US hit twice during 2018 hurricane season

Florence

30 Aug-17 Sep, USA
By Sancha Lancaster, Vesa Nietosvaara, HansPeter Roesli , Ivan Smiljanic 

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.

It was reported that Hurricane Florence had peak maximum sustained winds of at least 220km/h (140mph). Figure 1 shows the storm when it was a Category 4 storm with wind speeds of around 240km/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 1000hPa.

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

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.

Natural Colour RGB comparison

GOES-16 Meteosat-11

 

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).

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.

Rainfall records were broken in North Carolina when a reported 913mm (35.93in) fell over the state, almost eight times the September monthly average of 108mm (4.26in), beating the record of 611mm (24.06in) 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 1,000km at the leading edge of the hurricane (around the time of landfall).

Michael

10 Oct, Caribbean Sea, Cuba, Florida, Georgia, Gulf of Mexico
By Sancha Lancaster

Hurricane Michael was strongest hurricane on record in the Florida Panhandle, when it hit the US in early October 2018.

Michael was the 13th named storm, seventh hurricane, and second major hurricane of the 2018 Atlantic hurricane season. It originated from a broad low-pressure area in the western Caribbean Sea, becoming a tropical depression on 7 October.

It intensified into a hurricane on 8 October, near the western tip of Cuba. Strengthening continued in the Gulf of Mexico and on 9 October it became a Category 4 hurricane.

On 10 October Michael made landfall near Mexico Beach Florida with peak winds of 155mph (250km/h), becoming the region's first Category 4 hurricane and the third-most intense (in terms of central pressure) Atlantic hurricane to make landfall in the contiguous United States.

As it moved inland, the hurricane weakened and began to take a northeastward trajectory toward Chesapeake Bay, weakening to a tropical storm over Georgia and transitioning into an extratropical cyclone over the mid-Atlantic states on 12 October.

Impacts

• At least 12 people were reported to have died in the Florida Panhandle area.
• In Mexico Beach, many homes were flattened or completely swept away by storm surges.
• Debris on Interstate 10 resulted in the roadway being closed between Lake Seminole and Tallahassee (approx. 130km).
• More than 400,000 electrical customers in Georgia were left without power and at least 127 roads were blocked by fallen trees or debris.
• In Albany, 24,270 electrical customers lost power and several trees fell on homes and roads, blocking about 100 intersections.
• High winds left 200,651 people without power in the Cuban province of Pinar del Río.

Other image sources

Florence produces record rainfall in North Carolina and South Carolina (CIMSS Blog)
A View Inside Hurricane Florence (NASA Earth Observatory)
The Complex Evolution of Florence’s Winds (NASA Earth Observatory)
Hurricane Michael Makes Landfall (NOAA/NESDIS)
Hurricane Florence seen from the International Space Station (Ricky Arnold/Twitter)

Media reports

Live coverage of Florence and its aftermath (CBS News)
Photos: The Aftermath of Hurricane Florence (The Atlantic)
Florence Shatters North Carolina's Record for Most Rain Fallen from a Single Storm (Time)
Michael's death toll jumps; utter devastation in Panhandle -- live updates (CBS)