GOES-18 imagery/products, shown below, depict an area of enhanced instability and moisture centered on American Samoa, and convection develops and persists over/around the main island of American Samoa, Tutuila. The convection appears to form along an east-west line that drifts over the island between 0400 and 0800 UTC.

Total Precipitable Water fields, below, suggest the convection was at the leading edge of a somewhat more moist airmass: TPWs increased from 1.8″ (reddish in the enhancement) to 2.0+” (lilac/purple in the enhancement) as the convection developed on the leading edge of the enhanced moisture.

CSPP Geosphere visible/night microphysics RGB imagery, below, (click here for a faster animation, also from 0010 – 1210 UTC on 1 May 2025) shows the progression of the convection from low-level clouds to strong showers. The low-level outflow boundaries show up nicely as light blue/white/pink arcs that become progressively more dark red as the convection develops.

What other products are useful in diagnosing the atmosphere? The SSEC/CIMSS Tropical Weather page includes wind diagnostics over the South Pacific, and they can be retrieved from the data archive at that page. The vorticity analysis from 21 UTC/30 April 2025 to 120 UTC/1 May 2025, below, shows an increase in low-level vorticity over Samoa, reflecting the convective development that is occurring there. The center of the vorticity slowly drifts north as well during the animation. The vorticity fields have a structure that is in agreement with the observed convection; however, upper-level divergence and low-level convergence fields (not shown) do not.

MIMIC Total Precipitable Water fields (source), below, also show the subtle increase in TPW that accompanied the convection over Samoa.

Scatterometry data for this event was sparse. The toggle below — images that were taken from the manati website (https://manati.star.nesdis.noaa.gov/) — shows OSCAT imagery at 1128 UTC and MetopC imagery at 2013 UTC (both on 30 April 2025). There is speed convergence to the southeast of American Samoa.

LightningCast probabilities (available from an American Samoa Sector at this website) shows increasing probabilities from 0300 UTC to 0600 UTC, with GLM lightning observations starting at 0440 UTC.

LightningCast probabilities can also be calculated with CSPP Geo software, and the output for a longer period is shown below.

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NUCAPS profiles can give a good estimate of the thermodynamic state of the atmosphere. Gridded NUCAPS fields, not shown here, are very useful because they show horizontal fields that allow for a quicker diagnostic of thermodynamics than via the tedious clicking of multiple sounding points. These profiles can be compared to the 0000 UTC/1200 UTC 1 May 2025upper-air sounding from the balloon launched from the Pago Pago airport, shown below. These plots were retrieved from the University of Wyoming sounding site.

What did the NUCAPS profiles show? NUCAPS Sounding availability at 0008 UTC (NOAA-20) and 0059 UTC (NOAA-21) are shown below.

The four closest profiles to Tutuila, shown in the animation below, all have similar thermodynamic characteristics. Moisture is more abundant in the easternmost points.

The information from NOAA-21, at 0058 UTC, below, along a north-south line that passes north-south through the diagnosed maximum in instability, shows CAPE values are greatest just north of the American Samoa islands, and Total Precipitable Water is greatest over the islands.

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Storm Prediction Center storm reports for the 24 hours ending 1200 UTC on 30 April 2025 show a line of strong wind reports from the mid-Mississippi River valley east-northeastward into central Pennsylvania.

What satellite products could have been useful on 29 April to anticipate the arrival of these strong winds? The animation below shows Derived Lifted Index (a clear-sky only product) values plotted on top of visible imagery. Lifted Index values between -2 and -5 are widespread through much of the animation over western Pennsylvania. Convection moving in from the west will be able to access this instability.

NUCAPS profiles were also available over Pennsylvania in a cluster from 1500-1800 UTC, as shown in the animation below. The ‘Sounding Availability Plot’ includes more than one overpass, and selected profiles are shown over central Pennsylvania. Of particular note is the region in very rural Clearfield County where three profiles were sensed over the course of 3 hours! It’s worth repeating that the NUCAPS points are actually a volume of air, a tube about 50km in diameter that matches the ATMS footprint. Considerable destabilization is diagnosed over the three hours. Based on these profiles, a forecaster observing the severe weather reports upstream might not suspect that incoming convection will weaken. (Here’s an animation of just the three profiles over rural Clearfield County).

Clean window infrared imagery, below, spanning from 1900 UTC on 29 April through 0300 UTC on 30 April 2025, shows that coldest cloud tops warmed as the convection moved into central Pennsylvania. However, widespread power outages were reported, as shown here from this source.

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Were the power outages caused by the storms visible in the Day Night band, similar to this event in Spain? The toggle below compares Day Night Band imagery from early morning on the 29th (before the storms) and early morning on the 30th (just after the storms). It’s hard to tell for sure because a lot of the changes in illumination on 30 April are due to the thick cloud cover from the storms.

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1-minute Mesoscale Domain Sector GOES-19 (GOES-East) “Red” Visible (0.64 µm) and “Clean” Infrared Window (10.3 µm) images (above) showed thunderstorms that produced giant hail (as large as 5.00 inches in diameter) and damaging wind gusts (as high as 106 mph) across the Texas Panhandle and North Texas (SPC Storm Reports) on 29 April 2025. The Infrared images revealed pulses of thunderstorm overshooting tops that exhibited 10.3 µm brightness temperatures as cold as -78ºC (brighter white pixels embedded within dark black regions) — and the signature of an Above-Anvil Cirrus Plume (reference) was evident in Visible imagery (although the warmer AACP signature was less distinct in the lower-resolution Infrared images).

According to a plot of rawinsonde data (source) from Fort Worth, Texas at 0000 UTC on 30 April (below), the coldest GOES-19 cloud-top infrared brightness temperature of -78ºC represented a ~1.5 km overshoot of the Most Unstable air parcel Equilibrium Level (MU EL).

GOES-19 Visible images with plots of GLM Flash Points (below) displayed abundant lightning activity associated with these supercell thunderstorms — which were generally focused in the vicinity of a quasi-stationary frontal boundary.

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Power outages over the Iberian Peninsula on 28-29 April 2025 have caused wide-spread travel (and life) disruptions (Reuters link). Day Night Band imagery from the NASA Worldview Site (link), shown below, show a reduction in city lights especially over southern Spain on the 29th. The NOAA-20 overpass on 29 April was shortly after 0140 UTC. Skies at that time were mostly clear over southern Spain

The 3-day animation below includes imagery from 27 April as well.

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When NOAA-20 overflew Spain early on 30 April, Day Night Band imagery did not show widespread outages, as shown below.

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