Hurricane Florence has defied forecast models and the forecasts of the National Hurricane Center, rapidly intensifying to category 4 this afternoon. Shear, which was forecast to weaken the hurricane is evident in satellite imagery but the hurricane’s dome of warm air seems to have deflected the strong shear to the north side of the storm, allowing the core of the storm to strengthen. Forecasters are struggling to predict the intensity of Florence in the days to come but they continue to think that Florence will cross a strong southwest wind field that will shear and weaken the storm over the next few days. After that, as it moves towards Bermuda, Florence is expected to reintensify.
Here’s a simplified version of NHC’s 5pm discussion:
Since the shear is anticipated to increase during the next day or so, gradual weakening is shown in the short-term official forecast... Beyond 48 hours, vertical shear is anticipated to weaken, which would allow Florence to intensify and potentially regain major hurricane status.
The most recent satellite imagery, that displays water vapor levels in the mid atmosphere, shows dry air flowing towards Florence from the southwest and clouds flowing out rapidly from the storm on its northeast side. This mid-level shear should stop intensification even though Florence will be moving into progressively warmer water over the next few days.
The hurricane center’s forecast reflects the effects of shear for the next 2 days, followed by weakening shear and warming waters as Florence heads towards Bermuda. Given the difficulty in forecasting wind shear, this forecast has a higher than average degree of uncertainty.
FORECAST POSITIONS AND MAX WINDS
INIT 05/2100Z 22.7N 46.6W 115 KT 130 MPH
12H 06/0600Z 23.5N 48.1W 110 KT 125 MPH
24H 06/1800Z 24.5N 49.7W 105 KT 120 MPH
36H 07/0600Z 25.2N 51.3W 100 KT 115 MPH
48H 07/1800Z 25.5N 52.9W 95 KT 110 MPH
72H 08/1800Z 26.0N 55.3W 100 KT 115 MPH
96H 09/1800Z 27.0N 57.4W 105 KT 120 MPH
120H 10/1800Z 29.0N 60.0W 105 KT 120 MPH
The problem for next week is that there has been a strong ridge of high pressure under an upper atmospheric dome over the east coast that projects over the western north Atlantic. That high pressure may prevent Florence from harmlessly curving back out to sea in the middle of the ocean. Moreover, that persistent pattern of strong high pressure north of its normal position this summer has warmed waters of the U.S. east coast to record and near record levels. Thus a hurricane that moved into the waters off of the mid Atlantic or the northeast could be stronger than normal.
There is a high degree of uncertainty in Florence’s forecast track. However, both the American and European models have been consistently forecasting that Florence moving in the general direction of North Carolina’s outer banks towards the middle of next week and possibly moving towards the mid-Atlantic coast. Here is the latest 192 hour American model run for mid-day Thursday. Do not take this model run as a forecast. Follow the NHC’s forecast. Beyond 5 days “the butterfly effect” — small unpredictable variations early on can cause large deviations after 5 days or more — makes precise forecasting impossible. This model run is shown as a warning to people on the east coast to be prepared because this situation is possible next week.
Because this hurricane is stronger than expected it is developing swells with a longer period than normal for this part of the Atlantic. The speed that wave trains travel through the open ocean goes up dramatically as the period increases. Therefore, long period energy may begin to arrive on the east coast as early as Saturday. (There may be an error in the NHC discussion of when Florence may affect east coast surf.) Long period waves are more energetic than short period waves and are capable of producing rip currents that are dangerous to swimmers and inexperienced surfers.