{"id":70009,"date":"2026-04-22T13:55:21","date_gmt":"2026-04-22T13:55:21","guid":{"rendered":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/?p=70009"},"modified":"2026-04-22T15:24:33","modified_gmt":"2026-04-22T15:24:33","slug":"surprise-accas","status":"publish","type":"post","link":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/archives\/70009","title":{"rendered":"Surprise ACCAS"},"content":{"rendered":"\n

Altocumulus castellanus<\/a> 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<\/a> as they burst through a more stable layer.<\/p>\n\n\n\n

\"\"
The mid-level ACCAS clouds, with convective towers producing pileus, as seen from south UW-Madison campus. Courtesy of Jon Wade, UW Athletics.<\/figcaption><\/figure>\n\n\n\n

The rooftop cameras of the UW-AOSS building caught this development nicely.<\/p>\n\n\n\n

West view from rooftop camera<\/figcaption><\/figure>\n\n\n\n
South view from rooftop camera.<\/figcaption><\/figure>\n\n\n\n

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.<\/p>\n\n\n\n

\"Sounding
Skew-T \/ Log-P diagram and associated sounding metrics from the HRRR model, for Madison, Wisconsin.<\/figcaption><\/figure>\n\n\n\n

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

\"\"
700 mb height, wind, temperature, and frontogenesis from the Storm Prediction Center mesoanalysis. <\/figcaption><\/figure>\n\n\n\n

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

GOES-19 LightningCast contours, ABI true-color imagery, and GLM flash-extent density.<\/figcaption><\/figure>\n\n\n\n

Several cells produced 1″ hail in far southeastern Wisconsin.<\/p>\n\n\n\n

\"\"
SPC preliminary severe hail reports. All reports were for 1″-diameter hailstones.<\/figcaption><\/figure>\n\n\n\n

The ProbSevere v3 models<\/a> (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.<\/p>\n\n\n\n

\"\"
The image-based ML model (“PSCNN”) contours, MRMS MergedReflectivity, and NWS severe weather warnings valid at 17:50 UTC.<\/figcaption><\/figure>\n\n\n\n

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.<\/p>\n","protected":false},"excerpt":{"rendered":"

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 […]<\/p>\n","protected":false},"author":14,"featured_media":70021,"comment_status":"closed","ping_status":"","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[10,159,115],"tags":[111],"class_list":["post-70009","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-general-interpretation","category-goes-19","category-lightningcast","tag-lightningcast"],"acf":[],"_links":{"self":[{"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/posts\/70009","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/users\/14"}],"replies":[{"embeddable":true,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/comments?post=70009"}],"version-history":[{"count":4,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/posts\/70009\/revisions"}],"predecessor-version":[{"id":70023,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/posts\/70009\/revisions\/70023"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/media\/70021"}],"wp:attachment":[{"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/media?parent=70009"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/categories?post=70009"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/cimss.ssec.wisc.edu\/satellite-blog\/wp-json\/wp\/v2\/tags?post=70009"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}