Hurricane Sam reaches Category 4 intensity

September 25th, 2021 |

GOES-16 “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images [click to play animation | MP4]

1–minute Mesoscale Domain Sector GOES-16 (GOES-East) “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images (above) showed Hurricane Sam as it intensified from a Category 3 to a Category 4 storm (ADT | SATCON) in the central Atlantic Ocean on 25 September 2021. The eye became cloud-filled during the middle portion of the day, but Visible images revealed the presence of mesovortices within the eye both early and late in the day.

A DMSP-16 SSMIS Microwave (85 GHz) image at 1918 UTC from the CIMSS Tropical Cyclones site (below) displayed a fully closed eyewall, with several spiral bands wrapping inward toward the storm center.

DMSP-16 SSMIS Microwave (85 GHz) image at 1918 UTC [click to enlarge]

GOES-16 Infrared images with an overlay of deep-layer wind shear at 2200 UTC (below) indicated that Sam was in an environment of low shear — which favored intensification as the hurricane moved across relatively warm water (SST | OHC).

GOES-16 Infrared images, with an overlay of deep-layer wind shear at 2200 UTC [click to enlarge]

During the following nighttime hours, ample illumination from the Moon — which was in the Waning Gibbous phase, at 81% of Full — provided a “visible image at night” using the Suomi NPP VIIRS Day/Night Band (0.7 µm) (below).

Suomi NPP VIIRS Day/Night Band (0.7 µm) image [click to enlarge]

===== 26 September Update =====

GOES-16 “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.35 µm) images [click to play animation | MP4

On the following day, Sam exhibited a similar appearance on 1-minute GOES-16 Infrared and Visible images (above), with a small 7-15 mile diameter eye (containing mesovortices, as seen in Visible imagery). Both Infrared and Visible images revealed repeated pulses of gravity waves propagating away from the storm center. Sam’s intensity peaked at 135 knots late in the day (NHC advisory).

1-minute GOES-16 Visible images with plots of corresponding GLM Flashes (below) showed that Sam exhibited an Enveloped Eyewall Lightning signature (reference).

1-minute GOES-16 “Red” Visible (0.64 µm) images, with 1-minute GLM Flashes plotted in red [click to play animation | MP4]

Ephemeral Theresa

September 25th, 2021 |
Suomi-NPP Day Night Band Visible (0.7 µm) and M15 (10.8 µm) imagery, 0618 UTC on 25 September 2021 (Click to enlarge)

Theresa was a short-lived subtropical storm in the Atlantic Ocean north of Bermuda late on the 24th and early on the 25th of September. When Suomi NPP overflew the storm early on the 25th (imagery from this source), the low-level circulation was devoid of any significant convection; convection was displaced to the northeast of the storm. The toggle below of True-color imagery from CSPP Geosphere shows the storm structures at 1800 UTC on the 24th and 25th. Convection was nearly wrapped around the subtropical storm on the 24th; it was mostly absent near the storm center on the 25th.

CSPP Geosphere True Color Imagery at 1800 UTC on 24 and 25 September 2021 (Click to enlarge)

Note in the Day Night Band imagery at top that a dark region exists within the moonglint at around 29 N Latitude. Such features, discussed in this blog before (here and here, for example), suggest very light winds; an HY-2C overpass at 0330 UTC on 25 September, shown below (from this site; click here to see a similar image with wind flags), shows very light winds over/around 30 N.

HY-2C Scatterometry at 0330 UTC on 25 September 2021 (Click to enlarge)