A major outbreak of severe convection developed across the southern and central Plains states during the day on 28 March and 29 March 2007; large and long-lived thunderstorms produced widespread tornadoes (including fatal tornadoes in Texas and Oklahoma, and the first tornado-related fatality in Colorado since 1960), large hail (up to 4.5 inches in diameter in Texas), and damaging winds (28 March SPC storm reports). An AWIPS image of the MODIS 11.0µm InfraRed (“IR window”) channel with overlays of cloud-to-ground lightning strikes and surface wind streamlines (above) shows the large density of lightning strikes (1587 negative strikes and 61 positive strikes in a 15-minute period) that was generally aligned along a cold frontal boundary that was moving eastward across the Texas panhandle region. If we examine the MODIS IR image without any overlays (below), we see that there were 2 examples of a subtle “warm trench” signature (a ring of warmer cloud top temperatures surrounding the coldest temperatures associated with the overshooting tops) — this satellite signature suggests that there may have been some compensating subsidence immediately surrounding the vigorous overshooting tops as they penetrated the tropopause level. You can get a sense that such a “trench” can surround an overshooting top by examining astronaut photography of thunderstorms taken from the space shuttle (image courtesy of Earth Sciences and Image Analysis Laboratory, NASA Johnson Space Center). QuickTime animations of rapid-scan GOES-11 10.7µm IR images centered on Lubbock, Texas and Goodland, Kansas show large and persistent enhanced-v signatures exhibited by many of these storms.

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Widespread severe convection developed during the day on 23 March 2007 over parts of New Mexico, Texas, Oklahoma, Kansas, and Colorado. AWIPS imagery of the MODIS and GOES InfraRed (IR) and water vapor channels (above) indicates that six of these storms located over eastern New Mexico and the Texas/Oklahoma panhandle region exhibited “enhanced-v” signatures around 20:30-20:38 UTC (the enhanced-v signatures were better defined on the 1-km resolution MODIS images, compared to the 4-km resolution GOES images). This particular satellite signature is an indicator that thunderstorms are producing (or are about to produce) hail, tornadoes, or damaging winds. According to the SPC storm reports, the storms were indeed producing hail up to 1.75 inches in diameter in New Mexico around the time of the images above; hail up to 2 inches in diameter and several tornadoes were then reported within 1-4 hours of the image time.

A vigorous upper-level cutoff low was located over the Baja California region at 12 UTC that day. AWIPS imagery of the GOES sounder total column ozone product (below) depicted elevated ozone levels (green enhancement) within a broad tropopause anomaly in the vicinity of the upper low — the height of the 1.5 Potential Vorticity Unit (PVU) surface (which defines the dynamical tropopause) was as low 475 hPa . Dynamic forcing associated with this low began to increase over the southwestern US as the cutoff low moved northeastward during the day.

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The effect of the topography in western South Dakota and eastern Wyoming was very obvious on 19 March 2007, as low-altitude stratus clouds were forced to move around the higher elevation portions of the Black Hills (GOES-11 visible images: Java animation). AWIPS images of a few of the MODIS channels (above) revealed that a good deal of the cloud-free highest elevations were still snow-covered. This snow cover appeared brighter than the surrounding tree-covered (but snow-free) surface on the Band 1 visible image (above, upper left panel) — the darker appearance of that same area on the Band 7 Snow/Ice image (above, lower left panel) is a signal of snow on the ground (in contrast to the supercooled water droplet stratus cloud, which appeared much brighter on the Band 7 image). MODIS and GOES fog/stratus product images from the pre-dawn hours showed that this area of stratus cloud was forming due to upslope northerly/northeasterly flow in the wake of a cold frontal passage.

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A 500-meter resolution MODIS true color image (below) showed even better detail of the snow cover and the stratus clouds that surrounded the Black Hills. The light gray region just to the east of the stratus cloud edge is the Badlands National Park, whose sandy and rocky surface has a higher albedo than the surrounding grasslands. Snow depths at the highest elevations were still as great as 18 inches at North Rapid Creek (station NRPS2, near Rochford, South Dakota).

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GOES-11 (GOES-West) Visible (daytime) and 3.9 µm Shortwave Infrared (nighttime) imagery (above) revealed an interesting cyclonic vortex which was propagating westward across the eastern North Pacific Ocean on 13-14 March 2007. The radial banded cloud features that form such a cloud “swirl” are known as actinae or actinoform clouds, and they are occasionally seen in the marine stratocumulus cloud field over the Pacific Ocean (one such case was June 1997). This type of cloud pattern was first observed back in 1962 on TIROS V imagery near Hawaii.

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