Florence Unexpectedly Intensifies to a Major Hurricane; U.S. East Coast a Potential Target

September 5, 2018, 3:15 PM EDT

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Above: MODIS visible satellite image Hurricane Florence on Wednesday morning, September 5, 2018. Image credit: NASA Worldview.

An intense period of tropical cyclone activity is upon us in both the Atlantic and Pacific Oceans, with the U.S. East Coast, Hawaii, and the Caribbean all potentially at risk of impacts from named storms next week. The most immediate concern is Hurricane Florence, which residents of the U.S. East Coast need to pay close attention to. Florence surprised forecasters by intensifying into the season’s first major hurricane on Wednesday morning. Florence became a Category 3 storm with 125 mph winds at 11 am Wednesday, just 5 mph below Category 4 strength.

Florence’s ascension to major hurricane status on September 5 came very close to the usual September 4 date for the season’s first major hurricane. The Atlantic’s 2018 tally of 7 named storms and 3 hurricanes is about a week ahead of average; our accumulated cyclone energy (ACE) so far in 2018 is just 66% of average, though. That percentage is poised to jump much higher in the coming week as Hurricane Florence heads across the Atlantic towards the U.S.

Figure 1. Infrared image of Hurricane Florence as of 1450Z (10:50 am EDT) Wednesday, September 5, 2018. Image credit: tropicaltidbits.com.

Intensity forecast for Florence

Florence’s intensification into a Category 3 hurricane is a head-scratcher: the 36-hour intensity forecast from NHC from 11 pm EDT Monday called for Florence to be a weakening tropical storm with 65 mph winds at 8 am EDT Wednesday, because of marginal sea surface temperatures (SSTs) of 26.5° - 27°C (80° - 81°F), dry air with a mid-level relative humidity of 50 – 55%, and moderate to high wind shear of 15 – 25 knots. Florence shrugged off those seemingly hostile conditions, and satellite images on Wednesday morning showed a well-organized system with a prominent eye surround by an impressive eyewall with cold cloud tops.

Florence’s environment continued to show mostly hostile conditions for intensification on Wednesday morning, with strong upper-level winds out of the southwest creating a high 20 – 25 knots of wind shear, a dry atmosphere with a mid-level relative humidity of 50 – 55%, and (SSTs) of just 27°C (81°F). High wind shear remains in the forecast for the next 2 – 3 days, and our top intensity models continue to unanimously predict weakening through Friday--even though SSTs will be steadily warming. In their 11 am EDT Wednesday discussion, NHC called the intensity forecast “incredibly uncertain”, and speculated that the hurricane’s small size enabled it to find a small region of lower wind shear that our analysis techniques cannot resolve. Given Florence’s resilience in the face of hostile conditions so far, I expect the hurricane to keep Category 2 or 3 status the remainder of the week.

By Saturday, all the intensity models predict strengthening, as wind shear drops to a moderate 10 – 15 knots, SSTs warm to 29°C (84°F), and Florence moves into a moister atmosphere. Thus, expect Florence to intensify this weekend, and the storm’s power and proximity to the U.S. East Coast will cause a high surf event next week for much of the coast.

Florence forecast
Figure 2. Predicted tracks for Florence from the 50 members of the 0Z Wednesday run of the European ensemble forecast (left) and the 20 members of the 0Z Wednesday GFS ensemble forecast (right). About 50% of the lower-resolution European model ensemble forecasts (grey lines) along with the high-resolution deterministic forecast (red line) predicted that Florence would hit the U.S. East Coast next week. About 25% of the GFS ensemble members predicted a U.S. East Coast landfall, as well. Image credit: CFAN.

Track forecast for Florence: a concern for the U.S. East Coast

Florence was headed northwest at 12 mph across the Central Atlantic on Wednesday morning, and poses no threat to any land areas this week. The ridge of high pressure steering Florence will keep the hurricane moving northwest to west-northwest through Saturday. This ridge has a weakness in it, located several hundred miles to the northwest of the hurricane, which will cause Florence to slow down to a forward speed of about 7 mph Friday through Sunday.

By Sunday, Florence will have a major choice to make. A strong upper-level trough of low pressure passing to the north will have the opportunity to turn the hurricane to the north, allowing Florence to break through the weakness in the ridge. However, the ridge could also grow in strength, forcing Florence to resume a more west-northwesterly track at increased speed. The models have been flip-flopping in recent runs on which scenario is more likely, with the latest set of runs from 0Z Wednesday favoring the latter scenario: a more westerly track that would be a threat to Bermuda and the U.S. East Coast. In this scenario, Florence is likely to come dangerously close to the U.S. East Coast by the middle of next week. The steering currents so far in advance are difficult to accurately predict, but should grow more clear by Thursday, as Typhoon Jebi’s interaction with the large-scale atmospheric circulation in the Western Pacific becomes more defined. For now, it is highly uncertain if Florence will have impacts on any land areas, and it's a game of "what does the latest model run say: doom or dust?"

We’ll have an update on 92L, a system residents of the Caribbean should watch, and Olivia, a hurricane that Hawaii needs to watch, in posts later today.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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