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Capital Weather Gang | Analysis

Explainer: Florence’s final destination depends on many different players

By Philippe Papin

September 7, 2018 at 10:11 AM

Visible satellite view of Hurricane Florence on Friday morning. (CIRA/CSU/NOAA/)

The author, Philippe Papin, is an atmospheric scientist at the U.S. Naval Research Laboratory.


Florence weakened from a hurricane to a tropical storm Thursday over the central Atlantic Ocean. Despite the weaker intensity, the storm still poses a threat to the United States, as it is likely to intensify this weekend as it moves farther west.

The uncertainty over Florence’s track remains high, but, at least in the short term, computer models are coming into somewhat better agreement. The question is: Then what happens?

Animation showing convergence of forecast tracks for Florence from a suite of models since Thursday. ( U.S. Naval Research Laboratory -Monterey/)

The animation above shows how the track forecasts have evolved over the last 24 hours. The models have generally converged on a track that is safely south of Bermuda.

Despite this better agreement, beyond this period, high uncertainty remains, with some models curving Florence out to sea before it reaches the United States and others bringing the storm inland along the East Coast.

What causes this track divergence? Let’s take a moment to illustrate the different steering features that may influence the track in these models as annotated in the animated graphic below.

American Global Forecast System (GFS) model output showing the high altitude (500 millibar) flow around Florence from 8 p.m. Eastern Sept. 7 to 8 p.m. Eastern on Sept. 11. (Model charts from NOAA. Animation created by Philippe Papin./)

By Sunday, there will be three primary players influencing Florence’s track.

Florence is first situated between a ridge of high pressure to its southwest and another ridge to its northeast. The clockwise flow around both ridges roughly cancels out their impact on Florence’s motion.

To the north, however, there is a trough of low pressure creating a weakness in the flow to the north of Florence. This is a key feature, as its intensity could dictate whether: Florence gets tugged farther north, increasing the chance it curves out to sea; or continues westward, increasing the chances of a U.S. landfall.

In this latter scenario, the trough would be too weak and quickly move east, allowing a much stronger high-pressure ridge to build in behind it, likely blocking any path for Florence to move out to sea.

The latest American Global Forecast System (GFS) model forecast and most other models now follow this latter solution, allowing Florence to continue west.

The size and strength of the ridge that forms behind the trough, however, is also critical to the storm’s track and final destination. A larger, stronger ridge may move Florence west for a longer period, further increasing the odds of a U.S. landfall. A weaker ridge could allow it to narrowly stay offshore.

These are the features we will watch over the next few days as the forecast evolves. At this point, large uncertainty remains in the track of Florence, but keeping a close eye on these critical environmental features can help to inform which path the storm may ultimately take.

The U.S. Naval Research Laboratory provides the advanced scientific capabilities required to bolster our country’s position of global naval leadership. With more than 2,500 personnel scientists, engineers and support staff, it has served the U.S. Navy and the nation for nearly 100 years, advancing research further than you can imagine. For more information, visit the NRL website and join the conversation on Twitter, Facebook, and YouTube.

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Capital Weather Gang | Analysis

Explainer: Florence’s final destination depends on many different players

By Philippe Papin

September 7, 2018 at 10:11 AM

Visible satellite view of Hurricane Florence on Friday morning. (CIRA/CSU/NOAA/)

The author, Philippe Papin, is an atmospheric scientist at the U.S. Naval Research Laboratory.


Florence weakened from a hurricane to a tropical storm Thursday over the central Atlantic Ocean. Despite the weaker intensity, the storm still poses a threat to the United States, as it is likely to intensify this weekend as it moves farther west.

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