The Joint Typhoon Warning Center (JTWC) website has for more than a day been tracking invest number 91W, shown above. The feature is circled in yellow, meaning development is unlikely in the next 24 hours. What satellite data might be used to make that determination?

MIMIC Total Precipitable water fields, for the 24 hours ending at 1600 UTC on 24 July 2023, below, show an area rich in tropical moisture, and a broad, diffuse, cyclonic circulation is present between 140 and 150^o E, an between the Equator and 10^oN. In other words, there is plenty of moisture here and one can therefore infer a lack of dry air to impede convection. Are there observations to support the presence of a cyclonic circulation? Motion in the MIMIC fields is a by-product of advection of MIRS Total Precipitable Water fields (computed from individual data swaths from low-earth-orbit — LEO — satellites) by the GFS model (see this training on MIMIC).

Advanced Scatterometer (ASCAT) data from Metop-C and Metop-B (from this NOAA/NESDIS/STAR website), below, shows a cyclonic circulation with southwest winds between 2^o and 6^o N, and southeast winds from 6^oN to 10^oN, around near 147^o E at around 0000 UTC on 24 July 2023. The 1200 UTC imagery, below, doesn’t quite show a circulation in the same region, but it’s hard to tell how well it might have been sampled.

Tropical Cyclones typically form in regions of low shear. Estimates of shear are available at the SSEC/CIMSS Tropical Website (direct link), as are convergence/vorticity analyses, shown in a toggle below. For the region between 0 and 10^oN, 140 to 150 ^oE, there is low-level vorticity (not as concentrated as in the region of Typoon Doksuri, of course) near a narrow ribbon of low shear values (larger shear values are Equatorward of the vorticity center). Low-level convergence and upper-level divergence in the region are weak and do not appear to be linked however.

Himawari-9 Upper-level and lower-level Water Vapor imagery, below (Band 10, 7.34 µm, on the left and Band 8, 6.24 µm, on the right) from 1000 to 1500 UTC shows widespread convection in the square bordered by 140 and 150 oE and 0 and 10oN. No widespread dry air is present. Much of the convection is south and east of the vorticity center.

Future information on this invest (and on Typhoon Doksuri) can be viewed at the webpages of the Joint Typhoon Warning Center. The RSMC at Tokyo also has information on Western Pacific tropical cyclones. Forecasts are for 91W to move between Yap and Guam in the next days. The information presented here might help the reader decide for themselves whether or not the Invest will develop further.

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The image above shows International Space Station orbital paths over the western Pacific Ocean on 24 July 2023, computed from the TLE files and displayed using McIDAS. Note the orbital path at 1600 UTC that moves from mainland China southeastward over Taiwan and then to Indonesia. This is in close proximity to Typhoon Doksuri at that time. LIS lightning observations along this path, from 1602 to 1607 UTC, are shown below. Lightning strikes are detected within the outer rainbands of the typhoon, but no observations occur near the eyewall.

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GOES-18 (GOES-West) SO2 RGB and Ash RGB images (above) showed the northeastward drift of a volcanic cloud produced by an eruption of Mount Shishaldin that began shortly before 0730 UTC on 23 July 2023. The initial (higher-altitude) portion of the volcanic cloud likely contained moderate concentrations of SO2 (denoted by brighter shades of yellow in the SO2 RGB images) — while the trailing (lower-altitude) portion of the volcanic cloud likely contained concentrations of ash (shades of pink in the Ash RGB images). The ash signature dissipated after about 4 hours, while the SO2 signature persisted for the duration of the 11.5-hour animation.

A toggle between the SO2 RGB and Ash RGB images at 1000 UTC is shown below.

A pilot report issued at 0727 UTC (below) indicated that volcanic ash (VA) was rising to altitudes of 30000 feet (FL300).

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A longer animation of GOES-18 SO2 RGB images (above) showed the distinct SO2 signature (brighter shades of yellow) from the Shishaldin eruption, as it rotated counterclockwise within the circulation of a slow-moving middle tropospheric low in the Bering Sea — moving over far southwest Alaska and St. Lawrence Island on 23 July, and eventually becoming stretched into a long/narrow filament over far eastern Russia on 24 July.

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The animation below, showing GOES-16 Clean Window infrared imagery (Band 13, 10.3 µm; processed using geo2grid, v1.1), tracks the named storm Don, first named as Subtropical Storm Don at 0900 UTC on 14 July, reclassified as Tropical Depression Don at 1500 UTC on the 17th, and upgraded to a Tropical Storm at 0300 UTC on the 18th, and to a hurricane at 2100 UTC on the 22nd. The animation shows the storm periodically affected by shear (when the low-level circulation is separated from the deep convection), especially on the 14th of July.

A closer view using 10-minute GOES-16 Visible and Infrared images on 22 July (above) showed Tropical Storm Don developing a distinct eye, before being upgraded to a Hurricane at 2100 UTC. The coldest cloud-top infrared brightness temperatures were around -60ºC (darker shades of red).

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On 24 July, Long-lived Don, devoid of convection, was deemed extra-tropical. This occurred while Don was moving north of the domains shown above. The last tropical advisory was issued at 1500 UTC.

For more information on Don, refer to the National Hurricane Center’s website. Don presents no current threat to land.

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