1-minute Mesoscale Domain Sector GOES-16 (GOES-East) “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.3 µm) images (above) showed Lee as it intensified to become a Category 1 Hurricane in the Central Atlantic Ocean on 06 September 2023. The coldest cloud-top infrared brightness temperatures were around -90C (brighter white pixels embedded within darker black areas). 

Lee had been moving across warm water — and was traversing an environment having low deep-layer wind shear (below) — factors which were favorable for intensification. Water temperature and wind shear imagery was sourced from the CIMSS Tropical Cyclones site.

Meanwhile, in the East Pacific Hurricane Jova rapidly intensified to a Category 4 storm by 2100 UTC on 06 September, exhibiting a pinhole eye as seen in GOES-18 (GOES-West) Visible and Infrared imagery (below).

Similar to Lee, Jova had been moving across warm water — and was traversing an environment having low deep-layer wind shear (below) — factors which were favorable for intensification.

Jova was later upgraded to Category 5 intensity at 0900 UTC (Infrared images: animated GIF | MP4). 

===== 07 September Update =====

At 0300 UTC on 08 September, Hurricane Lee was upgraded to a Category 5 storm (the first Category 5 of the 2023 Atlantic Basin season). 6 hours of 1-minute GOES-16 Infrared images leading up to that time are shown above — and the 0100 UTC Infrared image with an overlay of Metop-C ASCAT surface scatterometer winds is shown below. Note: 25-km resolution ASCAT winds are available in AWIPS, which indicated a maximum speed of 75 knots — however, the Ultra-High Resolution (UHR) ASCAT wind product at that time (source) had a maximum speed of 136 knots.

Even though deep-layer wind shear was gradually increasing in its vicinity, Lee reached Category 5 intensity as it began to move across water with increasing values of Ocean Heat Content and Sea Surface Temperature (below).

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Northeastern Minnesota, northwestern Wisconsin and the adjacent waters of Lake Superior experienced severe weather late on 5 September 2023. The animation below shows the strong convection moving over western Lake Superior. GOES-16 Mesoscale Sector 1 imagery from 2300-2359 UTC shows numerous convective towers bubbling up and collapsing. Storm Reports from SPC include a 2315 UTC report of a 61-knot gust from the ship Dirk S. VanEnkevort 25 miles southeast of Lutsen (satellite image at that time; note the overshooting top!).

The Storm Prediction Center had a Slight Risk over the western Great Lakes on the 5th, consistently from the 1200 outlook through the 2000 UTC outlook. Watch box #669 was issued for the region at 2100 UTC; the primary threat was strong winds and hail. See also SPC”s Mesoscale Discussions 2084 and 2087 on that day.

The Plains were under the influence of an Elevated Mixed Layer (EML) on this day, as shown in the GOES-16 Band 10 (Low-level water vapor) infrared animation below; EMLs in this enhancement are dark red. The 1200 UTC sounding from Topeka (here, or here), shows very steep mid-level lapse rates as expected with an EML. That is the type of airmass moving northeastward towards Lake Superior during the day.

The upper-air station at Chanhassen MN released a balloon at 1800 UTC, and the toggle below shows the airmass change between 1200 and 1800 UTC on the 5th: the atmosphere moistened and destabilized in those 6 hours. Notice that the shear has changed as well.

Upper-air information with better time resolution was available 5 September for the Minneapolis airport from ACARS data, as shown below. On this day however, no water vapor information was available, so the erosion of the dry layer that is apparent in the two soundings above is not sampled.

NOAA-20 overflew the region shortly after 1800 UTC on 5 September. NUCAPS profiles derived from CrIS and ATMS data on board NOAA-20 could be used to diagnose the thermodynamics of the atmosphere at the time. The image below shows lapse rates (850-700; 700-300) and the Total Totals index; all parameters show a corridor of reduced stability stretching from south-central Minnesota up to western Lake Superior. Quality Flags show green, many complete infrared retrievals, in this region, as might be expected from the visible satellite image. Data from both GOES and JPSS satellites suggest strong convection is possible.

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The slider below compares the 2349 UTC visible imagery with SAR winds derived from RCM3 Normalized Radar Cross Section (NRCS) observations. Note that SAR wind observations over land (including the Apostle Islands) are not valid. Numerous wind maxima in the 25-30 m/s range are apparent in the region of convection extending northeast of the Apostle Islands.

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Thanks to Patrick Ayd, Science and Operations Officer at WFO Duluth, for alerting us to this very interesting event!

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GOES-18 (GOES-West) SO2 RGB and Ash RGB images (above) showed the east-southeastward drift of a volcanic cloud produced by an explosive eruption of Mount Shishaldin that began shortly before 1600 UTC on 05 September 2023. The initial volcanic cloud exhibited pale shades of yellow in both RGB types, suggesting a mixture of SO2 and Ash; after 1900 UTC, the trailing edge of the volcanic cloud exhibited brighter shades of pink in the Ash RGB (indicating higher concentrations of ash).

A Pilot Report issued at 1642 UTC (below) highlighted a volcanic cloud extending to an altitude of 25000 feet. The initial Volcanic Ash Advisory and Forecast advised of ash between the surface and 32000 feet. A Special Marine Statement mentioned the possibility of ash fall to the southeast of Umiak Island.

GOES-18 True Color RGB images from the CSPP GeoSphere site (below) helped to highlight the ash-rich volcanic cloud (shades of tan to brown) moving east-southeast from the summit of Shishaldin.

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As has been noted before, sunglint can occasionally show the parallel lines of internal waves within the ocean. On 4 September, this occurred again north of Sulawesi in the Celebes Sea, as shown in the animation above (created using geo2grid software and Himawari-9 HSD — Himawari Standard Data — files). Different sets of internal waves are apparent, propagating mostly to the southeast towards Sulawesi. However, intersecting sets of internal waves moving in different directions are also visible, especially at 0540 UTC on the 4th, as highlighted below.

Note that there are regions where the glint region is darker than its surroundings. This occurs north of Sulawesi where gap winds from the south are perturbing the ocean surface, reducing the amount of solar reflection. That is shown in the annotated image below, courtesy of Brandon Aydlett and Michael Ziobro, WFO Guam.

Advanced Scatterometer (ASCAT) data from Metop-C early on 4 September 2023, below, (source) show very strong south winds to the east of Sulawesi.

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Many thanks to Brandon Aydlett and Michael Ziobro, WFO Guam, for alerting us to this very interesting event. Note also: there is a long history of using sunglint in MODIS imagery to view internal waves: Link 1; Link 2.

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