Altocumulus castellanus clouds (ACCAS) unexpectedly developed in southern Wisconsin yesterday morning, producing beautiful cloud features and eventually a severe thunderstorm. ACCAS form due to mid-level instability and manifest in towering billows that can often produce pileus clouds as they burst through a more stable layer.

The rooftop cameras of the UW-AOSS building caught this development nicely.

A model sounding from the High-Resolution Rapid Refresh (HRRR) did hint at mid-level instability due to an elevated mixed layer atop a temperature inversion. The model shows a mid-level lapse rate of 8.7 C/km and 155 J/kg of most-unstable CAPE (MUCAPE). However, the model did not forecast convection or precipitation.

There is some evidence that frontogenesis at 700mb may have served as a forcing mechanism for the convection, but it remains uncertain.

The NOAA/CIMSS LightningCast model caught this ACCAS development, and predicted elevated probabilities of lightning 20-45 minutes prior to first flashes (depending on position in the line).

Several cells produced 1″ hail in far southeastern Wisconsin.

The ProbSevere v3 models (PSv3) never exceeded 19% on these cells. However, an experimental image-based AI model produced > 50% probability of severe for several time steps (it predicts the probability of any severe hazards within the next 45 minutes and 20 km). The image-based approach may be able to better exploit spatial context and multi-modal signals from meteorological observations than the tree-based PSv3.

As a meteorologist, it is not always pleasant to be surprised by unexpected weather, but it is always a delight to behold beauty and wonder in the sky.

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1-minute Mesoscale Domain Sector GOES-19 (GOES-East) True Color RGB images from the CSPP GeoSphere site (above) highlighted notable smoke plumes from two large wildfires in southeast Georgia (the Pineland Road Fire near the Florida border, and the Brantley Highway 82 Fire farther to the northeast), in addition to another wildfire in northern Florida on 21 April 2026. Those particular wildfires were burning in parts of Georgia and Florida that were experiencing Exceptional Drought conditions.

1-minute GOES-19 GeoColor RGB images with an overlay of Next Generation Fire System (NGFS) Fire Detection polygons (below) provided a close-up view of the smoke plume and thermal signature associated with the Brantley Highway 82 wildfire in southeast Georgia — which was nearly under control at 700 acres in size during the early morning hours on 21 April, but then quickly grew out of control as surface winds abruptly shifted to the northeast then to the east (occasionally gusting to 22-23 mph), with the fast-moving wildfire burning an area of 5000 acres by the end of the day. Numerous evacuation orders were issued, about 87 homes were destroyed and schools throughout Brantley County were closed due to air quality concerns.

At 1758 UTC, the wildfire first exhibited a 3.9 µm shortwave infrared brightness temperature of 138ºC (below) — which is the saturation temperature of GOES-19 ABI Band 7 detectors. Also of note, at that time the Fire Radiative Power of the thermal signature was a rather high 6594 MW.

Periodic bursts of brighter-white pyrocumulus clouds rose above the hazy gray smoke plume, such as was seen just west of the fire’s thermal anomaly at 2019 UTC (below).

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5-minute CONUS Sector GOES-19 (GOES-East) imagery (above) revealed the brief development of what appeared to be a standing wave cloud — caused by a vertically-propagating gravity wave — that was anchored near the north coast of Saginaw Bay, Michigan on 20 April 2026. At first glance, this author was reminded of similar-appearing standing wave clouds that form along the Minnesota coast of Lake Superior.

A toggle between GOES-19 images and Topography (below) seemed to show that the northwestern edge of the cloud feature lined up with the final (subtle) drop in topography near the coast — however, since there was not the strong NW offshore surface wind flow common to the aforementioned Minnesota example, perhaps the onshore (and slightly upslope) lake breeze played a role in vertical gravity wave initiation? Until a sound explanation rooted in science is stumbled upon, this event will fall into the coveted “What the heck is this?” blog post category.

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5-minute CONUS Sector GOES-18 (GOES-West) and GOES-19 (GOES-East) Nighttime Microphysics RGB + daytime True Color RGB images from the CSPP GeoSphere site (above) showed a lake effect cloud plume that developed over the southern half of Great Salt Lake on 17 April 2026 — which produced a swath of light snowfall downwind (southeast) of the lake that was apparent in True Color RGB images after sunrise.

Day Snow-Fog RGB images from GOES-18 and GOES-19 created using Geo2Grid (below) were helpful in discriminating between the lake effect clouds (shades of white) and the fresh snow cover (shades of red) immediately downwind of the Great Salt Lake after sunrise. Bare ground appeared as shades of green.

A plot of the 24-hour total observed snowfall (below) indicated that amounts were generally 1 inch or less at lower-elevation sites downwind of the Great Salt Lake (with 2 sites near topographical features reporting 3.0 inches).

GOES-18 Infrared images (below) depicted cloud-top infrared brightness temperatures within the lake effect cloud plume that were as cold as -35ºC. Note that Salt Lake City International Airport (KSLC) was reporting light snow which reduced the surface visibility to 8 miles at 0454 UTC, as the lake effect cloud was moving over that location.

According to a plot of 0000 UTC rawinsonde data from Salt Lake City (below), an air temperature of -35ºC occurred at an elevation of 5900 m (19358 ft). Since the water temperature of the lake was around 50ºF or 10ºC that day, the surface temperature of the sounding was modified to match that value.

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