Democracy Dies in Darkness

Capital Weather Gang

Florence weakens to tropical storm but forecast to regain strength and draw close to East Coast

September 6, 2018 at 11:44 PM

Tropical Storm Florence looms about 1,800 miles east of the U.S. East Coast on Thursday night. (NOAA/)

(This article, first published at 11:15 a.m. and updated at 5 p.m., was revised at 11:15 p.m. based on Florence’s downgrade to a tropical storm.) 

After rapidly intensifying into the season’s first major (Category 3 or higher) hurricane Wednesday, Florence substantially weakened Thursday and was downgraded to a tropical storm with winds of 70 mph.

But the storm is predicted to re-strengthen to a hurricane over the weekend and cautionary alarm bells are ringing that it may have some effect on the East Coast in about a week. Whether it is a direct hit, a graze or near miss is unknowable for an event so far into the future. The storm is still 1,800 miles from the East Coast.

Wind shear on Thursday significantly disrupted Florence’s thunderstorm development and structure and could continue to weaken the storm into Friday. But this destructive wind shear is then forecast to relax this weekend and, as storm moves over warm waters, steady intensification is forecast.  By Monday or Tuesday, the National Hurricane Center predicts it will regain Category 3 hurricane strength with 125 mph winds.

It is too soon to say with certainty that Florence will reach the United States. If it does indeed make it to U.S. shores, it would likely be between Wednesday and Friday.  Although the intensity forecast for the storm is extremely uncertain, it could approach the East Coast as a major hurricane, rated Category 3 or higher.

For the past few days, the American and European computer models have been relatively consistent on a track perilously close to the East Coast, though with wobbles and jumps, as expected in a forecast seven to 10 days into the future. But now we are within six or seven days of a potential impact, and models are starting to become more reliable.

Group of simulations from American (blue) and European (red) computer models from Wednesday night (previous model run) and Thursday (most recent model run). Each color strand represents a different model simulation with slight tweaks to initial conditions. Note that the strands are clustered together where the forecast track is most confident, but they diverge where the course of the storm is less certain. The thick bold red line is the average of all of the European model simulations, while the blue is the average of all the American model simulations. The thinner bold lines are the main or operational simulations from each model. (StormVistaWxModels.com/)

In general, these models have shifted the track westward, closer to the United States but the average of all the simulations is still offshore.

Of the 50 simulations in the European modeling system (shown in red above), some bring the storm inland, some hug the coastline, others curl it more out to sea.

In broad strokes, an inland track would bring multiple hazards, including torrential rain and damaging winds over coastal and some inland areas, and a substantial rise in water above normally dry land in coastal zones. A storm hugging the coast offshore would result in heavy rain and strong winds near the coast, dangerous surf, beach erosion and coastal flooding. A track farther out to sea would confine effects to the Atlantic coast, mainly big waves and rip currents.

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

It is too soon to focus on any specific track forecast but rather closely monitor the trends in the overall group of model simulations.

Since 1851, 67 named storms have passed within 200 nautical miles of Florence’s present location, and not a single one has ever hit the United States. So if this one does, it would be a remarkable outlier.

Tracks of the 67 known tropical storms and hurricanes that passed within 200 nautical miles of Florence’s position as of the 5 a.m. advisory.

However, the weather pattern steering this storm, dominated by a strong area of high pressure over the north Atlantic Ocean, is very unusual and capable of bringing this hurricane into the East Coast.

Residents along the entire East Coast should closely monitor forecasts for this storm to remain informed, but no one should panic. Disregard social media posts and news stories that proclaim doom and gloom over a specific model scenario. It is simply impossible to determine exactly where this storm will end up, how strong it will be and how profoundly it will affect or not affect any area. It will take until the weekend, at the earliest, to speculate about these specifics and discuss particular outcomes, given different model scenarios.

But it’s never too soon to be prepared. Have trees that could fall on your house during a windstorm? Trim them now. Declutter drains and gutters. Make sure you have an emergency kit with supplies for at least three days. These are among the basic things that anyone living along or near the East or Gulf coasts should already be doing during hurricane season. This is an excellent opportunity to do them.

Elsewhere in the tropical Atlantic

A wave that exited the West African coast on Sunday is also close to becoming a tropical depression or storm. It’s just west of the Cabo Verde islands. Models keep this system at a low latitude throughout the coming week, so we will need to keep a close eye on it, as it could affect the Leeward Islands by next Thursday. If named, this will become Helene.

Additionally, another potent wave is just leaving Africa today, but models generally do not favor that one to develop much.

Visible satellite image over the far eastern Atlantic and western Africa. (EUMETSAT/)

As of this morning, the season has racked up 71 percent of the normal Accumulated Cyclone Energy for the date, which is an integrated measure of the intensity and duration of all the storms that have formed.

We are now entering the climatologically most active week of hurricane season. We’ve had seven named storms, three hurricanes and one major hurricane; the average by today is six named storms, two hurricanes and one major hurricane.

Accumulated Cyclone Energy values to date in 2018 (yellow), compared with the 1981-2010 average (purple./)

Brian McNoldy works in cyclone research at the University of Miami’s world-renowned Rosenstiel School of Marine and Atmospheric Science (RSMAS). His website hosted at RSMAS is also quite popular during hurricane season.

Jason Samenow is The Washington Post’s weather editor and Capital Weather Gang's chief meteorologist. He earned a master's degree in atmospheric science and spent 10 years as a climate change science analyst for the U.S. government. He holds the Digital Seal of Approval from the National Weather Association.

Post Recommends
Outbrain

We're glad you're enjoying The Washington Post.

Get access to this story, and every story, on the web and in our apps with our Basic Digital subscription.

Welcome to The Washington Post

Thank you for subscribing
Keep reading for $10 $1
Show me more offers