As discussed on the CIMSS Satellite Blog on July 23rd, active tropical weather continues in the western Pacific Ocean. Specifically, the storm previously known as Invest 98W has organized into Tropical Storm Krosa, now located northwest of Guam and directly west of the Northern Mariana Islands, broadly moving north. Low-Earth Orbiting (LEO) satellite observations collected via direct broadcast by SSEC’s antenna system in Guam provides a valuable perspective on this intensifying storm in near-real time.

Above on the left is a sequence of JPSS VIIRS I-05 imagery centered over Guam between 15:30-16:30 UTC on Friday, July 25th. This is the “Legacy IR Window” band at 11.45 microns, very similar to the traditional IR imagery routinely available from geostationary satellites, albeit with higher spatial resolution. While the broad cloud shield of Krosa is easily visible, it is hard to diagnose what is going on in the lower levels of the storm. Since the time of these observations is during the local nighttime, traditional visible imagery is unavailable from both LEO and GEO platforms. However, VIIRS Adaptive Day Night band imagery (above, on the right) from the same passes, does a great job highlighting the low-level circulation of Krosa, displaced just to the north of the strongest convection.

Another valuable tool for monitoring tropical systems is the AMSR2 microwave imaging instrument aboard GCOM-W1. Above is the Level 1 36.5 GHz Horizontal Polarization brightness temperature observations over the region, on the left at 3:40 UTC on July 25th, and on the right at 15:53 UTC July 25th. This channel is particularly sensitive to the structure of precipitation in the lower portion of the atmosphere. As such, the tightening of the concentric features near the low level circulation is easy to spot over this approximately 12 hour period.

AMSR2 microwave data can also be processed to a number of Level 2 products utilizing the CSPP GAASP software package. Again, the image on the left is from 3:40 UTC July 25th, and on the right from 15:53 UTC July 25th. Comparing the two, there appears to be greater organization of the precipitation bands on the eastern and southern side of the storm in the later pass, with longer, more circular structure evident.

Another GAASP product, Ocean Wind Speed, provides an estimate of wind speeds over water. The time sequence matches the other products shown above. This product makes it clear that both the maximum strength and overall size of Tropical Storm Krosa’s wind field grew substantially in the roughly 12 hours between overpasses at 3:40 and 15:53 UTC. Interests in the region should continue to follow updates on the storm from the National Weather Service. All of the products featured in this post were collected and processed by SSEC’s direct broadcast antenna system in Guam (utilizing software such as CSPP Polar2Grid) and provided to the National Weather Service with low latency.

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1-minute Mesoscale Domain Sector GOES-19 (GOES-East) Visible images (above) showed the rapid development of thunderstorms that produced strong wind gusts across parts of northeast Illinois and northwest Indiana on 24 July 2025 — forcing ground stops for all scheduled inbound flights to Chicago’s Midway and O’Hare airports (beginning at 1945 UTC). The peak wind gust at Midway airport (KMDW) was 56 kts or 64 mph at 2002 UTC. In addition, GLM Flash Points indicated that abundant lightning activity was associated with these thunderstorms.

The corresponding GOES-19 Infrared imagery (below) displayed cloud-top infrared brightness temperatures as cold as -75ºC with some of the more robust thunderstorm overshooting tops.

According to a plot of rawinsonde data from Lincoln, Illinois (below) the coldest GOES-19 cloud-top infrared brightness temperatures of -75ºC represented a ~1 km overshoot of the Most Unstable (MU) air parcel’s Equilibrium Level (EL).

1-minute GOES-19 Visible and Infrared images (below) included plots of SPC Storm Reports — which showed a wind gust to 80 mph (W80) at 1946 UTC (caused by a thunderstorm microburst) in northeast Illinois, and a wind gust to 63 mph (W63) at 2034 UTC in far northwest Indiana.

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It is mid to late July, and the western Pacific Ocean is starting to see its share of tropical activity. While Tropical Storm Francisco is spinning toward the Ryuku Islands of southwestern Japan, the people of Guam have their eyes on Invest 98W, an intensifying system closely approaching their island. As of 0900 UTC on 23 July 2025, Invest 98W is exhibiting winds between 18 and 23 kts. However, the Joint Typhoon Warning Center is projecting a high potential for this system to intensify to a tropical cyclone within 24 hours. Initial satellite imagery using Band 13 (the infrared window) from the Himawari-9 Advanced Himawari Imager (AHI) shows the system as of 1900 UTC on the 23rd. This can be seen on SSEC Real Earthas the group of cold brightness temperatures (therefore deep convection) between 10 and 15 deg N and 140 and 150 deg W.

Note that the invest does not appear to have the clear cyclonic circulation of a more developed tropical storm like what can be seen in Tropical Storm Francisco in the northwest part of the image. However, low leevl winds resolved by scatterometry indicate that a circulation is in place just ot the southwest of Guam.

The biggest concern for Guam right now is significant rainfall. Satellite imagery can help tell that story by indicating the large quantity of water vapor available for precipitation. The AHI Band 8 (6.2 microns) shows large regions of cold brightness temperatures in yellow, indicating dep layers of the atmosphere with high water vapor content.

This can be further seen in the CIMSS MIMIC-TPW2 product, which quantifies the total precipitable water through compositing microwave imagery from several different polar orbiting satellites. While MIMIC does not have the temporal resolution of the geostationary orbiter, its innovative morphological compositing technique is still able to produce hourly observations. The microwave instruments are able to penetrate the upper level clouds and thus the cyclonic circulation of Invest 98W is more readily apparent. This product shows that TPW values near Guam are exceeding 70 mm.

These high values for water vapor are going to contribute to significant rainfall totals for Guam in the coming days. The Global Ensemble Forecast System (GEFS) has 21 members and gives a general assessment of the breadth of potential forecast outcomes over the next week. While the mean outcome is a total of 4 inches of rain and the greatest clustering is between 2 and 3 inches, individual members are showing anywhere between 2 and 14 inches of rainfall between the 23rd and the 31st of July.

More information about this system can be obtained from the Joint Typhoon Warning Center at https://www.metoc.navy.mil/jtwc/jtwc.html.

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5-minute PACUS Sector GOES-18 (GOES-West) daytime True Color RGB and Nighttime Microphysics RGB images created using Geo2Grid (above) revealed the cyclonic circulation of a Mesoscale Convective Vortex (MCV) that migrated west-northwestward across southern Arizona into southeast California (not far south of the California/Nevada border) during the 2-day period from 16 July to 18 July 2025. This MCV emerged from a dissipating cluster of thunderstorms over far southeast Arizona after about 0900 UTC on 16 July — and was moving through an environment of enhanced moisture and instability that was favorable for additional convective development.

This MCV was highlighted in NWS/WPC Mesoscale Precipitation Discussions on 16 July, 17 July and 18 July — since this middle-tropospheric feature was helping to force the development of new thunderstorms, further enhancing flash flooding risks in the vicinity of the MCV.

GOES-18 Shortwave Infrared images visualized using RealEarth (below) included outlines of the numerous Flash Flood Watches/Advisories/Warnings that were issued as the MCV propagated from southern Arizona to southeast California.

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