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.

---

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.

---

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

View only this post Read Less

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.

View only this post Read Less

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.

---

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.

View only this post Read Less

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.

View only this post Read Less