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.

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.

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).

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.

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

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.

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There have been many episodes of Saharan Air over the tropical Atlantic within the past months, and another episode is in progress on 20 September. The Saharan Air Layer (SAL) analysis, above, from the CIMSS Tropical Weather website (Direct Link), shows dry air north and east of the Caribbean. The Clean Window ABI Band 13 (10.3 µm) Full-Disk ABI infrared imagery, below, from 0500 UTC, overlain with NUCAPS sounding points, shows where data were available from that morning overpass of Suomi NPP.

The stepping animation below shows NUCAPS Soundings at a selection of points that starts north of the Saharan Air Layer and ends up within the SAL. The underlying figure is the Dust RGB from AWIPS, an RGB that combines the Split Window Difference (12.3 µm -10.3 µm; Red Component), Split Cloud Top Phase Brightness Temperature Difference (11.2 µm – 8.5 µm; Green Component) and 10.3 µm Infrared Imagery (Blue Component). Typically, regions with dust as might accompany a SAL have a pink tinge. The soundings are annotated to include Total Precipitable Water measurements, and mid-level Relative humidity. NUCAPS soundings identify the region where the SAL is present.

The SAL air continued its movement west during the day on 20 September.  The toggle below shows the Dust RGB, ABI Band 3 (0.86 µm) and the Baseline Aerosol Detection Product (in blue) at about the same time as the afternoon NUCAPS Sounding overpass (from Suomi NPP).  Suomi NPP overflew the eastern half of the SAL air (the overpass from NOAA-20 was more centered on the SAL air approaching the Caribbean, but NOAA-20 NUCAPS soundings are not yet in AWIPS;  they should be by the end of the year).

NUCAPS Soundings at 3 points (North of the SAL, within the SAL, and south of the SAL), below, show much different thermodynamics within the SAL.

NOAA’s G-IV flew through this outbreak, deploying dropsondes to sample the event. The path of the aircraft (with the dropsonde locations) is here. Sonde #26, below, in the heart of the SAL, is shown below, with a nearby NOAA-20 NUCAPS sounding. (Flight path and Sonde imagery courtesy Chris Barnet, STC/NOAA) Refer to the caption for details.  Recall that the Dropsonde shows values at a point.  The NUCAPS profile is sampling a volume that is approximately a 50-km cylinder!  There is nevertheless excellent agreement.

SAL outbreaks cause a significant deterioration in air quality over the Caribbean. The image below, courtesy Ernesto Rodriguez, SOO for the National Weather Service office in San Juan, Puerto Rico, compares Air Quality before and during a SAL outbreak in July, and during the current outbreak.

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After Hurricane Florence made landfall in North Carolina during the morning hours on 14 September, it moved very slowly (at times only 2-3 mph) southwestward into South Carolina during 15-16 September (surface analyses). Prolonged heavy rainfall resulted (WPC summary), with new state records (see below) for precipitation from a tropical cyclone being set in North Carolina (35.93 inches) and South Carolina (23.63 inches). GOES-16 (GOES-East) “Clean” Infrared Window (10.3 µm) images every 5 minutes during the 4-day period of 13-16 September (above) showed the evolution of banding and the development of new convection that produced the heavy rainfall — widespread flooding along with strong winds caused power outages across portions of the 2 states (NC | SC), and closed sections of Interstates 95 and 40. Note that the power outages caused extended dropouts of the plotted surface reports — especially in eastern North Carolina; reports were missing when the gray 4-letter station identifiers disappeared — even though many of those sites were likely experiencing heavy rainfall during those dropout times.

Florence also spawned a few tornadoes on 14, 15 and 16 September — SPC storm reports are plotted in cyan on the GOES-16 Infrared images.

Hourly images of the MIMIC Total Precipitable Water product (below) showed tropical moisture associated with Florence as it moved inland during the 13-17 September period.

Animations of plots of rawinsonde data from the coastal sites of Newport/Morehead City, North Carolina and Charleston, South Carolina (below) revealed the increase in deep tropical moisture from 13-16 September — Total Precipitable Water values were as high as 68.6 mm (2.70 inches) at Newport and 67.8 mm (2.67 inches) at Charleston.

As the remnants of Florence moved from Kentucky to West Virginia during the daylight hours of 17 September, numerous tornadoes occurred in central Virginia (SPC storm reports | NWS Wakefield summary). 1-minute GOES-16 “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.3 µm) images (below) showed the development of thunderstorms which produced these tornadoes.

Today, it’s two updates. After some chatter with the @CoCoRaHS observer, we’ve accepted the reading. There was uncertainty in house about the reading. Just trying to be careful. pic.twitter.com/7bF76eeGLQ

— David Roth (@DRmetwatch) September 17, 2018

Radar loops have been finalized for #Florence, and include a 24-hour loop for landfall and a 104-hour loop to cover its slow trek through #NorthCarolina and #SouthCarolina. Please visit https://t.co/vWICHS7kHl @UMiamiRSMAS pic.twitter.com/8jixg0mR1C

— Brian McNoldy (@BMcNoldy) September 17, 2018

 

===== 18 September Update =====

A comparison of before/after (26 August/18 September) Terra MODIS False Color Red-Green-Blue (RGB) images from the MODIS Today site (above) showed areas of inland flooding (increasing water coverage appears as darker shades of blue) in the wake of Florence across far southeastern North Carolina and far northeastern South Carolina.

Looking slightly to the south, a similar before/after comparison of Terra MODIS True Color RGB images (below) revealed areas of sediment runoff into the Atlantic Ocean.

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Hurricane Florence made landfall near Wrightsville Beach, North Carolina at 1115 UTC (7:15 am EDT) with estimated maximum winds of 78 knots (90 mph) and a minimum central pressure estimate of 958 hPa (28.29″). Overlapping GOES-16 (GOES-East) Mesoscale Domain Sectors provided images every 30 seconds — “Red” Visible (0.64 µm) images (above) and “Clean” Infrared Window (10.3 µm) images (below) showed the storm as it slowly moved inland after sunrise. A peak wind gust of 105 mph was recorded at Wilmington NC (which is located at the center of the GOES-16 images); in northeastern North Carolina, winds gusted to 105 mph at Fort Macon and 112 mph at the New River Inlet Buoy.

The MIMIC Total Precipitable Water product (below) showed abundant moisture associated with Florence moving inland during the 48-hour period ending at 23 UTC on 14 September.

Toggles between Visible and Infrared Window images from Terra/Aqua MODIS and Suomi NPP VIIRS are shown below.

Over the western Atlantic Ocean, strong winds associated with Florence created large waves which induced upwelling of colder water from below the ocean surface, as seen in Ocean Heat Content data (below).

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