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Decaying Mesoscale Convective System’s outflow boundary produces a low-level undular bore, along with a vertically-propagating gravity wave

By Scott Bachmeier •

5-minute GOES-19 Nighttime Microphysics RGB and daytime True Color RGB images, from 0501-1601 UTC on 28 July [click to play MP4 animation]

5-minute CONUS Sector GOES-19 (GOES-East) Nighttime Microphysics RGB images from the CSPP GeoSphere site (above) depicted a nocturnal arc of low-level cloud bands (pale shades of white) associated with an undular bore produced by the outflow boundary from a decaying Mesoscale Convective System (MCS) over northern Iowa on 28 July 2025 — however, the boundary layer cloud bands of this undular bore began to dissipate around or shortly after sunrise, as seen in the subsequent daytime True Color images.

GOES-19 Mid-level Water Vapor images (below) showed that the MCS outflow boundary produced a few wind gusts >50 kts (red) in Nebraska and Iowa, including a gust to 62 kts at Mason City IA — and this outflow boundary / undular bore also acted as the forcing mechanism for a vertically-propagating gravity wave that continued to move south and southwest for about 12 hours. The notable shift of surface wind direction as the outflow boundary passed began to diminish after about 1401-1501 UTC.

5-minute GOES-19 Mid-level Water Vapor (6.9 µm) images, with/without overlays of hourly surface wind barbs (white) and 30-minute Peak Wind Gusts (cyan/red), from 0501-1701 UTC on 28 July [click to play MP4 animation]

The vertically-propagating nature of the gravity wave was evident due to its signature in both GOES-19 Mid-level Water Vapor (6.9 µm) as well as Upper-level Water Vapor (6.2 µm) images (below). This feature was somewhat analogous to leeside cold frontal gravity waves that have been previously documented on this blog.

5-minute GOES-19 Upper-level Water Vapor images (6.2 µm, top) and Mid-level Water Vapor images (6.9 µm, bottom), from 0501-1701 UTC on 28 July [click to play animated GIF]

The train of vertically-propagating gravity waves was not as apparent farther to the south, in more moisture-rich areas like much of Kansas and Missouri — but due to this moisture, an arc of mid-level clouds was forming along and just behind its leading edge (below). A Pilot Report noted Light to Moderate Turbulence (TB LGT-MOD) at altitudes of 11-15 kft during its eastward climb from the St. Louis airport at 1322 UTC; about 30 minutes earlier, the 1251 UTC METAR surface report from St. Louis KSTL included the mention of Altocumulus castellanus (ACC) from the northwest to the northeast (Altocumulus castellanus is a cloud type that indicates the presence of mid-level instability, which can cause light to moderate turbulence).

GOES-19 Mid-level Water Vapor (6.9 µm) image at 1321 UTC, with cursor samples of a Pilot Report (green) and the METAR surface report at St. Louis MO (white) [click to enlarge]

Categories: Aviation, GOES-19, Red-Green-Blue (RGB) images, Severe convection