The models were seriously wrong on Hurricane Florence

Author: Adam Button | Category: News

Forecasters got just about everything wrong

A day ago the US National Hurricane Center announced Florence had reached hurricane status but it forecast a weakening back to a tropical storm in the next 24-72 hours.

That hasn't happened. Instead, the storm is now an extremely power, albeit small Category 4 storm.

The NHC notice is amazing reading. It hints at how the storm has confounded the models and the new forecast track shows this may be a threat to be a damaging storm and hit the US. That said, it's still too far north to be a threat to gulf oil production.

Florence has continued to intensify during the day, with colder cloud tops completely encircling a clearer eye. Both subjective and objective Dvorak estimates have unanimously increased to T6.0/115 kt, and on this basis, the initial intensity is raised to 115 kt, making Florence a category 4 hurricane.

Florence appears to be right along the southern edge of a zone of stronger vertical shear and continues to defy the intensity guidance, which has consistently been predicting weakening. That said, the last few satellite images indicate that the eye may be becoming slightly disrupted by the shear. Since the shear is anticipated to increase during the next day or so, gradual weakening is shown in the short-term official forecast, which lies along the upper end of the guidance through 48 hours. Beyond 48 hours, vertical shear is anticipated to weaken, which would allow Florence to intensify and potentially regain major hurricane status. However, Florence will need to thread the needle between areas of stronger shear for this to happen, and there is significant uncertainty in the intensity forecast. The new official intensity forecast is little changed from before except to adjust upward to account for the higher initial intensity.

The initial motion estimate remains northwestward, or 305/11 kt. Strong upper-level ridging is forecast to strengthen north of Florence over the next few days, forcing the hurricane to turn toward the west-northwest through about 48 hours. Track guidance is good agreement during this period, and the official forecast is essentially an update to the previous one. Beyond day 2, a break forms in the ridge, which results in a weakening of the steering currents, a slowdown in Florence's forward speed, and a turn back toward the northwest. The new official forecast is shifted south a bit on days 3-5 to account for recent model trends and is closest to the TVCN consensus.

It should be noted that there is considerable model ensemble spread and run-to-run variability for Florence's track beyond day 5. Given the large uncertainty at these time ranges, it is far too soon to speculate what, if any, impacts Florence may have on the U.S. East Coast next week. Regardless of Florence's eventual track, large swells emanating from the hurricane will reach Bermuda beginning on Friday, resulting in life-threatening surf and rip currents on the island. Swells from Florence could also reach the U.S. East Coast by early next week.

At this point you might as well have economists making these forecasts.


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