During a period when American Samoa had been under a prolonged Flood Watch, the Weather Service Office at Pago Pago requested that a GOES-18 (GOES-West) Mesoscale Domain Sector be positioned over the region (due to their lack of radar). 1-minute Infrared imagery with GLM Flash Points (above) displayed deep convection with intermittent lightning activity that moved southward across the main island of Tutuila (where Pago Pago, METAR identifier NSTU, is located) and the smaller Manu’a Islands (~65 miles to the east) during a 10-hour period on 29 March 2026. The development of rain showers and thunderstorms was enhanced by the presence of a surface trough of low pressure that was draped across the Samoan islands (surface analyses: 0600 UTC | 1230 UTC | 1500 UTC | 1800 UTC).

Rawinsonde data from Pago Pago (below) revealed that the atmosphere was rather moist and unstable, with parameters that were favorable for the development of deep convection. The coldest cloud-top infrared brightness temperature exhibited by storms in the vicinity of NSTU was -78ºC — which represented a small overshoot of the Most Unstable (MU) air parcel’s Equilibrium Level (EL).

Although rainfall across American Samoa on 29 March was not exceptionally heavy (and no Flash Flood Warnings were issued), there was a period when NSTU received 1.11″ of rain within about 2 hours as a thunderstorm with moderate rain showers passed over Tutuila (below).

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1-minute Mesoscale Domain Sector GOES-19 (GOES-East) Visible, Water Vapor and Infrared images (above) showed the development of vertically-propagating standing wave clouds over far northeastern South Dakota on 28 March 2026 — initiated by strong SW winds interacting with the topography of the Coteau des Prairies.

The coldest cloud-top infrared brightness temperatures of the standing wave cloud features were around -45ºC — which roughly corresponded to the 300 hPa pressure level (or an altitude just below 9 km), according to rawinsonde data from Aberdeen SD (below).

A toggle between the GOES-19 images at 1600 UTC and topography (below) included plots of surface wind barbs (white) and RAP model 850 hPa wind barbs (beige) at that time. This helped to visualize the strong flow across the higher terrain that was responsible for generating the standing wave clouds immediately downwind of the Coteau des Prairies (the terrain elevation along the eastern edge abruptly drops from about 2.0 kft to around 1.1 kft — making the formation mechanism of these cloud features similar to those that develop along the coast of northeastern Minnesota).

Wind gusts across and to the lee of the Coteau reached the 60-70 mph range (below).

1-minute GOES-19 True Color RGB images from the CSPP GeoSphere site (below) revealed the quasi-stationary nature of the orographically-forced wave clouds.

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During the day of 27 March 2026, a band of strong strorms propagated from north of the Samoan islands southward to the island themselves. These storm were notable for deep convection and strong environmental instability and were the cause of flood warnings across American Samoa.

A good first look at the environment supporting these storm comes from the NUCAPS vertical profile retrievals. Here’s a profile from the NOAA-20 satellite from just south of the area where convection formed. This is from 13:23 UTC, which corresponds to 2:23 AM in American Samoa Standard Time. Even though it’s the middle of the night, 4822 J/kg of surface-based CAPE is present. The downdraft CAPE is also significant at 895 J/kg, and the sounding is moist (although not atypically high for this region) at 1.87 inches of precipitable water. Together, these values indicate a strong possibility of deep convection and significant rainfall.

The CIMSS MIMIC-TWP2 brings some larger-scale context to the high amounts of water vapor. This loop spans from 0400 UTC on the 27th to 0500 UTC on the 28th. Looking at the Samoan Islands (southwest of the intersection of 10 S and 170 W), it’s clear that they lie in the heart of a strong plume of atmospheric moisture.

The Band 13 (10.3 micron) imagery from GOES-18 also helps show the intensity of the convection, with numerous overshooting tops seen throughout this field of view. These are seen as the grey regions embedded in the darker black areas in the enhanced color scale on the Band 13 imagrey.

It may be easier to recognize the areas of intense convection using the Day Convection RGB. Here, the most vigorous updrafts are visible as bright yellows. Note that the colors seem to fade to a more pastel tone as the loop continues. This is because this particular loop runs between 0100 to 0450 UTC on the 28th. That’s 2:00 to 5:50 PM in local time, and sunset is at 6:28 PM. This product depends on several different shortwave channels (0.64, 1.6, and 3.9 microns) and thus the fading daylight at the end of this loop makes this product increasingly unreliable.

An interesting phenomenon can be seen in the visible imagery during the morning. Lots of outflow boundaries are seen propagating southward of the main convection located to the north of the Samoan islands; note the thin bands of clouds that form in the middle of the image at the end of the loop. Here is where we are seeing the impact of the strong downdraft CAPE (DCAPE) from the NUCAPS sounding at the top of this post. DCAPE represents the tendency of a parcel that is perturbed downwards to keep going down. Areas with high DCAPE are prone to strong evaporatively-cooled downdrafts which force strong winds when they collide with the surface and propagate outward. These downbursts fizzled out before they reached land which makes it hard to determine just how strong they are given the lack of surface observations in the ocean.

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2.5-minute Target Sector JMA Himawari-9 AHI Infrared images (above) showed Cyclone Narelle after it had rapidly intensified to a 115 kt Category 4 storm (JTWC discussion | ADT | SATCON) off the coast of Western Australia on 26 March 2026. As Narelle moved southwestward toward the Exmouth Peninsula, it weakened slightly to a 110 kt Category 3 storm by 2100 UTC.  The coldest cloud-top infrared brightness temperatures were in the -90 to -95C range.

A plot of surface report data from Learmonth Airport (below) depicted a wind gust to 104 kts at 2200 UTC.

Himawari-9 Infrared images with an overlay of Deep-Layer Wind Shear (below) indicated that Narelle was moving through an environment of very low shear — which, along with warm Sea Surface Temperatures favored the intensification that Narelle experienced.

Microwave images from ATMS (above) and DMSP-18 (below) displayed signatures of a partial inner eyewall surrounded by a more complete outer eyewall (with evidence of a moat-like structure separating the two) — indicating that an eyewall replacement cycle was occurring, which in addition to increasing land interaction likely halted further intensification.

Wind shear and microwave imagery were sourced from the CIMSS Tropical Cyclones site.

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