Remnants of Post-Tropical Cyclone Florence north of Bermuda

September 20th, 2018 |

GOES-16

GOES-16 “Red” Visible (0.64 µm) images, with plots of Derived Motions Winds [click to play animation | MP4]

GOES-16 (GOES-East) “Red” Visible (0.64 µm) images and their Derived Motion Winds (above) revealed the partially exposed low-level circulation associated with the indirect remnants of Post-Tropical Cyclone Florence north of Bermuda on 20 September 2018 (surface analyses). The strongest Visible winds — calculated by tracking cloud features having a height assignment at or below the 700 hPa pressure level — located west and northwest of the circulation center were generally in the 35-40 knot range during the later part of the day, with one target being tacked at 56 knots (though this seemed to be an anomalous outlier).

However, ASCAT scatterometer data from an overpass of the Metop-A satellite at 1335 UTC (below) only sensed surface winds speeds (deduced from ocean surface roughness) as high as 25 knots around the center of the circulation.

Metop-A ASCAT surface scatterometer winds [click to enlarge]

Metop-A ASCAT surface scatterometer winds [click to enlarge]

Using a GOES-16 satellite-winds-derived 850 hPa Relative Vorticity product from the CIMSS Tropical Cyclones site (below), motion of the lower-tropospheric vorticity associated with Florence could be followed from landfall on 14 September to the current position north of Bermuda 6 days later. While the bulk of the vorticity became elongated (as Post-Tropical Cyclone Florence transformed into more of a weak baroclinic frontal wave over the Northeast US on 18 September: surface analyses), a small portion of the remnant 850 hPa vorticity became separated and then moved southeastward across the Atlantic.

GOES-16 Relative Vorticity product [click to play animation | MP4]

GOES-16 Relative Vorticity product [click to play animation | MP4]

Sea Surface Temperature and Ocean Heat Content [click to enlarge]

Sea Surface Temperature and Ocean Heat Content [click to enlarge]

A tropical Invest (98L) was initiated by the National Hurricane Center to gather additional data and more closely monitor this feature. Although the circulation had been moving over the Gulf Stream where warm Sea Surface Temperature and modest Ocean Heat Content existed (above), deep-layer wind shear was increasing over the area due to the approach of a branch of the polar jet stream (below).

GOES-16 Mid-level Water Vapor (6.9 µm) images, with deep-layer wind shear analyzed at 22 UTC [click to enlarge]

GOES-16 Mid-level Water Vapor (6.9 µm) images, with deep-layer wind shear analyzed at 22 UTC [click to enlarge]

Although deep convection was displaced to the southeast of the low-level circulation center, the GOES-16 Total Precipitable Water derived product (below) showed that ample moisture remained in place farther to the northwest over the Invest 98L.

GOES-16 Low-level Water Vapor (7.3 µm) images + Total Precipitable Water derived product [click to play MP4 animation]

GOES-16 Low-level Water Vapor (7.3 µm) images + Total Precipitable Water derived product [click to play MP4 animation]

===== 21/22 September Update =====

GOES-16

GOES-16 “Red” Visible (0.64 µm) images [click to play animation | MP4]

GOES-16 “Red” Visible (0.64 µm) images (above) showed the cyclonic spin of Invest 98L as it moved south of Bermuda on 21 September.

On 22 September, the circulation continued to drift a bit farther south of Bermuda (below), a few hundred miles north of an area of Saharan Air Layer dust (discussed here) — note the hazy signature of the dust on Visible imagery, along with elevated Aerosol Optical Depth values of 0.6 to 0.7 having a good coverage of medium to high confidence Dust Detection.

GOES-16

GOES-16 “Red” Visible (0.64 µm) images, along with Aerosol Optical Depth and Dust Detection products [click to play MP4 animation]

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