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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Sentinel-1A overflew the Samoan Islands on 4 September — as it does every 12 days — and the Normalized Radar Cross Section data (as at this website) was used to create wind data at very high spatial resolution. Sentinel-1A derived wind data can also be viewed at this NOAA/STAR website that has links to both NRCS data and wind speeds. The wind data shown in the animation above was plotted in AWIPS to facilitate comparison to infrared imagery. Sometimes, there are obvious relationships between the SAR wind features and ABI imagery. In the case shown above, that’s a challenge. The ABI data does show high clouds sinking south towards Samoa and low-level cloud streets moving west-northwest. Peak winds near 30 knots are north of Upolu and east of Savai’i. There is an ABI feature associated with that wind, but it would be difficult to relate reliably all similar ABI features to wind maxima. It is far more challenging to relate the smaller wind maxima that suggest a relationship with cloud streets/showers to ABI structures. It is possible to estimate the wind direction however given the presence of wind shadows downwind of the islands, especially Tutuila.

The Slider below that compares the SAR Wind speeds shows the difficulty in relating the SAR winds to Band 13 imagery.

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Himawari-9 Clean Window infrared (10.4 µm, Band 13) imagery (every 30 minutes) in late August and early September 2023, above, show the initially looping and later linear path of Typhoon Saola from its development east of the Philippines to its landfall in southern China. Saola was very strong just east of the Philippines before weakening on 27-28 August, then strengthening again as the storm slipped between the Philippines and Taiwan. The temporary weakening is apparent in the SATCON wind plot shown below (taken from the SSEC/CIMSS tropical website).

The animation of Total Precipitable Water from the MIMIC site, below, suggests a reason for the weakening on 27/28 August: dry air was entrained into the storm on 27/28 August. The relatively dry airstream is highlighted in this toggle from 0000 UTC 28 August 2023.

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Deep-layer shear estimates (also from the SSEC/CIMSS Tropical Weather website), below, show Saola developing and traversing a region with relatively low shear values. Increased shear does not appear to have been a big factor in the weakening on 27/28 August 2023.

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