Severe convection developed ahead of a frontal boundary that was moving southward across the Rio Grande Valley late in the day on 24 April 2007. GOES-12 was in Rapid Scan Operations, providing images at 5-minute intervals during much of the event; 10.7 µm Infrared (IR) images (above; Java animation) showed that these supercells began to exhibit an “enhanced-v” signature about 30 minutes prior to the initial SPC storm reports (although the storm could very well have been producing severe weather before the time of the SPC reports, while it was still over Mexico). This storm produced large hail (up to 2.75 inches in diameter), strong winds (up to 76 mph), and the EF-3 tornado that killed 10 and injured 120 persons in the Eagle Pass, Texas / Piedras Negras, Mexico area.

A view of the storm with the 1-km resolution IR image from the polar orbiting NOAA-16 satellite revealed a “warm trench” signature surrounding the most intense overshooting top (above). You can get a sense that such a trench feature 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). The anvil top temperatures were as cold as -67º C on the GOES-12 IR image, which corresponded to the tropopause temperature near the 141 mb pressure level (at an altitude of 14.3 km / 47,000 ft) on the Del Rio, Texas rawinsonde report — however, the overshooting top temperatures were significantly colder (-78º C) on the NOAA-16 IR image. In addition, a false-color RGB composite using NOAA-16 AVHRR visible channels 1 and 2 along with IR channel 4 (below) showed a well-defined above-anvil cirrus plume streaming northeastward from the primary overshooting top region; this anvil plume was also a persistent feature on GOES-12 visible images (animated GIF | MP4).

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An intense upper-tropospheric cutoff low was moving across the southern Rocky Mountain region on 24 April 2007, which was very evident on GOES-12 water vapor imagery (QuickTime animation). An AWIPS image of the 17 UTC operational “GOES high density winds” overlaid on a 17:10 UTC water vapor image (above) did not adequately resolve the closed circulation; in contrast, the CIMSS “GOES mesoscale winds” (or “Atmospheric Motion Vectors”) generated at 16:15 UTC (below) did show the closed circulation of the upper low over eastern Colorado. These mesoscale winds are part of a suite of products available on the Satellite-based Nowcasting and Aviation Application Program (SNAAP) site.

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No…just a weak cyclonic vortex off the Atlantic coast of Florida that just happens to look like a tropical cyclone with an eye! GOES-12 visible imagery (above; Java animation) shows the cloud swirl as it developed what appeared to be an “eye” on 20 April 2007. While this vortex was responsible for some weak banded offshore rain features (radar reflectivity image), the GOES-12 10.7 µm IR brightness temperatures were still very warm (around 0º C), indicating a lack of deep convection. The vortex existed in a high-shear environment — around 90 knots on the CIMSS wind shear product (below) — which was not favorable for tropical cyclone development. In addition, the MODIS sea surface temperatures in that region were still below the 80º F (26.7º C) threshold generally considered necessary for tropical cyclone genesis, and GOES sounder total precipitable water values were only in the 20-30 mm (0.79-1.18 inch) range.

The history of this particular swirl is rather interesting. A Weather Channel Blog posting discussed a possible Mesoscale Convective Vortex (MCV) origin; if we follow satellite imagery back in time for a day or two, the development of this feature appears to have been tied to a fairly large mid/upper tropospheric cyclone that was just south of the Great Lakes on 18 April (QuickTime animations: GOES water vapor | GOES IR). A potential vorticity (PV) anomaly associated with the cyclone shows up as a local maximum of GOES Sounder total column ozone (light green to red enhancement on this QuickTime animation); a GFS model cross section through the region of the swirl at 18 UTC on 20 April (below) suggests that a PV “tail” lagging behind the main PV anomaly farther offshore may have played a role in helping to spin up the swirl.

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What would eventually become the largest wildfire in Georgia history (the 472,000 acre Sweat Farm Road / Big Turnaround fire) began to burn in southeastern Georgia (in central Ware county, just south of Waycross, station identifier KAYS) during the afternoon hours on 16 April 2007. Much of that region had been experiencing moderate drought conditions, and high winds that day (gusting to 38 mph at Waycross) helped the fire to grow quickly, forcing some local residents to evacuate their homes. AWIPS images of the MODIS and GOES shortwave IR (3.7/3.9 µm) and IR window (10.7/11.0 µm) channels (above) revealed a significant “hot spot” associated with this fire complex at 03 UTC on 17 April (11 PM on 16 April, local time). Compared to the 4-km resolution GOES image, the 1-km resolution MODIS image offered a more accurate depiction of the shape of the active fire (reported to be 1 mile in width, and 12 miles in length), and also showed another smaller fire that was burning to the east of KAYS at that time. Note that portions of the larger fire were hot enough to saturate the shortwave IR detectors on both MODIS and GOES, yielding black pixels (“NO DATA” pixels) on the AWIPS imagery. McIDAS imagery of the fire hot spots depicted maximum brightness temperatures of 329.0 K (132.5 F) on GOES-12, 330.0 K (134.3 F) on MODIS channels 20/22, and 354.9 K (179.15 F) on MODIS channel 23. In addition, the fire was even hot enough to exhibit “warm pixels” on the IR window channel images (42º C on MODIS, 15º C on GOES). The automated CIMSS Wildfire ABBA product indicated saturated fire pixels over that area as early as 20:15 UTC on 16 April (4:15 PM local time).

It was not possible to locate the hottest fire pixel within the cluster of black “NO DATA” pixels on the AWIPS MODIS Band 20 (3.7 µm) IR images; however, one could use the MODIS Band 7 (2.1 µm) image (below) to pinpoint where the hottest portion of the fire was at that particular time (denoted by the small cluster of “bright” pixels in the upper right panel, near the center of Ware county in Georgia). Although the Band 7 imagery is primarily used for snow/ice detection, this near-IR channel (available during daytime only) will exhibit a signal where the hottest pixels are located — this is due to the fact that higher temperatures shift the peak of the Planck function to shorter wavelengths.

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GOES-12 visible and 3.9 µm shortwave IR imagery (above; Java animation) showed a thick smoke plume drifting southeastward during the day on 17 April; the smoke reduced surface visibilities (as low as 1/2 mile at Jacksonville, Florida) and slowed traffic on area roadways. On McIDAS, the saturated fire pixels “wrapped around” and were displayed as very cold pixels (white enhancement) on the 3.9 µm IR imagery. The land surface beneath the thick smoke plume exhibited a slightly cooler 3.9 µm brightness temperature (lighter gray enhancement), due to a reduction in solar radiation arriving at the surface.MODIS true color imagery (below) indicated that the smoke from this fire had drifted as far as the northern Bahama Islands.

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