Hurricane Florence

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Hurricane Florence was the first major hurricane of the 2018 North Atlantic hurricane season and caused major devastation when it hit the US east coast in mid-September.

Hurricane Florence
Date & Time
30 August 2018 12:00 UTC–17 September 23:00 UTC
Satellites
Meteosat-11, Metop-A & B, GOES-16
Instruments
SEVIRI, ASCAT, ABI
Channels/Products
Visible, ASCAT Winds, Dust RGB, Natural Colour RGB, Infrared, High Resolution Visible (HRV)

By Ivan Smiljanic (SCISYS), HansPeter Roesli (Switzerland), Vesa Nietosvaara (EUMETSAT) and Sancha Lancaster (Pactum)

It was reported that Hurricane Florence had peak maximum sustained winds of at least 220 km/h (140 mph). Figure 1 (top right, click to expand) shows the storm when it was a Category 4 storm with wind speeds of around 240 km/h.

Florence originated from a strong tropical wave off the west coast of Africa on 30 August, forming into a tropical depression the next day.

After moving in a west-northwest direction, the system became a tropical storm on 1 September 1. On 4 and 5 September unexpected rapid intensification led to Florence becoming a Category 4 hurricane.

The sequence of Dust RGB images looping backwards in time (Figure 2) points vividly to the birthplace of Hurricane Florence. It initially started as a subtropical depression in the late afternoon of 30 August, not far from the Senegal coast. The end of the loop reveals the big tropical convective system responsible for the development of the initial depression.

The initial subtropical depression deepened over the next two days and was categorised as a tropical storm around noon of 1 September, with pressure slightly higher than 1000 hPa.

The convective system then became a hurricane in the early hours of 4 September (ca. beginning of the loop), with pressure around 990 hPa and wind speeds of approximately 120 km/h.


Figure 2: Meteosat-11 Dust RGB animation, 30 August 12:00 UTC–04 September 06:00 UTC. Download animation (MP4, 4 MB).

Comparing the Natural Colour RGBs from two different satellites, GOES-16 and Meteosat-11, at the moment when Florence was roughly mid-way between the two, provides a good insight into instrument characteristics/capabilities.

Aside from the difference in resolution, the parallax shift for high clouds is apparent, together with the fact that forward scattering in the morning brings more photons to the lens of the GOES-16 ABI imager (versus the backscattering towards the SEVIRI instrument). The comparison was done only hours after Florence was categorised as a hurricane.

Image comparison
GOES-16 Natural Color RGB, 4 Sept 09:15 UTC Meteosat-11 Natural Colour RGB, 4 Sept 09:15 UTC
Figure 3: Comparison of Meteosat-11 and GOES-16 Natural Colour images of Hurricane Florence, 9 Sept 09:15 UTC.

For days the storm's intensity fluctuated up and down, but by 9 September it was again a Category 4 hurricane and remained so for a further three days (Figure 4).


Figure 4: Clear eye of the storm seen on the GOES-16 Visible, 12 September 14:20–15:20 UTC
Figure 5
 
Figure 5: Meteosat-11 HRV animated gif, 11 September 20:30–22:30 UTC

The last reasonable view from the Meteosat-11 satellite (with regards to the satellite field of view) is shown using the High resolution Visible (HRV) channel loop (Figure 5).

The slant view from the east towards the hurricane in the evening hours reveals best the complex structures on top of the convective clouds. At that time Florence was a Category 4 hurricane. HRV scans this area because that area (the HRV upper window) follows the Sun terminator (the moving line that divides the day lit side and the dark night side of Earth). In the past it did not, so we can see the advantage of the EUMETSAT's decision to move this HRV window during the day (see Scan Modes web section).

As it headed closer to the south-eastern coastal areas of the United States, on 13 September, its strength reduced and it was downgraded to a Category 2 storm (Figure 6).

However, it remained a powerful storm as it made landfall on 14 September and and slowly moved inland over the weekend, bringing heavy rainfall, which caused severe flooding and subsequent devastation in parts of North and South Carolina. At least 37 people died in storm-related incidents — the majority in North Carolina, with some also in South Carolina and Virginia. Wilmington, one of North Carolina's largest cities, was completely cut-off for days due to the floods.

Figure 6
 
Figure 6: Enhanced Metop-B infrared image , with cloud tops colder than -40 °C colour coded, 13 September 00:41 UTC

Rainfall records were broken in North Carolina when a reported 913 mm (35.93 in) fell over the state, almost eight times the September monthly average of 108 mm (4.26 in), beating the record of 611 mm (24.06 in) set by Hurricane Floyd in 1999.

The GOES-16 infrared window channel loop (Figure 7) covers the hurricane making landfall over North Carolina and the follow-on decay of the system over the continent. At the final stage the system was introduced into the main mid-latitude jet stream where it lost the momentum and faded out.

This colour-coded imagery nicely shows the cold and high reaching cloud bands responsible for the heavy rainfall events. An impressive elongated cirrus band stretched more than 1,000 km at the leading edge of the hurricane (around the time of landfall).


Figure 7: GOES-16 enhanced infrared animation, 13 September 00:00 UTC–17 September 23:00 UTC.
Download animation (MP4, 4 MB).
 
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