Tropical Storm Bolaven formed southeast of the Mariana Islands at 1200 UTC on 07 October 2023 (about 540 miles southeast of Guam, according to the initial advisory issued by JTWC). With sufficient illumination from the Moon (which was in the Waning Crescent phase, at 41% of Full), a nighttime Suomi-NPP VIIRS Day/Night Band (0.7 µm) image valid at 1625 UTC (above) showed Bolaven as it was centered about 200 miles northeast of Chukk International Airport (METAR identifier PTKK) on Weno Island.

JMA Himawari-9 AHI Infrared Window (11.2 µm) images from the CIMSS Tropical Cyclones site (below) showed that Bolaven was moving through an environment of low to moderate deep-layer wind shear — and deep convection was beginning to increase south of the storm center.

A Himawari-9 Infrared / Water Vapor Difference product (reference) (below) highlighted areas of deep convection where overshooting tops were likely penetrating the local tropopause (yellow to red to violet enhancement).

Bolaven was also traversing very warm warm water, where Sea Surface Temperature and Ocean Heat Content values were rather high (below).

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Tropical Storm Bolaven has formed in the western Pacific and is moving towards the Marianas Islands. Were there satellite products useful in monitoring the atmosphere prior to the storm’s development? One such useful product is the Advanced Scatterometer (ASCAT) that is on both MetopB and MetopC polar orbiters at this website (or this one). The stepped animation above, from late on 05 October through late on 07 October, shows 6 different MetopB/MetopC observations of the developing system. At around 2300 UTC on 5 October, a belt of strong westerlies between 2^oN and 6^oN Latitude is Equatorward of easterly winds closer to 10^oN. Twelve hours later, near 1100 UTC on 6 October, a cyclonic circulation is apparent near 9^oN, 156^oE, with winds approaching 25 knots away from the cyclonic center. By 2200 UTC on 7 October, a well-defined storm is shown near 153^oE with tropical-storm forced winds diagnosed by the ASCAT.

MIMIC Total Precipitable Water fields from this same time show a band of rich moisture in the same location of the storm’s formation. The observations of moisture in these fields are from microwave instruments on polar orbiters that sample the Equator occasionally. The movement of the fields is determined by GFS wind output (as described in mp4 / PowerPoint training) A slow increase in cyclonic motion in the moisture fields is apparent south of 10^oN latitude around 155^oE longitude.

Moisture and a cyclonic circulation can mean that a storm is present. For a tropical cyclone to develop and strengthen, low values of vertical wind shear are also required. A wind shear analysis from 0000 UTC on 3 October to 2100 UTC on 5 October, showing wind shear fields from the SSEC/CIMSS Tropical Weather website, below, shows a band of low shear values between the Equator and 10^oN Latitude between 150^oE Longitude and the Dateline. Low shear values, abundant moisture, and a cyclonic circulation all co-located over the warm waters of the tropical Pacific all suggest a storm might develop.

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Seasonably cool Fall-like temperatures overspread the western Great Lakes and northern Plains on 5 October. The animation above shows the Day Cloud Phase Distinction overlain with LightningCast probability from 1321-1721 UTC on 5 October (that is, 8:21-12:21 CDT). SPC on this day was anticipating general thunder over northern Minnesota. Given that, and the autumnal temperatures, as shown below (dewpoints in the 40s, temperature in the 50s) — do you think lightning occurred?

Of course, you should expect lightning to occur. The training data set that was used to create the LightningCast probabilities via Machine Learning includes data from all months. Day Cloud Phase Distinction RGB is showing glaciation, and LightningCast is highlighting regions where lightning subsequently occurs. Recall that the probabilities are for lightning (in the form of a GLM observation) to occur in the next 60 minutes.

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LightningCast probabilities are available online here.

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Looking ahead for the next 2 hours, GOES-16 Day Cloud Phase Distinction RGB images with overlays of GLM Flash Extent Density, LightningCast Probability and Local Storm Reports (above) revealed that some of the convection reached severe levels, producing wind gusts as high as 78 mph at Stannard Rock Lighthouse (identifier STDM4) in western Lake Superior at 1943 UTC (SPC Storm Reports) along with several reports of small (sub-severe) hail.

Given the relatively low GLM Flash Extent Density values associated with this cool-season convection, the default Color Table Min/Max values were changed to 0.5/5.0 (to help identify subtle lightning jumps that preceded some of the high wind reports).

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Target Sector (2.5-minute interval) JMA Himawari-9 AHI Clean Infrared Window (10.4 µm) images (above) showed Typhoon Koinu after it reached Category 4 intensity at 0600 UTC (SATCON) on 04 October 2023. The coldest cloud-top infrared brightness temperatures (-80 to -89C, shades of violet to purple) were generally confined to the southern semicircle of the storm. A faster animation (GIF | MP4) revealed a bit of trochoidal motion (wobble) to Koinu’s eye.

Even though Koinu was traversing an area of warmer water (Sea Surface Temperature | Ocean Heat Content), it was moving through an environment of increasing deep-layer wind shear (source). The general satellite presentation began to deteriorate — with the eye becoming more cloud-filled — as it approached the far southern tip of Taiwan, making landfall around 2332 UTC.

Microwave (85 GHz) images from DMSP and GMI satellites (below) provided 3 views of the evolving eyewall structure at 0934 UTC, 1651 UTC and 2032 UTC.

A toggle between daytime (0450 UTC) and nighttime (1719 UTC) Suomi-NPP VIIRS Day/Night Band (0.7 µm) images (below) also showed that Koinu’s eye became more cloud-filled during that 12-hour period.

It is worth noting that the eye of Koinu passed directly over Orchid Island around 1334 UTC — where an unconfirmed wind gust of 213 mph (95.2 m/s) was measured at the Lanyu Weather Station within the rear eyewall at 1351 UTC.

Statistics released by CWA confirmed that Lanyu observed a gust of 95.2m/s in the last hour, and the data was missing after the 10-minute sustained wind speed rose to 47.6m/s. The current MSLP at Lanyu has dropped to 939hpa, which also broke the historical record? pic.twitter.com/N7eccj3YP5

— Jim yang (@yangyubin1998) October 4, 2023

Unofficially, this 213 mph wind gust is the 3rd highest highest wind gust ever measured on Earth. https://t.co/WgbqoxmB1W

— Colin McCarthy (@US_Stormwatch) October 4, 2023

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