Portions of the Upper Mississippi River Valley region received very heavy rainfall during the last half of August 2007 — 10-20 inches (250-500 mm) of rain was observed in a 30-day period across parts of southeastern Minnesota and southwestern Wisconsin, and 30-day precipitation departures there were as much as 300-400% above normal. A MODIS true color image (using the CIMSS/SSEC “MODIS True Color Imagery Viewer” AWIPS application) centered over southwestern Wisconsin on 12 August (above) shows the Mississippi River (flowing north to south) and the Wisconsin River (flowing east to west) several days before the onset of the heavy rain period. Note the appearance of the Mississippi River “flood plains” (the brighter green areas located just downstream of Lock and Dam Number 8 near Genoa, WI and Lock and Dam Number 9 near Harpers Ferry, IA: Google map).

Two weeks later, a corresponding MODIS true color image from 25 August (below) reveals that much of the Mississippi River flood plains had become flooded by water rich with sediment (lighter brown colors). The Wisconsin River also appears notably wider on the 25 August MODIS image, and one of its major tributaries (the Kickapoo River, flowing north to south) is very evident due to an abundance of sediment-laden water. Use this Java applet to interactively fade (or toggle) between the 12 August and 25 August MODIS true color images.

A new record 24-hour precipitation amount for the state of Minnesota was set when 15.10 inches (384 mm) of rain fell on 18-19 August one mile south of Hokah (which is located near the upper left corner of these MODIS images: Google map). In addition, some locations set new records for the highest total precipitation for any calendar month, including 15.18 inches (386 mm) at Madison WI, and 13.75 inches (349 mm) at La Crosse WI.

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After crossing the Yucatan Peninsula of Mexico, Hurricane Dean emerged into the Gulf of Mexico as a Category 1 storm late in the day on 21 August 2007. GOES-12 10.7µm IR imagery (above) shows a ragged eye associated with Dean; also note the burst of convection to the north and northwest of the eye (beginning around 00:45 UTC on 22 August). This area of convection continued to increase in areal coverage, with IR brightness temperatures eventually cooling to -80º C / -112º F (black enhancement).

An AWIPS image of the MODIS Sea Surface Temperature (SST) product from 20 August (above) reveals a region of warmer SST values in the general area where the convective burst was seen to form on the IR imagery above. SST values were as warm as 86.7º F / 30.4º C (dark red enhancement), and this pocket of deep, warm water may have helped to fuel the burst of convection.

MODIS true color imagery of the Yucatan Peninsula before Dean (above) and after Dean (below) show that the hurricane winds increased the levels of turbidity in the waters of the Gulf of Mexico, leading to widespread areas of cyan and milky-colored water off the west coast of the Yucatan (due to a large amount of suspended particulate matter in the water).

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GOES-12 IR images (above) from the CIMSS Tropical Cyclones site show Hurricane Dean making landfall along the Yucatan Peninsula region of Mexico during the early hours of 21 August 2007, with sustained surface winds near 165 mph. Dean was the first Category 5 storm to make landfall in the Atlantic Basin since Hurricane Andrew struck Florida back in 1992, and Dean’s central pressure at landfall (906 hPa) was the 3rd lowest pressure at landfall (behind 892 hPa with the 1935 Labor Day Hurricane in the Florida Keys, and 888 hPa with Hurricane Gilbert near Cancun, Mexico in 1988). While the appearance on satellite imagery (and the loss of a well-defined eye) indicated a rapid weakening after landfall, Dean is expected to maintain hurricane intensity as it traverses the Yucatan Peninsula.

An IR image from polar-orbiting NOAA-18 satellite (below) shows the eye of Hurricane Dean about 1 hour prior to landfall. Station identifier MMCM is Chetumal, Mexico. The coldest IR brightness temperatures in the northern eyewall region were -83º C / -117 º F (dark red enhancement), while IR brightness temperatures within the eye region were as warm as +20º C/ 68 º F.

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The GOES-12 IR image (above) and the SSM/I 85GHz brightness temperature image (below) from the CIMSS Tropical Cyclones site show Hurricane Dean shortly after it was upgraded to Category 5 intensity late in the day on 20 August 2007, with surface winds estimated to be about 160 mph. Dean is the first Category 5 storm in the Atlantic basin since Hurricane Wilma in October 2005.

Note the double eyewall structure indicated on the SSM/I image below.

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