five days for good reason: beyond that the forecasts just aren’t that reliable and rapidly decline in accuracy, not that’s going to stop the following analysis. In the case of Lee, the models are all pretty much in agreement, the storm will turn north:

Colored lines are major models, gray lines are ensemble members … cook your spaghetti …

Why is this? To understand this we need to look at the upper level winds that “steer” storms. One of the most important charts for weather forecasting is the 500mb height and wind chart. This is looking at the winds and heights (a measure of pressure) at about 18,000 feet. The winds at this level are a major factor in hurricanes. Here is what the chart (in this case based on the GFS model) looks like this morning:

The prevailing winds, caused by the clockwise rotation of wind around the high pressure just north of the storm, are pushing Lee rapidly west. Now let’s look at the forecast for next Tuesday:

You’ll notice several features. First, that high pressure ridge is gone. That means that the “steering” is gone as well, thus the slowing and near-stall of the storm. Second, there is a new storm (AL95, by then should be at least Tropical Depression 14 and maybe a new tropical storm, Margot). The prevailing winds to the north of the storm are blowing west to east. Now let’s look at next Thursday:

A front (represented by the dip in the west-east winds over the southeast) should have shifted south, preventing Lee from approaching the coast. All of the major global models are showing this scenario to various degrees. On this track the Bahamas will be in the same situation the Leeward Islands are today, watching a near miss (and some big incoming waves). By the way, all of these charts are from NOAA’s Model Analysis and Guidance site, and update every six hours with the forecast going out two weeks.

The major question at this point is timing of these surrounding systems. The later the ridge breaks down, the farther west Lee can travel; earlier, the turn starts earlier. Of course, if the western side of the ridge doesn’t break down, or the front doesn’t materialize or extend as far south as forecast, that could result in a threat to Florida and the southeast, but so far, this looks like the way things are shaping up.

In any case, the official forecast is showing Lee (other than waves) staying out of the Caribbean, which is good. After that, we’ll have to wait and see, although at the moment the indications are it will turn. It will be a couple more days before the official forecast starts to show the turn. Check back Sunday for a better picture of how things are materializing, and if the turn still seems solid.

Wow. Made it through without any snarky comments about Florida 😛 , which was super hard this morning with this story in the news about Florida Man(tm) trying to cross the Atlantic in a hamster wheel. No, you read that right …


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