Hurricane Walaka

October 1st, 2018 |

GOES-15 Infrared Window (10.7 µm) images [click to play animation | MP4]

GOES-15 Infrared Window (10.7 µm) images [click to play animation | MP4]

GOES-15 (GOES-West) Infrared Window (10.7 µm) images (above) showed the formation of a well-defined eye of Hurricane Walaka during a period of rapid intensification (ADT | SATCON) from 0000-2330 UTC on 01 October 2018; Walaka was classified a Category 5 hurricane as of the 02 October 00 UTC advisory. Walaka was moving over very warm water with Sea Surface Temperatures of 30ºC.

A 1536 UTC DMSP-16 SSMIS Microwave (85 GHz) image from the CIMSS Tropical Cyclones site (below) revealed a small eye (reported to be 20 nautical miles in diameter at 21 UTC).

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

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

A side-by-side comparison of JMA Himawari-8 and GOES-15 Infrared Window images (below) showed Walaka from 2 different satellite perspectives — the superior spatial resolution of Himawari-8 (2 km, vs 4 km for GOES-15) was offset by the much larger viewing angle. Cloud-top infrared brightness temperatures were -80ºC and colder (shades of violet) from both satellites early in the animation, but warmed somewhat into the -70 to -75ºC range by 00 UTC on 02 October.

Infrared Window images from Himawari-8 (10.3 µm, left) and GOES-15 (10.7 µm, right) [click to play animation | MP4]

Infrared Window images from Himawari-8 (10.3 µm, left) and GOES-15 (10.7 µm, right) [click to play animation | MP4]

===== 02 October Update =====

NOAA-20 VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images [click to enlarge]

NOAA-20 VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images [click to enlarge]

Walaka remained classified as a Category 5 hurricane until the 15 UTC advisory on 02 October, when it was assigned Category 4 status after some weakening as a result of an overnight eyewall replacement cycle. A toggle between NOAA-20 VIIRS Day/Night Band (0.7 µm) and Infrared Window (11.45 µm) images (above; courtesy of William Straka, CIMSS) showed the storm at 1240 UTC or 2:40 am local time.

GOES-15 Infrared Window (10.7 µm) images (below) showed the northward motion of Waleka. Given that the storm was forecast to pass very close to Johnston Atoll, the US Coast Guard was dispatched to evacuate personnel on Johnston Island.

GOES-15 Infrared Window (10.7 µm) images; the white circle shows the location of Johnston Atoll [click to play animation | MP4]

GOES-15 Infrared Window (10.7 µm) images; the white circle shows the location of Johnston Atoll [click to play animation | MP4]

The MIMIC-TC product (below) showed the eyewall replacement cycle during the 0000-1445 UTC period.

MIMIC-TC morphed microwave product [click to play animation]

MIMIC-TC morphed microwave product [click to play animation]

Around 1830 UTC, a toggle between GOES-15 Infrared (10.7 µm) and GPM GMI Microwave (85 GHz) images (below) showed a small eye, with evidence of a larger outer eyewall suggesting that another eyewall replacement cycle was taking place.

GOES-15 Infrared Window (10.7 µm) and GPM GMI Microwave (85 GHz) images [click to enlarge]

GOES-15 Infrared Window (10.7 µm) and GPM GMI Microwave (85 GHz) images [click to enlarge]

Medicane “Zorbas”

September 29th, 2018 |

NOAA-20 and Suomi NPP VIIRS Day/Night Band (0.7 µm) images [click to enlarge]

NOAA-20 and Suomi NPP VIIRS Day/Night Band (0.7 µm) images [click to enlarge]

Medicane “Zorbas” — as named by Freie Universität Berlin (surface analyses) — developed in the Mediterranean Sea late in the day on 27 September 2018. A toggle between VIIRS Day/Night Band (0.7 µm) images from NOAA-20 and Suomi NPP (above; courtesy of William Straka, CIMSS) revealed the well-defined circulation of the storm a few hours after Midnight local time on 28 September. Note the bright streak north of the storm center on the NOAA-20 image — this was an area of clouds illuminated by intense lightning activity. Other less prominent lightning streaks were evident in thunderstorms farther to the east over the Mediterranean Sea. On the Suomi NPP image, a small bright spot could be seen, evidence of minor volcanic activity at Mount Etna on the island of Sicily, as well as the hazy signature of a plume of blowing dust/sand that was moving northward off the coast of Libya. The corresponding VIIRS Infrared images are available here.

During the following daylight hours of 28 September, EUMETSAT Meteosat-11 High Resolution Visible (0.8 µm) images (below) showed the storm as it became better organized and increased intensity. Another dense plume of blowing dust/sand began to move off the coast of Libya late in the day.

Meteosat-11 Visible (0.8 µm) images, with hourly plots of wind barbs (yellow) and wind gusts (red) [click to play animation | MP4]

Meteosat-11 Visible (0.8 µm) images, with hourly plots of wind barbs (yellow) and wind gusts (red) [click to play animation | MP4]

On 29 September, Meteosat-11 Visible (0.8 µm) images (below) showed the Medicane moving inland along the Peloponnese coast of southern Greece — shortly after the storm center passed, winds gusted to 48 knots at Kalamata at 1220 UTC (while a heavy thunderstorm was being reported).

Meteosat-11 Visible (0.8 µm) images, with hourly plots of winds (yellow) and gusts in knots (red) [click to play animation | MP4]

Meteosat-11 Visible (0.8 µm) images, with hourly plots of wind barbs (yellow) and gusts in knots (red) [click to play animation | MP4]

A sequence of Terra and Aqua MODIS True Color Red-Green-Blue (RGB) images from 28 and 29 September from RealEarth (below) showed another view of the Zorbas on those 2 days (the valid time of the Terra MODIS image showing the eye-like feature on 29 September was 0911 UTC). Sea Surface Temperatures were near 25ºC in the central Mediterranean Sea where Zorbas was intensifying.

Terra/Aqua MODIS True Color RGB images on 28 and 29 September [click to enlarge]

Terra/Aqua MODIS True Color RGB images on 28 and 29 September [click to enlarge]

Hourly images of the MIMIC Total Precipitable Water product (below) showed moisture associated with the storm, which produced heavy rainfall and flash flooding in parts of southern Greece — the NESDIS Blended TPW Anomaly product indicated that this moisture was as much as 200% of normal for the region and date. Additional information and videos can be found here.

MIMIC morphed Total Precipitable Water images, 27-29 September [click to play animation | MP4]

MIMIC morphed Total Precipitable Water images, 27-29 September [click to play animation | MP4]

Hurricane Rosa

September 28th, 2018 |
GOES-15 Ifrared Window (10.7 µm, left) and GOES-17

GOES-15 Infrared Window (10.7 µm, left) and GOES-17 “Clean” Infrared Window (10.3 µm, right) images [click to play animation | MP4]

* GOES-17 images shown here are preliminary and non-operational *

GOES-15 (GOES-West) Infrared Window (10.7 µm) and GOES-17 “Clean” Infrared Window (10.3 µm) images (above) showed Hurricane Rosa on the morning of 28 September 2018, after it had rapidly intensified to Category 4 intensity overnight (ADT | SATCON). Since GOES-17 was operating in a Mode 6 scan strategy, images were available every 10 minutes (compared to every 15 minutes from GOES-15, with 30-minute gaps during Full Disk scans every 3 hours). A notable warming trend was seen in the cloud tops surrounding the eye.

A toggle between DMSP-18 SSMIS Microwave (85 GHz) and GOES-15 Infrared Window (10.7 µm) from the CIMSS Tropical Cyclones site (below) showed the bands of heavier precipitation withing the central dense overcast surrounding the eye at 1333 UTC.

DMSP-18 SSMIS Microwave (85 GHz) and GOES-15 Infrared Window (10.7 µm) images [click to enlarge]

DMSP-18 SSMIS Microwave (85 GHz) and GOES-15 Infrared Window (10.7 µm) images [click to enlarge]

After sunrise, a comparison of GOES-15 Visible (0.63 µm) and GOES-17 “Red” Visible (0.64 µm) images (below) revealed an eye that was filled with low-level clouds.

GOES-15 Visible (0.63 µm, left) and GOES-17 "Red" Visible (0.64 µm, right) images [click to play animation | MP4]

GOES-15 Visible (0.63 µm, left) and GOES-17 “Red” Visible (0.64 µm, right) images [click to play animation | MP4]

An animation of the MIMIC-TC product (below) showed that Rosa went through an eyewall replacement cycle during the morning, and was downgraded to a Category 3 intensity at 15 UTC.

MIMIC-TC morphed microwave product, 0000-1545 UTC [click to enlarge]

MIMIC-TC morphed microwave product, 0000-1545 UTC [click to enlarge]

Extratropical transition of Leslie

September 26th, 2018 |

GOES-16 Low-level (7.3 µm, left), Mid-level (6.9 µm, center) and Upper-level (6.2 µm, right) Water Vapor images [click to play animation | MP4]

GOES-16 Low-level (7.3 µm, left), Mid-level (6.9 µm, center) and Upper-level (6.2 µm, right) Water Vapor images [click to play animation | MP4]

GOES-16 (GOES-East) Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images (above) showed Leslie as it transitioned from a Subtropical Storm at 00 UTC on 25 September to a Subtropical Depression at 03 UTC, then to a Post-Tropical Cyclone at 15 UTC, and eventually to a warm seclusion hurricane force cyclone with a classic “scorpion tail” sting jet signature by 00 UTC on 27 September.

Surface analyses during this 48-hour period are shown below; Leslie is located in the lower left corner. A longer animation (from 21-27 September) revealed the slowly-meandering circulation centers of pre- and post-Leslie (animated GIF | MP4).

Surface analyses at 6-hour intervals [click to play animation | MP4]

Surface analyses at 6-hour intervals [click to play animation | MP4]

The period of warm seclusion intensification of the remnants of Leslie, beginning after about 12-15 UTC on 26 September, was in response to the approach of an upper-level Potential Vorticity (PV) anomaly from the west-northwest (below). The “dynamic tropopause” — taken to be the pressure of the 1.5 Potential Vorticity Unit (PVU) surface — then descended to the 660 hPa pressure level (around 10,000 feet or 3 km) at 18 UTC on 26 September, according to GFS90 model fields.

GOES-16 Upper-level (6.2 µm) Water Vapor images, with contours of the PV1.5 pressure surface plotted in red [click to play animation | MP4]

GOES-16 Upper-level (6.2 µm) Water Vapor images, with contours of the PV1.5 pressure surface plotted in red [click to play animation | MP4]

GOES-16 Mid-level (6.9 µm) Water Vapor images, with contours of the PV1.5 pressure surface plotted in red [click to play animation | MP4]

GOES-16 Mid-level (6.9 µm) Water Vapor images, with contours of the PV1.5 pressure surface plotted in red [click to play animation | MP4]

GOES-16 Low-level (7.3 µm) Water Vapor images, with contours of the PV1.5 pressure surface plotted in red [click to play animation | MP4]

GOES-16 Low-level (7.3 µm) Water Vapor images, with contours of the PV1.5 pressure surface plotted in red [click to play animation | MP4]

This lowered tropopause brought ozone-rich air from the stratosphere down to very low altitudes — on GOES-16 Air Mass Red-Green-Blue (RGB) imagery (below), this ozone-rich air was highlighted by varying shades of red (the Air Mass RGB uses the 9.6 µm Ozone band to calculate the Green component).

GOES-16 Air Mass RGB images, with contours of PV1.5 pressure [click to play animation | MP4]

GOES-16 Air Mass RGB images, with contours of the PV1.5 pressure surface plotted in green [click to play animation | MP4]

A larger-scale view of the GOES-16 Air Mass RGB from the NWS Ocean Prediction Center (below) extends to 1115 UTC on 27 September.


An animation of the GOES-16 Air Mass RGB at 6-hour intervals from 00 UTC on 25 September to 12 UTC on 27 September is shown below, with and without contours of PV1.5 pressure. The dynamic tropopause descended to 850 hPa at 06 UTC on 27 September (when the storm was producing hurricane force winds) and eventually to 925 hPa at 12 UTC on 27 September (when it was producing storm force winds). The Air Mass RGB images highlighted the signature of the PV anomaly (shades of red) as it approached from the northwest then wrapped around the western and southern portion of the storm circulation.

GOES-16 Air Mass RGB images, with and without contours of GFS PV1.5 pressure [click to play animation | MP4]

GOES-16 Air Mass RGB images, with and without contours of GFS PV1.5 pressure [click to play animation | MP4]

The corresponding 6-hourly GOES-16 Split Water Vapor Difference (6.2 µm – 7.3 µm) images (below) show that a signature of the dry air aloft — associated with the aforementioned PV anomaly — was evident as a tongue of negative values in the -10 to -15ºC range (green to gray enhancement) that initially approached the storm from the northwest.

GOES-16 Split Water Vapor Difference (6.2 µm - 7.3 µm) images, with and without contours of PV1.5 pressure [click to play animation | MP4]

GOES-16 Split Water Vapor Difference (6.2 µm – 7.3 µm) images, with and without contours of PV1.5 pressure [click to play animation | MP4]

In cases such as this, the Air Mass RGB and Split Water Vapor Difference can be used in tandem to identify and track PV anomalies (18 UTC / 25 September | 12 UTC / 26 September). Note that east of the storm there also another small PV anomaly moving northward, associated with an upper-level low pressure feature — but this second PV anomaly played no role in the development/intensification of the post-tropical remnants of Leslie.

===== 27 September Update =====

Sequence of GOES-16 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images from 09 UTC on 27 September to 00 UTC on 28 September [click to play animation | MP4]

Sequence of GOES-16 Low-level (7.3 µm), Mid-level (6.9 µm) and Upper-level (6.2 µm) Water Vapor images from 09 UTC on 27 September to 00 UTC on 28 September [click to play animation | MP4]

A sequence of GOES-16 Low-level, Mid-level and Upper-level Water Vapor images from 09 UTC on 27 September to 00 UTC on 28 September (above) showed the classic wrapped dry/moist bands often seen with occluded mid-latitude cyclones.