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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A lee-side frontal gravity wave was seen on GOES-12 6.5µm “water vapor” imagery (above; Java animation) — this feature was associated with a cold frontal boundary that was moving southward across eastern New Mexico and western Texas on 03 April 2007. These types of vertically-propagating gravity waves are stationary with respect to the flow (they follow the cold front) and are forced by nongeostrophic and nonhydrostatic accelerations in the frontal zone (these waves above frontal surfaces are dynamically equivalent to stationary “trapped lee waves” that appear over mountains). Wave breaking induces narrow zones of subsidence that can be sensed as warm/dry bands on the water vapor imagery. Note that the “wave signature” was not readily apparent on the 8-km resolution GOES-11 water vapor channel image; the gravity wave packet eventually became less obvious on GOES-12 4-km resolution water vapor imagery after about 17 UTC, but was still evident on 1-km resolution MODIS water vapor channel imagery at that time. A comparison of GOES-12 visible and water vapor imagery indicates that no clouds were forming along the cold frontal zone in the New Mexico / Texas border region; however, the vertically-propagating gravity waves along the far western end of the front appeared to generate cirrus cloud patches in the foothills of New Mexico.
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AWIPS images of NOAA wind profiler data (below) show that the depth of the cold air was fairly shallow, with winds shifting to northeasterly within the lowest 1 km of the atmosphere at Tucumcari, New Mexico (TCUN5) and Jayon, Texas (JTNT2) as the front moved southward through those locations. Even though the GOES-12 water vapor channel weighting function (calculated using Amarillo, Texas rawinsonde data) peaked at around 450 hPa (about 6.5 km above the surface), a signal of the vertically-propagating wave breaking above the top of the cold front was still evident on the water vapor channel imagery.

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A shallow bank of fog and stratus cloud was moving southward along the Delaware/Maryland/Virginia coast during the pre-dawn and early morning hours on 03 April 2007. GOES-12 3.9µm InfraRed (IR) imagery (above; Java animation) shows that the fog/stratus feature appeared to be “channeled” along the colder waters that were located immediately offshore (between the coastline and the warmer Gulf Stream waters farther offshore) — sea surface temperatures the previous afternoon were 45-55º F on the MODIS SST product. The fog dissipated over the warmer water (and also over land as the warm April sun heated the surface), while the fog immediately off the coast continued to move southward over the patch of colder waters. A comparison of the GOES and MODIS fog/stratus product around 03/04 UTC revealed that the edges of the fog/stratus were much sharper on the 1-km resolution MODIS imagery (and the 4-km resolution GOES fog/stratus product exhibited some “false signal” off the Virginia coast at that time).

GOES-12 visible imagery (below; Java animation) showed better details of the fog/stratus with the 1-km resolution channel; it is interesting to note that the fog/stratus appeared to thicken as a “shock wave” formed along the leading edge upon reaching Cape Henry in the Norfolk/Virginia Beach area (MODIS true color images: Terra | Aqua).

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An animation of 3-hourly water vapor channel image composites from AWIPS during the entire month of March 2007 (41 MB QuickTime animation; 1280×1024 screen resolution required) shows the diverse variety of synoptic scale features that affected the Northern Hemisphere during that particular month.

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