Due to a lack of radar coverage over American Samoa, WSO Pago Pago requested 1-minute Mesoscale Domain Sector coverage 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 showers and thunderstorms that developed in the general vicinity of the American Samoa island of Tutuila (where Pago Pago International Airport NSTU is located) on 08 May 2025. The coldest cloud-top infrared brightness temperatures associated with some of these thunderstorms were in the -80 to -85ºC range (shades of violet to purple embedded within brighter white regions). GLM Flash Points indicated that there was only intermittent lightning activity with this convection (and the lightning occurred north of Tutuila — no thunderstorms or lightning was reported at Pago Pago).

A plot of rawinsonde data from Pago Pago at 0000 UTC (below) displayed a Total Precipitable Water (PW) value of 2.35″ — and a GOES-18 derived Total Precipitable Water value near Tutuila around that time was somewhat higher at 2.52″ (GOES-18 derived Total Precipitable Water values in the vicinity of the island were as high as 2.61″ at 0451 UTC).

The MIMIC Total Precipitable Water product (below) showed that American Samoa (centered at 14.3ºS, 170.7ºW) was situated along the southern edge of a broad east-to-west oriented band of high moisture just north of the islands.

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East-southeast surface winds at Pago Pago during this time period were gusting as high as 36 kts or 41 mph (METAR list | Decoded surface reports) — and these winds were creating Significant Wave Heights of 15-16 feet just east and southeast of the island of Tutuila at 0931 UTC (above). Widespread moderate southeasterly winds across the Samoan Islands region were caused by the pressure gradient between a trough of low pressure just north of the islands (along which the more pronounced convective activity was focused) and high pressure to the south (mean sea level analyses: 0000 UTC | 0300 UTC | 0600 UTC | 1230 UTC | 1500 UTC | 1800 UTC | 2100 UTC) — and the broad areal coverage of these winds was depicted in OSCAT-3 scatterometer data at 1158 UTC (below).

The combination of these winds and periodic heavy rainfall was responsible for some flooding and wind damage across parts of Tutuila (Local Storm Reports).

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GREMLIN (GOES Radar Estimation via Machine Learning to Inform NWP) fields use ABI and GLM data to estimate what radar reflectivity might look like. From 0700-1200 UTC on 8 May (below), GREMLIN showed heavier rains northwest of Samoa, but coverage is slowly increasing.

Between 1400 and 1700 UTC (below), rains have overspread American Samoa, although the bulk of the rains are to the north. (Note: GREMLIN fields are not parallax-corrected).

Between 1720 and 1920 UTC (below), rains continue, and they are widespread.

Widespread rains start to dissipate over Tutuila between 2100 and 2330 UTC, as shown below.

GREMLIN fields are available at the CIRA Slider.

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5-minute CONUS Sector GOES-18 (GOES-West) Shortwave Infrared (3.9 µm) and Fire Mask derived product images (above) displayed a pronounced thermal signature associated with Episode 20 of the ongoing eruption in the Halema’uma’u crater (located within the Kilauea summit caldera) on the Big Island of Hawai’i, which began around 0328 UTC on 07 May 2025. Shortwave Infrared 3.9 µm brightness temperatures exhibited values of 137.88ºC — the saturation temperature of GOES-18 ABI Band 7 detectors — for several hours, beginning at 0336 UTC. This prolonged multi-episode Kilauea eruption began on 23 December 2024.

The sustained period of lava fountaining during this episode ended around 0758 UTC — but a GOES-18 thermal signature of prolonged surface lava flow was still apparent several hours later at 1156 UTC, and the bright nighttime glow of this lava flow could be seen in NOAA-21 VIIRS Day/Night Band imagery (below).

GOES-18 SO₂ RGB images from the RealEarth site (below) indicated that a low-altitude plume of SO₂ (pale shades of green) was drifting southwest from Kilauea — the cluster of dark blue pixels denoted the thermal anomaly associated with the eruption site.

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Dust RGB imagery (here is a larger version) from GK2A at 1720 UTC on 5 May 2025, above, show the distinct bright pink signature of dust along the east coast of China. Airmass RGB imagery (larger version) at the same time shows a rust/orange color as might be expected from a potential vorticity anomaly (GK2A imagery is from this KMA website). NOAA-21 overflew eastern China just after the imagery above. What did the atmospheric spectra from the CrIS instrument on NOAA-21 show? As discussed here and here, Hydra software within McIDAS-V can be used to display CrIS spectra. The partial NOAA-21 data swath is shown below within Hydra.

The plots below show two (identical) longwave infrared spectra (in white and in cyan) as a function of wavenumber at the two points that are plotted on the two different greyscale plots at the bottom of the figure. One point, depicted by a red cross-hatch, corresponds to the white line in the spectral plots; the second, depicted by the cyan cross-hatch, corresponds to the cyan line in the spectral plots. The probed brightness temperature near the region of the bright pink dust RGB signature is warmer at 12.2 µm (283.84 K) than at 10.3 µm (279.99 K); the brightness temperature over the western Pacific at some distance from that pink dust RGB signature is colder at 12.2 µm (283.12 K) than at 10.3 µm (284.7 K). This difference in the differences reflects more absorption by dust of 10.3 µm radiation at the red cross-hatch, and more absorption by water vapor of 12.2 µm radiation at the cyan cross-hatch. Note in the spectral plot how the spectra that has the warmed brightness temperatures is a function of wavenumber. If you had a geostationary sounder viewing this event, the spectrum you see as dust starts to move over will be affected in noticeable ways, just as in this case.

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GOES-19 True-Color animations on 5 May and 4 May 2025 (from the CSPP Geosphere site) show a circulation cut off from the main westerlies and slowly meandering across the Ohio River Valley. The system has become a bit more symmetric on the 5th as it separates more completely from the front along the east coast. A MIMIC Total Precipitable Water animation for the 24 hours ending 1600 UTC on 5 May, below, shows the system surrounded by relatively dry air.

Advected Layer Precipitable Water (ALPW) fields, below, from ca. 0300 UTC 4 May 2025 and 1500 UTC 5 May 2025 (from this site) shows that the system is stacked in the vertical and moving very very slowly.

Airmass RGB shows the system drifting across northern Kentucky in the 38 hours ending at 1400 UTC 5 May 2025. It is spawning clouds and showers over a large portion of the eastern United States as the storm meanders, as shown in this radar animation from here.

The animation below shows the evolution of the surface system from 2100 UTC 3 May through 1200 UTC 5 May 2025.

500-mb heights on 5 May 2025 (below, at 0000 and 1200 UTC), clearly show the system over Kentucky. A system over the southwestern United States is poised to kick the storm over the Ohio River valley to the east.

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