The late-season snow event that affected southern Wisconsin on 11 April 2007 dropped as much as 6-12 inches of snow across much of the region (the 7.0 inches at Milwaukee and the 5.4 inches at Madison were both daily snowfall records). However, an AWIPS image of the GOES-12 visible channel (above) shows a good deal of mesoscale variability to the resulting snow cover 2 days following the storm. Of particular interest was the rather large “snow hole” over some of the eastern counties, as well as the lack of snow cover in the lower elevations of the Wisconsin River Valley (MODIS true color images: Wisconsin | Madison hi-res). An AWIPS high-resolution topography image (below; Java applet to fade between the visible and topography images) shows that the large snowfall minimum (NOHRSC 72 hour snowfall reports) was in the area to the west of the higher terrain of the Kettle Moraine; with strong easterly to northeasterly winds during the event (winds gusted to 52 mph at Milwaukee), perhaps a downsloping effect may have helped to keep the area west of the higher terrain slightly warmer (surface air temperatures were near freezing during much of the snowfall period), therefore limiting snow accumulations there.

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An AWIPS image of the 12 April 2007 MODIS Sea Surface Temperature (SST) product over the northwestern Atlantic Ocean (above) showed a very sharp SST gradient between the cold waters of the Labrador Current (30-40º F, blue colors) and the warm waters of the Gulf Stream (55-65º F, green colors). An interactive Java applet allows you to explore the SST values over that region (click on the SST image in the applet to activate the cursor readout). A MODIS SST image farther to the south (below) reveals SST values that were significantly warmer (75-80º F, orange to red colors) in the Gulf of Mexico and along the southern portion of the Gulf Stream. SST and other Level 2 MODIS Science Products are available daily on the CIMSS MODIS Direct Broadcast Products site.

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GOES-12 visible channel images (above; Java animation) revealed an undular bore that was propagating southeastward across the Gulf Coast region of the US on 11 April 2007. This feature was aligned along a pre-frontal trough axis that was located ahead of an advancing cold front. A closer view of the southwestern end of the bore (below; Java animation) shows the fine wave cloud structure as the feature moved over the Gulf of Mexico (the wave train contained as many as 15-20 individual, narrow “roll cloud” bands). Rawinsonde data from Corpus Christi and Brownsville Texas showed that a very pronounced temperature inversion was present in the 1200-2300 ft (360-700 m) layer above the surface, which likely acted as a ducting mechanism for the bore; GOES-12 10.7µm InfraRed (IR) imagery exhibited fairly warm (+12 to +16 C) brightness temperatures in the area of the bore clouds, suggesting that those cloud features were at or below the top of the inversion. Note how the northward-moving marine layer cloud features that were present offshore appeared to dissipate with the passage of the undular bore. A similar case was seen in that same general region on GOES-10 rapid-scan imagery on 19 March 1998.

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MODIS true color imagery (above) revealed a wide swath of fresh snow cover across much of the northcentral US on 04 April 2007. Snowfall during the 03-04 April period included 9 inches in North Dakota, 11 inches in South Dakota, 13 inches in Minnesota (the 12.1 inches that fell in Duluth was the highest daily snowfall for the month of April), 18 inches in Wisconsin, and 25 inches in Michigan. The deep snow cover helped minimum temperatures drop to -3º F at Beulah in North Dakota and +2º F at Roscoe in South Dakota. On the corresponding MODIS false color image (below), snow cover (and ice crystal clouds) appear as varying shades of red, while supercooled water droplet clouds appear as shades of white (bare ground appears as cyan colors). A Java applet allows you to interactively fade between these 2 MODIS images.

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