Due to a lack of radar coverage over American Samoa, WSO Pago Pago requested a 1-minute Mesoscale Domain Sector over the islands to monitor convective development and the potential for flash flooding. GOES-18 (GOES-West) Clean Infrared Window (10.3 µm) images (above) showed rain showers that developed in the general vicinity of the American Samoa island of Tutuila (where Pago Pago International Airport NSTU is located) on 09 October 2025. GLM Flash Points indicated that intermittent lightning occurred very near the island of Tutuila — although no thunderstorms or lightning were explicitly reported during that particular 10-hour period at NSTU. The northwest-to-southeast orientated band of deep convection was aligned along a trough of low pressure that was located across the islands.

A Flash Flood Warning was issued at 0335 UTC (25 minutes prior to the start of the animation shown above), which was valid until 0700 UTC — 0.60″ of rainfall was recorded during the 6-hour period ending at 0600 UTC, but there no reports of flooding on Tutuila that were received by WSO Pago Pago.

GOES-18 Total Precipitable Water derived product values in the vicinity of American Samoa were generally in the 2.0-2.4 inch range — and a toggle between Pago Pago rawinsonde data at 0000 UTC and 1200 UTC (below) indicated that Precipitable Water (PW) increased from 2.19 inches to 2.40 inches during that 12-hour period. In addition, after 1200 UTC the coldest cloud-top infrared brightness temperatures associated with some of the rain showers and thunderstorms across the region were around -80ºC (darker shades of purple embedded within brighter white areas) — which represented a small overshoot of the Most Unstable (MU) air parcel’s Equilibrium Level (EL).

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An explosion followed by a large fire occurred at the Chevron El Segundo refinery near Los Angeles (media story) around 0430 UTC on 03 October 2025 (9:30 PM PDT on 02 October). A thermal signature of the fire was evident in 5-minute CONUS Sector GOES-18 (GOES-West) Near-Infrared and Shortwave Infrared images (above), beginning at 0431 UTC — however, this thermal signature began to become less apparent after about 0701 UTC.

In contrast, GOES-18 Microphysics RGB images with an overlay of Next Generation Fire System (NGFS) Fire Detection polygons (below) exhibited a distinct thermal signature for a significantly longer time period (ending at 1626 UTC).

A cursor sample of GOES-18 NGFS Fire Detection parameters at 0431 UTC is shown below.

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1-minute Mesoscale Domain Sector GOES-19 (GOES-East) Visible and Infrared images (above) showed Hurricane Imelda as it intensified from a Category 1 to a Category 2 storm (at 1800 UTC) while approaching Bermuda on 01 October 2025. GLM Flash Points revealed very little satellite-detected lightning activity within the eyewall of Imelda.

About an hour prior to the beginning of the GOES-19 Visible/Infrared image animation above, a DMSP-17 SSMIS Microwave image at 1057 UTC (below) depicted the eye and eyewall of Imelda.

After sunset, 1-minute GOES-19 Infrared images (below) showed Imelda as it moved across the island of Bermuda. The strongest wind gust at Bermuda/Wade International Airport (TXKF) was on the back side of Imelda, with 49 kts occurring at 0400 UTC. Soon after the core of Imelda passed over Bermuda, a series of convective bursts exhibited cloud-top infrared brightness temperatures as cold as -87ºC. Interestingly, brief clusters of GLM Flash Points were associated with some of these convective bursts northeast of Bermuda — until that point, there was a general void of lighting activity with Imelda after sunset.

According to a plot of rawinsonde data from Bermuda International Airport (below), the coldest cloud-top infrared brightness temperature of -87ºC corresponded to an overshoot of the Most Unstable (MU) air parcel’s Equilibrium Level (EL) of at least 1 km.

Category 2 Imelda moved across a patch of warmer Sea Surface Temperature and higher Ocean Heat Content as it approached Bermuda (below) — that warmer water could have played a role in fueling the development of the aforementioned convective bursts (with lighting activity) seen in GOES-19 Infrared imagery.

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1-minute Mesoscale Domain Sector GOES-19 (GOES-East) Visible images (above) showed Hurricane Humberto as it intensified from a Category 4 to a Category 5 storm on 27 September 2025 (Humberto became the second Category 5 storm in the Atlantic Basin this season — with Erin being the first). Low-altitude mesovortices were evident within the eye, along with cloud-top gravity waves in the surrounding eyewall region.

In an animation of 1-minute GOES-19 Infrared images (below), the eye began to exhibit a slight amount of trochoidal motion as Humberto intensified later in the day.

A DMSP-18 SSMIS Microwave image at 2013 UTC — shortly before Humberto was upgraded to Category 5 at 2100 UTC (below) displayed a fully-closed inner eyewall.

Humberto’s intensification to Category 5 occurred as the hurricane was moving through an environment of weak deep-layer wind shear (below) and traversing warm water.

Microwave, wind shear and sea surface temperature imagery was sourced from the CIMSS Tropical Cyclones site.

A RCM-2 Synthetic Aperture Radar (SAR) image (source) at 2214 UTC (below) sensed wind speeds as high as 126.94 kt in the NW quadrant of Humberto.

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